aboutsummaryrefslogtreecommitdiffstats
path: root/system/doc/reference_manual/expressions.xml
blob: fa8f9b2e8f8ab0b2254fd4cd4bbfae8a784ef03d (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.

      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.

    </legalnotice>

    <title>Expressions</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>expressions.xml</file>
  </header>
  <p>In this chapter, all valid Erlang expressions are listed.
    When writing Erlang programs, it is also allowed to use macro-
    and record expressions. However, these expressions are expanded
    during compilation and are in that sense not true Erlang
    expressions. Macro- and record expressions are covered in
    separate chapters: <seealso marker="macros">Macros</seealso> and
    <seealso marker="records">Records</seealso>.</p>

  <section>
    <title>Expression Evaluation</title>
    <p>All subexpressions are evaluated before an expression itself is
      evaluated, unless explicitly stated otherwise. For example,
      consider the expression:</p>
    <code type="none">
Expr1 + Expr2</code>
    <p><c>Expr1</c> and <c>Expr2</c>, which are also expressions, are
      evaluated first - in any order - before the addition is
      performed.</p>
    <p>Many of the operators can only be applied to arguments of a
      certain type. For example, arithmetic operators can only be
      applied to numbers. An argument of the wrong type will cause
      a <c>badarg</c> run-time error.</p>
  </section>

  <section>
    <marker id="term"></marker>
    <title>Terms</title>
    <p>The simplest form of expression is a term, that is an integer,
      float, atom, string, list, map or tuple.
      The return value is the term itself.</p>
  </section>

  <section>
    <title>Variables</title>
    <p>A variable is an expression. If a variable is bound to a value,
      the return value is this value. Unbound variables are only
      allowed in patterns.</p>
    <p>Variables start with an uppercase letter or underscore (_)
      and may contain alphanumeric characters, underscore and @.
      Examples:</p>
    <pre>
X
Name1
PhoneNumber
Phone_number
_
_Height</pre>
    <p>Variables are bound to values using
      <seealso marker="patterns">pattern matching</seealso>. Erlang
      uses <em>single assignment</em>, a variable can only be bound
      once.</p>
    <p>The <em>anonymous variable</em> is denoted by underscore (_) and
      can be used when a variable is required but its value can be
      ignored. Example:</p>
    <pre>
[H|_] = [1,2,3]</pre>
    <p>Variables starting with underscore (_), for example
      <c>_Height</c>, are normal variables, not anonymous. They are
      however ignored by the compiler in the sense that they will not
      generate any warnings for unused variables. Example: The following
      code</p>
    <pre>
member(_, []) ->
    [].</pre>
    <p>can be rewritten to be more readable:</p>
    <pre>
member(Elem, []) ->
    [].</pre>
    <p>This will however cause a warning for an unused variable
      <c>Elem</c>, if the code is compiled with the flag
      <c>warn_unused_vars</c> set. Instead, the code can be rewritten
      to:</p>
    <pre>
member(_Elem, []) ->
    [].</pre>
    <p>Note that since variables starting with an underscore are
      not anonymous, this will match:</p>
    <pre>
{_,_} = {1,2}</pre>
    <p>But this will fail:</p>
    <pre>
{_N,_N} = {1,2}</pre>
    <p>The scope for a variable is its function clause.
      Variables bound in a branch of an <c>if</c>, <c>case</c>, 
      or <c>receive</c> expression must be bound in all branches 
      to have a value outside the expression, otherwise they
      will be regarded as 'unsafe' outside the expression.</p>
    <p>For the <c>try</c> expression introduced in 
      Erlang 5.4/OTP-R10B, variable scoping is limited so that 
      variables bound in the expression are always 'unsafe' outside 
      the expression. This will be improved.</p>
  </section>

  <section>
    <marker id="pattern"></marker>
    <title>Patterns</title>
    <p>A pattern has the same structure as a term but may contain
      unbound variables. Example:</p>
    <pre>
Name1
[H|T]
{error,Reason}</pre>
    <p>Patterns are allowed in clause heads, <c>case</c> and
      <c>receive</c> expressions, and match expressions.</p>

    <section>
      <title>Match Operator = in Patterns</title>
      <p>If <c>Pattern1</c> and <c>Pattern2</c> are valid patterns,
        then the following is also a valid pattern:</p>
      <pre>
Pattern1 = Pattern2</pre>
      <p>When matched against a term, both <c>Pattern1</c> and
        <c>Pattern2</c> will be matched against the term. The idea
        behind this feature is to avoid reconstruction of terms.
        Example:</p>
      <pre>
f({connect,From,To,Number,Options}, To) ->
    Signal = {connect,From,To,Number,Options},
    ...;
f(Signal, To) ->
    ignore.</pre>
      <p>can instead be written as</p>
      <pre>
f({connect,_,To,_,_} = Signal, To) ->
    ...;
f(Signal, To) ->
    ignore.</pre>
    </section>

    <section>
      <title>String Prefix in Patterns</title>
      <p>When matching strings, the following is a valid pattern:</p>
      <pre>
f("prefix" ++ Str) -> ...</pre>
      <p>This is syntactic sugar for the equivalent, but harder to
        read</p>
      <pre>
f([$p,$r,$e,$f,$i,$x | Str]) -> ...</pre>
    </section>

    <section>
      <title>Expressions in Patterns</title>
      <p>An arithmetic expression can be used within a pattern, if
        it uses only numeric or bitwise operators, and if its value
        can be evaluated to a constant at compile-time. Example:</p>
      <pre>
case {Value, Result} of
    {?THRESHOLD+1, ok} -> ...</pre>
      <p>This feature was added in Erlang 5.0/OTP R7.</p>
    </section>
  </section>

  <section>
    <title>Match</title>
    <pre>
Expr1 = Expr2</pre>
    <p>Matches <c>Expr1</c>, a pattern, against <c>Expr2</c>.
      If the matching succeeds, any unbound variable in the pattern
      becomes bound and the value of <c>Expr2</c> is returned.</p>
    <p>If the matching fails, a <c>badmatch</c> run-time error will
      occur.</p>
    <p>Examples:</p>
    <pre>
1> <input>{A, B} = {answer, 42}.</input>
{answer,42}
2> <input>A.</input>
answer
3> <input>{C, D} = [1, 2].</input>
** exception error: no match of right hand side value [1,2]</pre>
  </section>

  <section>
    <marker id="calls"></marker>
    <title>Function Calls</title>
    <pre>
ExprF(Expr1,...,ExprN)
ExprM:ExprF(Expr1,...,ExprN)</pre>
    <p>In the first form of function calls,
      <c>ExprM:ExprF(Expr1,...,ExprN)</c>, each of <c>ExprM</c> and
      <c>ExprF</c> must be an atom or an expression that evaluates to
      an atom. The function is said to be called by using the
      <em>fully qualified function name</em>. This is often referred
      to as a <em>remote</em> or <em>external function call</em>.
      Example:</p>
    <code type="none">
lists:keysearch(Name, 1, List)</code>
    <p>In the second form of function calls,
      <c>ExprF(Expr1,...,ExprN)</c>, <c>ExprF</c> must be an atom or
      evaluate to a fun.</p>

    <p>If <c>ExprF</c> is an atom the function is said to be called by
      using the <em>implicitly qualified function name</em>.  If the
      function <c>ExprF</c> is locally defined, it is called.
      Alternatively if <c>ExprF</c> is explicitly imported from module
      <c>M</c>, <c>M:ExprF(Expr1,...,ExprN)</c> is called. If
      <c>ExprF</c> is neither declared locally nor explicitly
      imported, <c>ExprF</c> must be the name of an automatically
      imported BIF. Examples:</p>

    <code type="none">
handle(Msg, State)
spawn(m, init, [])</code>
    <p>Examples where ExprF is a fun:</p>
    <code type="none">
Fun1 = fun(X) -> X+1 end
Fun1(3)
=> 4

fun lists:append/2([1,2], [3,4])
=> [1,2,3,4]</code>

    <p>Note that when calling a local function, there is a difference
    between using the implicitly or fully qualified function name, as
    the latter always refers to the latest version of the module. See
    <seealso marker="code_loading">Compilation and Code Loading</seealso>.</p>

    <p>See also the chapter about
      <seealso marker="functions#eval">Function Evaluation</seealso>.</p>

    <section>
      <title>Local Function Names Clashing With  Auto-imported BIFs</title>
    <p>If a local function has the same name as an auto-imported BIF,
    the semantics is that implicitly qualified function calls are
    directed to the locally defined function, not to the BIF. To avoid
    confusion, there is a compiler directive available,
    <c>-compile({no_auto_import,[F/A]})</c>, that makes a BIF not
    being auto-imported. In certain situations, such a compile-directive
    is mandatory.</p>

    <warning><p>Before OTP R14A (ERTS version 5.8), an implicitly
    qualified function call to a function having the same name as an
    auto-imported BIF always resulted in the BIF being called. In
    newer versions of the compiler the local function is instead
    called. The change is there to avoid that future additions to the
    set of auto-imported BIFs does not silently change the behavior
    of old code.</p>

    <p>However, to avoid that old (pre R14) code changed its
    behavior when compiled with OTP version R14A or later, the
    following restriction applies: If you override the name of a BIF
    that was auto-imported in OTP versions prior to R14A (ERTS version
    5.8) and have an implicitly qualified call to that function in
    your code, you either need to explicitly remove the auto-import
    using a compiler directive, or replace the call with a fully
    qualified function call, otherwise you will get a compilation
    error. See example below:</p> </warning>

    <code type="none">
-export([length/1,f/1]).

-compile({no_auto_import,[length/1]}). % erlang:length/1 no longer autoimported

length([]) ->
    0;
length([H|T]) ->
    1 + length(T). %% Calls the local funtion length/1

f(X) when erlang:length(X) > 3 -> %% Calls erlang:length/1,
                                  %% which is allowed in guards
    long.</code>

    <p>The same logic applies to explicitly imported functions from
    other modules as to locally defined functions. To both import a
    function from another module and have the function declared in the
    module at the same time is not allowed.</p>

    <code type="none">
-export([f/1]).

-compile({no_auto_import,[length/1]}). % erlang:length/1 no longer autoimported

-import(mod,[length/1]).

f(X) when erlang:length(X) > 33 -> %% Calls erlang:lenght/1,
                                   %% which is allowed in guards

    erlang:length(X);              %% Explicit call to erlang:length in body

f(X) ->
    length(X).                     %% mod:length/1 is called</code>


    <p>For auto-imported BIFs added to Erlang in release R14A and thereafter,
    overriding the name with a local function or explicit import is always
    allowed. However, if the <c>-compile({no_auto_import,[F/A])</c>
    directive is not used, the compiler will issue a warning whenever
    the function is called in the module using the implicitly qualified
    function name.</p>
    </section>
  </section>

  <section>
    <title>If</title>
    <pre>
if
    GuardSeq1 ->
        Body1;
    ...;
    GuardSeqN ->
        BodyN
end</pre>
    <p>The branches of an <c>if</c>-expression are scanned sequentially
      until a guard sequence <c>GuardSeq</c> which evaluates to true is
      found. Then the corresponding <c>Body</c> (sequence of expressions
      separated by ',') is evaluated.</p>
    <p>The return value of <c>Body</c> is the return value of
      the <c>if</c> expression.</p>
    <p>If no guard sequence is true, an <c>if_clause</c> run-time error
      will occur. If necessary, the guard expression <c>true</c> can be
      used in the last branch, as that guard sequence is always true.</p>
    <p>Example:</p>
    <pre>
is_greater_than(X, Y) ->
    if
        X>Y ->
            true;
        true -> % works as an 'else' branch
            false
    end</pre>
  </section>

  <section>
    <marker id="case"></marker>
    <title>Case</title>
    <pre>
case Expr of
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
end</pre>
    <p>The expression <c>Expr</c> is evaluated and the patterns
      <c>Pattern</c> are sequentially matched against the result. If a
      match succeeds and the optional guard sequence <c>GuardSeq</c> is
      true, the corresponding <c>Body</c> is evaluated.</p>
    <p>The return value of <c>Body</c> is the return value of
      the <c>case</c> expression.</p>
    <p>If there is no matching pattern with a true guard sequence,
      a <c>case_clause</c> run-time error will occur.</p>
    <p>Example:</p>
    <pre>
is_valid_signal(Signal) ->
    case Signal of
        {signal, _What, _From, _To} ->
            true;
        {signal, _What, _To} ->
            true;
        _Else ->
            false
    end.</pre>
  </section>

  <section>
    <marker id="send"></marker>
    <title>Send</title>
    <pre>
Expr1 ! Expr2</pre>
    <p>Sends the value of <c>Expr2</c> as a message to the process
      specified by <c>Expr1</c>. The value of <c>Expr2</c> is also
      the return value of the expression.</p>
    <p><c>Expr1</c> must evaluate to a pid, a registered name (atom) or
      a tuple <c>{Name,Node}</c>, where <c>Name</c> is an atom and
      <c>Node</c> a node name, also an atom.</p>
    <list type="bulleted">
      <item>If <c>Expr1</c> evaluates to a name, but this name is not
       registered, a <c>badarg</c> run-time error will occur.</item>
      <item>Sending a message to a pid never fails, even if the pid
       identifies a non-existing process.</item>
      <item>Distributed message sending, that is if <c>Expr1</c>
       evaluates to a tuple <c>{Name,Node}</c> (or a pid located at
       another node), also never fails.</item>
    </list>
  </section>

  <section>
    <marker id="receive"></marker>
    <title>Receive</title>
    <pre>
receive
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
end</pre>
    <p>Receives messages sent to the process using the send operator
      (!). The patterns <c>Pattern</c> are sequentially matched
      against the first message in time order in the mailbox, then
      the second, and so on. If a match succeeds and the optional
      guard sequence <c>GuardSeq</c> is true, the corresponding
      <c>Body</c> is evaluated. The matching message is consumed, that
      is removed from the mailbox, while any other messages in
      the mailbox remain unchanged.</p>
    <p>The return value of <c>Body</c> is the return value of
      the <c>receive</c> expression.</p>
    <p><c>receive</c> never fails. Execution is suspended, possibly
      indefinitely, until a message arrives that does match one of
      the patterns and with a true guard sequence. </p>
    <p>Example:</p>
    <pre>
wait_for_onhook() ->
    receive
        onhook ->
            disconnect(),
            idle();
        {connect, B} ->
            B ! {busy, self()},
            wait_for_onhook()
    end.</pre>
    <p>It is possible to augment the <c>receive</c> expression with a
      timeout:</p>
    <pre>
receive
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
after
    ExprT ->
        BodyT
end</pre>
    <p><c>ExprT</c> should evaluate to an integer. The highest allowed
      value is 16#ffffffff, that is, the value must fit in 32 bits.
      <c>receive..after</c> works exactly as <c>receive</c>, except
      that if no matching message has arrived within <c>ExprT</c>
      milliseconds, then <c>BodyT</c> is evaluated instead and its
      return value becomes the return value of the <c>receive..after</c>
      expression.</p>
    <p>Example:</p>
    <pre>
wait_for_onhook() ->
    receive
        onhook ->
            disconnect(),
            idle();
        {connect, B} ->
            B ! {busy, self()},
            wait_for_onhook()
    after
        60000 ->
            disconnect(),
            error()
    end.</pre>
    <p>It is legal to use a <c>receive..after</c> expression with no
      branches:</p>
    <pre>
receive
after
    ExprT ->
        BodyT
end</pre>
    <p>This construction will not consume any messages, only suspend
      execution in the process for <c>ExprT</c> milliseconds and can be
      used to implement simple timers.</p>
    <p>Example:</p>
    <pre>
timer() ->
    spawn(m, timer, [self()]).

timer(Pid) ->
    receive
    after
        5000 ->
            Pid ! timeout
    end.</pre>
    <p>There are two special cases for the timeout value <c>ExprT</c>:</p>
    <taglist>
      <tag><c>infinity</c></tag>
      <item>The process should wait indefinitely for a matching message
       -- this is the same as not using a timeout. Can be
       useful for timeout values that are calculated at run-time.</item>
      <tag>0</tag>
      <item>If there is no matching message in the mailbox, the timeout
       will occur immediately.</item>
    </taglist>
  </section>

  <section>
    <title>Term Comparisons</title>
    <pre>
Expr1 <input>op</input> Expr2</pre>
    <table>
      <row>
        <cell align="left" valign="middle"><em>op</em></cell>
        <cell align="left" valign="middle"><em>Description</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle">==</cell>
        <cell align="left" valign="middle">equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">/=</cell>
        <cell align="left" valign="middle">not equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">=&lt;</cell>
        <cell align="left" valign="middle">less than or equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">&lt;</cell>
        <cell align="left" valign="middle">less than</cell>
      </row>
      <row>
        <cell align="left" valign="middle">&gt;=</cell>
        <cell align="left" valign="middle">greater than or equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">&gt;</cell>
        <cell align="left" valign="middle">greater than</cell>
      </row>
      <row>
        <cell align="left" valign="middle">=:=</cell>
        <cell align="left" valign="middle">exactly equal to</cell>
      </row>
      <row>
        <cell align="left" valign="middle">=/=</cell>
        <cell align="left" valign="middle">exactly not equal to</cell>
      </row>
      <tcaption>Term Comparison Operators.</tcaption>
    </table>
    <p>The arguments may be of different data types. The following
      order is defined:</p>
    <pre>
number &lt; atom &lt; reference &lt; fun &lt; port &lt; pid &lt; tuple &lt; list &lt; bit string</pre>
    <p>Lists are compared element by element. Tuples are ordered by
      size, two tuples with the same size are compared element by
      element.</p>
      <p>When comparing an integer to a float, the term with the lesser
      precision will be converted into the other term's type, unless the
      operator is one of =:= or =/=. A float is more precise than
      an integer until all significant figures of the float are to the left of
      the decimal point. This happens when the float is larger/smaller than
      +/-9007199254740992.0. The conversion strategy is changed
      depending on the size of the float because otherwise comparison of large
      floats and integers would lose their transitivity.</p>

    <p>Returns the Boolean value of the expression, <c>true</c> or
      <c>false</c>.</p>
    <p>Examples:</p>
    <pre>
1> <input>1==1.0.</input>
true
2> <input>1=:=1.0.</input>
false
3> <input>1 > a.</input>
false</pre>
  </section>

  <section>
    <title>Arithmetic Expressions</title>
    <pre>
<input>op</input> Expr
Expr1 <input>op</input> Expr2</pre>
    <table>
      <row>
        <cell align="left" valign="middle"><em>op</em></cell>
        <cell align="left" valign="middle"><em>Description</em></cell>
        <cell align="left" valign="middle"><em>Argument type</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle">+</cell>
        <cell align="left" valign="middle">unary +</cell>
        <cell align="left" valign="middle">number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">-</cell>
        <cell align="left" valign="middle">unary -</cell>
        <cell align="left" valign="middle">number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">+</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
        <cell align="left" valign="middle">number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">-</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
        <cell align="left" valign="middle">number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">*</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
        <cell align="left" valign="middle">number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">/</cell>
        <cell align="left" valign="middle">floating point division</cell>
        <cell align="left" valign="middle">number</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bnot</cell>
        <cell align="left" valign="middle">unary bitwise not</cell>
        <cell align="left" valign="middle">integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">div</cell>
        <cell align="left" valign="middle">integer division</cell>
        <cell align="left" valign="middle">integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">rem</cell>
        <cell align="left" valign="middle">integer remainder of X/Y</cell>
        <cell align="left" valign="middle">integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">band</cell>
        <cell align="left" valign="middle">bitwise and</cell>
        <cell align="left" valign="middle">integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bor</cell>
        <cell align="left" valign="middle">bitwise or</cell>
        <cell align="left" valign="middle">integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bxor</cell>
        <cell align="left" valign="middle">arithmetic bitwise xor</cell>
        <cell align="left" valign="middle">integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bsl</cell>
        <cell align="left" valign="middle">arithmetic bitshift left</cell>
        <cell align="left" valign="middle">integer</cell>
      </row>
      <row>
        <cell align="left" valign="middle">bsr</cell>
        <cell align="left" valign="middle">bitshift right</cell>
        <cell align="left" valign="middle">integer</cell>
      </row>
      <tcaption>Arithmetic Operators.</tcaption>
    </table>

    <p>Examples:</p>
    <pre>
1> <input>+1.</input>
1
2> <input>-1.</input>
-1
3> <input>1+1.</input>
2
4> <input>4/2.</input>
2.0
5> <input>5 div 2.</input>
2
6> <input>5 rem 2.</input>
1
7> <input>2#10 band 2#01.</input>
0
8> <input>2#10 bor 2#01.</input>
3
9> <input>a + 10.</input>
** exception error: an error occurred when evaluating an arithmetic expression
     in operator  +/2
        called as a + 10
10> <input>1 bsl (1 bsl 64).</input>
** exception error: a system limit has been reached
     in operator  bsl/2
        called as 1 bsl 18446744073709551616</pre>
  </section>

  <section>
    <title>Boolean Expressions</title>
    <pre>
<input>op</input> Expr
Expr1 <input>op</input> Expr2</pre>
    <table>
      <row>
        <cell align="left" valign="middle"><em>op</em></cell>
        <cell align="left" valign="middle"><em>Description</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle">not</cell>
        <cell align="left" valign="middle">unary logical not</cell>
      </row>
      <row>
        <cell align="left" valign="middle">and</cell>
        <cell align="left" valign="middle">logical and</cell>
      </row>
      <row>
        <cell align="left" valign="middle">or</cell>
        <cell align="left" valign="middle">logical or</cell>
      </row>
      <row>
        <cell align="left" valign="middle">xor</cell>
        <cell align="left" valign="middle">logical xor</cell>
      </row>
      <tcaption>Logical Operators.</tcaption>
    </table>
    <p>Examples:</p>
    <pre>
1> <input>not true.</input>
false
2> <input>true and false.</input>
false
3> <input>true xor false.</input>
true
4> <input>true or garbage.</input>
** exception error: bad argument
     in operator  or/2
        called as true or garbage</pre>
  </section>

  <section>
    <title>Short-Circuit Expressions</title>
    <pre>
Expr1 orelse Expr2
Expr1 andalso Expr2</pre>
    <p>Expressions where <c>Expr2</c> is evaluated only if
      necessary. That is, <c>Expr2</c> is evaluated only if <c>Expr1</c>
      evaluates to <c>false</c> in an <c>orelse</c> expression, or only
      if <c>Expr1</c> evaluates to <c>true</c> in an <c>andalso</c>
      expression. Returns either the value of <c>Expr1</c> (that is,
      <c>true</c> or <c>false</c>) or the value of <c>Expr2</c>
      (if <c>Expr2</c> was evaluated).</p>

    <p>Example 1:</p>
    <pre>
case A >= -1.0 andalso math:sqrt(A+1) > B of</pre>
    <p>This will work even if <c>A</c> is less than <c>-1.0</c>,
      since in that case, <c>math:sqrt/1</c> is never evaluated.</p>
    <p>Example 2:</p>
    <pre>
OnlyOne = is_atom(L) orelse
         (is_list(L) andalso length(L) == 1),</pre>

    <p>From R13A, <c>Expr2</c> is no longer required to evaluate to a
    boolean value. As a consequence, <c>andalso</c> and <c>orelse</c>
    are now tail-recursive.  For instance, the following function is
    tail-recursive in R13A and later:</p>

    <pre>
all(Pred, [Hd|Tail]) ->
    Pred(Hd) andalso all(Pred, Tail);
all(_, []) ->
    true.</pre>
  </section>

  <section>
    <title>List Operations</title>
    <pre>
Expr1 ++ Expr2
Expr1 -- Expr2</pre>
    <p>The list concatenation operator <c>++</c> appends its second
      argument to its first and returns the resulting list.</p>
    <p>The list subtraction operator <c>--</c> produces a list which
      is a copy of the first argument, subjected to the following
      procedure: for each element in the second argument, the first
      occurrence of this element (if any) is removed.</p>
    <p>Example:</p>
    <pre>
1> <input>[1,2,3]++[4,5].</input>
[1,2,3,4,5]
2> <input>[1,2,3,2,1,2]--[2,1,2].</input>
[3,1,2]</pre>

     <warning><p>The complexity of <c>A -- B</c> is
     proportional to <c>length(A)*length(B)</c>, meaning that it
     will be very slow if both <c>A</c> and <c>B</c> are
     long lists.</p></warning>
   </section>

  <section>
    <marker id="map_expressions"></marker>
	<title>Map Expressions</title>
	  <section>
	  <title>Creating Maps</title>
	  <p>
		  Constructing a new map is done by letting an expression <c>K</c> be associated with
		  another expression <c>V</c>:
	  </p>
	  <code>#{ K => V }</code>
	  <p>
		  New maps may include multiple associations at construction by listing every
		  association:
	  </p>
	  <code>#{ K1 => V1, .., Kn => Vn }</code>
	  <p>
		  An empty map is constructed by not associating any terms with each other:
	  </p>
	  <code>#{}</code>
	  <p>
		  All keys and values in the map are terms. Any expression is first evaluated and
		  then the resulting terms are used as <em>key</em> and <em>value</em> respectively.
	  </p>
	  <p>
		  Keys and values are separated by the <c>=></c> arrow and associations are
		  separated by <c>,</c>.
	  </p>

	  <p>
		  Examples:
	  </p>
	  <code>
M0 = #{},                 % empty map
M1 = #{a => &lt;&lt;"hello"&gt;&gt;}, % single association with literals
M2 = #{1 => 2, b => b},   % multiple associations with literals
M3 = #{k => {A,B}},       % single association with variables
M4 = #{{"w", 1} => f()}.  % compound key associated with an evaluated expression</code>
	  <p>
		  where, <c>A</c> and <c>B</c> are any expressions and <c>M0</c> through <c>M4</c>
		  are the resulting map terms.
	  </p>
	  <p>
		  If two matching keys are declared, the latter key will take precedence.
	  </p>
	  <p>
		  Example:
	  </p>

<pre>
1> <input>#{1 => a, 1 => b}.</input>
#{1 => b }
2> <input>#{1.0 => a, 1 => b}.</input>
#{1 => b, 1.0 => a}
</pre>
	  <p>
		  The order in which the expressions constructing the keys and their
		  associated values are evaluated is not defined. The syntactic order of
		  the key-value pairs in the construction is of no relevance, except in
		  the above mentioned case of two matching keys.
	  </p>
  </section>

  <section>
	  <title>Updating Maps</title>
	  <p>
		  Updating a map has similar syntax as constructing it.
	  </p>
	  <p>
		  An expression defining the map to be updated is put in front of the expression
		  defining the keys to be updated and their respective values.
	  </p>
	  <code>M#{ K => V }</code>
	  <p>
		  where <c>M</c> is a term of type map and <c>K</c> and <c>V</c> are any expression.
	  </p>
	  <p>
		  If key <c>K</c> does not match any existing key in the map, a new association
		  will be created from key <c>K</c> to value <c>V</c>.  If key <c>K</c> matches
		  an existing key in map <c>M</c> its associated value will be replaced by the
		  new value <c>V</c>. In both cases the evaluated map expression will return a new map.
	  </p>
	  <p>
		  If <c>M</c> is not of type map an exception of type <c>badmap</c> is thrown.
	  </p>
	  <p>
		  To only update an existing value, the following syntax is used,
	  </p>
	  <code>M#{ K := V } </code>
	  <p>
		  where <c>M</c> is an term of type map, <c>V</c> is an expression and <c>K</c>
		  is an expression which evaluates to an existing key in <c>M</c>.
	  </p>
	  <p>
		  If key <c>K</c> does not match any existing keys in map <c>M</c> an exception
		  of type <c>badarg</c> will be triggered at runtime. If a matching key <c>K</c>
		  is present in map <c>M</c> its associated value will be replaced by the new
		  value <c>V</c> and the evaluated map expression returns a new map.
	  </p>
	  <p>
		  If <c>M</c> is not of type map an exception of type <c>badmap</c> is thrown.
	  </p>
	  <p>
		  Examples:
	  </p>
	  <code>
M0 = #{},
M1 = M0#{a => 0},
M2 = M1#{a => 1, b => 2},
M3 = M2#{"function" => fun() -> f() end},
M4 = M3#{a := 2, b := 3}.  % 'a' and 'b' was added in `M1` and `M2`.</code>
	  <p>
		  where <c>M0</c> is any map. It follows that <c>M1 .. M4</c> are maps as well.
	  </p>
	  <p>
		  More Examples:
	  </p>
<pre>
1> <input>M = #{1 => a}.</input>
#{1 => a }
2> <input>M#{1.0 => b}.</input>
#{1 => a, 1.0 => b}.
3> <input>M#{1 := b}.</input>
#{1 => b}
4> <input>M#{1.0 := b}.</input>
** exception error: bad argument
</pre>
	  <p>
		  As in construction, the order in which the key and value expressions
		  are evaluated is not defined. The
		  syntactic order of the key-value pairs in the update is of no
		  relevance, except in the case where two keys match, in which
		  case the latter value is used.
	  </p>
  </section>

  <section>
	  <title>Maps in Patterns</title>
	  <p>
		  Matching of key-value associations from maps is done in the following way:
	  </p>

	  <code>#{ K := V } = M</code>
	  <p>
		  where <c>M</c> is any map. The key <c>K</c> has to be an expression with bound
		  variables or a literals, and <c>V</c> can be any pattern with either bound or
		  unbound variables.
	  </p>
	  <p>
		  If the variable <c>V</c> is unbound, it will be bound to the value associated
		  with the key <c>K</c>, which has to exist in the map <c>M</c>. If the variable
		  <c>V</c> is bound, it has to match the value associated with <c>K</c> in <c>M</c>.
	  </p>
	  <p> Example: </p>
<code>
1> <input>M = #{"tuple" => {1,2}}.</input>
#{"tuple" => {1,2}}
2> <input>#{"tuple" := {1,B}} = M.</input>
#{"tuple" => {1,2}}
3> <input>B.</input>
2.</code>
	  <p>
		  This will bind variable <c>B</c> to integer <c>2</c>.
	  </p>
	  <p>
		  Similarly, multiple values from the map may be matched:
	  </p>
	  <code>#{ K1 := V1, .., Kn := Vn } = M</code>
	  <p>
		  where keys <c>K1 .. Kn</c> are any expressions with literals or bound variables. If all
		  keys exist in map <c>M</c> all variables in <c>V1 .. Vn</c> will be matched to the
		  associated values of their respective keys.
	  </p>
	  <p>
		  If the matching conditions are not met, the match will fail, either with
	  </p>
	  <list>
		  <item>
			  a <c>badmatch</c> exception, if used in the context of the matching operator
			  as in the example,
		  </item>
		  <item>
			  or resulting in the next clause being tested in function heads and
			  case expressions.
		  </item>
	  </list>
	  <p>
		  Matching in maps only allows for <c>:=</c> as delimiters of associations.
		  The order in which keys are declared in matching has no relevance.
	  </p>
	  <p>
		  Duplicate keys are allowed in matching and will match each pattern associated
		  to the keys.
	  </p>
	  <code>#{ K := V1, K := V2 } = M</code>
	  <p>
		  Matching an expression against an empty map literal will match its type but
		  no variables will be bound:
	  </p>
	  <code>#{} = Expr</code>
	  <p>
		  This expression will match if the expression <c>Expr</c> is of type map, otherwise
		  it will fail with an exception <c>badmatch</c>.
	  </p>
	  <section>
		  <title>Matching syntax: Example with literals in function heads</title>
		  <p>
			  Matching of literals as keys are allowed in function heads.
		  </p>
		  <code>
%% only start if not_started
handle_call(start, From, #{ state := not_started } = S) ->
...
    {reply, ok, S#{ state := start }};

%% only change if started
handle_call(change, From, #{ state := start } = S) ->
...
    {reply, ok, S#{ state := changed }};</code>
	  </section>
  </section>
  <section>
	  <title>Maps in Guards</title>
	  <p>
		  Maps are allowed in guards as long as all sub-expressions are valid guard expressions.
	  </p>
	  <p>
		  Two guard BIFs handles maps:
	  </p>
	  <list>
		  <item>
			  <seealso marker="erts:erlang#is_map/1">is_map/1</seealso>
		  </item>
		  <item>
			  <seealso marker="erts:erlang#map_size/1">map_size/1</seealso>
		  </item>
	  </list>
	</section>
  </section>

  <section>
    <marker id="bit_syntax"></marker>
    <title>Bit Syntax Expressions</title>
    <code type="none"><![CDATA[<<>>
<<E1,...,En>>]]></code>
    <p>Each element <c>Ei</c> specifies a <em>segment</em> of
      the bit string. Each element <c>Ei</c> is a value, followed by an
      optional <em>size expression</em> and an optional <em>type specifier list</em>.</p>
    <pre>
Ei = Value |
     Value:Size |
     Value/TypeSpecifierList |
     Value:Size/TypeSpecifierList</pre>
    <p>Used in a bit string construction, <c>Value</c> is an expression
    which should evaluate to an integer, float or bit string.  If the
    expression is something else than a single literal or variable, it
    should be enclosed in parenthesis.</p>

    <p>Used in a bit string matching, <c>Value</c> must be a variable,
    or an integer, float or string.</p>

    <p>Note that, for example, using a string literal as in
    <c><![CDATA[<<"abc">>]]></c> is syntactic sugar for
    <c><![CDATA[<<$a,$b,$c>>]]></c>.</p>

    <p>Used in a bit string construction, <c>Size</c> is an expression
    which should evaluate to an integer.</p>
    
    <p>Used in a bit string matching, <c>Size</c> must be an integer or a 
    variable bound to an integer.</p>

    <p>The value of <c>Size</c> specifies the size of the segment in
    units (see below). The default value depends on the type (see
    below). For <c>integer</c> it is 8, for
    <c>float</c> it is 64, for <c>binary</c> and <c>bitstring</c> it is
    the whole binary or bit string. In matching, this default value is only
    valid for the very last element. All other bit string or binary 
    elements in the matching must have a size specification.</p>

    <p>For the <c>utf8</c>, <c>utf16</c>, and <c>utf32</c> types,
    <c>Size</c> must not be given. The size of the segment is implicitly
    determined by the type and value itself.</p>
    
    <p><c>TypeSpecifierList</c> is a list of type specifiers, in any
    order, separated by hyphens (-). Default values are used for any
    omitted type specifiers.</p>
    <taglist>
      <tag><c>Type</c>= <c>integer</c> | <c>float</c> | <c>binary</c> |
             <c>bytes</c> | <c>bitstring</c> | <c>bits</c> |
	     <c>utf8</c> | <c>utf16</c> | <c>utf32</c> </tag>
      <item>The default is <c>integer</c>. <c>bytes</c> is a shorthand for 
      <c>binary</c> and <c>bits</c> is a shorthand for <c>bitstring</c>.
      See below for more information about the <c>utf</c> types.
      </item>

      <tag><c>Signedness</c>= <c>signed</c> | <c>unsigned</c></tag>
      <item>Only matters for matching and when the type is <c>integer</c>. 
      The default is <c>unsigned</c>.</item>

      <tag><c>Endianness</c>= <c>big</c> | <c>little</c> | <c>native</c></tag>
      <item>Native-endian means that the endianness will be resolved at load
       time to be either big-endian or little-endian, depending on
       what is native for the CPU that the Erlang machine is run on.
       Endianness only matters when the Type is either <c>integer</c>,
       <c>utf16</c>, <c>utf32</c>, or <c>float</c>. The default is <c>big</c>.
       </item>

      <tag><c>Unit</c>= <c>unit:IntegerLiteral</c></tag>
      <item>The allowed range is 1..256. Defaults to 1 for <c>integer</c>,
       <c>float</c> and <c>bitstring</c>, and to 8 for <c>binary</c>.
       No unit specifier must be given for the types 
       <c>utf8</c>, <c>utf16</c>, and <c>utf32</c>.
       </item>
    </taglist>
    <p>The value of <c>Size</c> multiplied with the unit gives
      the number of bits. A segment of type <c>binary</c> must have 
      a size that is evenly divisible by 8.</p>

    <note><p>When constructing binaries, if the size <c>N</c> of an integer
    segment is too small to contain the given integer, the most significant
    bits of the integer will be silently discarded and only the <c>N</c> least
    significant bits will be put into the binary.</p></note>

    <p>The types <c>utf8</c>, <c>utf16</c>, and <c>utf32</c> specifies
    encoding/decoding of the <em>Unicode Transformation Format</em>s UTF-8, UTF-16,
    and UTF-32, respectively.</p>

    <p>When constructing a segment of a <c>utf</c> type, <c>Value</c>
    must be an integer in the range 0..16#D7FF or
    16#E000....16#10FFFF. Construction
    will fail with a <c>badarg</c> exception if <c>Value</c> is
    outside the allowed ranges. The size of the resulting binary
    segment depends on the type and/or <c>Value</c>. For <c>utf8</c>,
    <c>Value</c> will be encoded in 1 through 4 bytes. For
    <c>utf16</c>, <c>Value</c> will be encoded in 2 or 4
    bytes. Finally, for <c>utf32</c>, <c>Value</c> will always be
    encoded in 4 bytes.</p>

    <p>When constructing, a literal string may be given followed
    by one of the UTF types, for example: <c><![CDATA[<<"abc"/utf8>>]]></c>
    which is syntatic sugar for 
    <c><![CDATA[<<$a/utf8,$b/utf8,$c/utf8>>]]></c>.</p>

    <p>A successful match of a segment of a <c>utf</c> type results
    in an integer in the range 0..16#D7FF or  16#E000..16#10FFFF.
    The match will fail if returned value
    would fall outside those ranges.</p>

    <p>A segment of type <c>utf8</c> will match 1 to 4 bytes in the binary,
    if the binary at the match position contains a valid UTF-8 sequence.
    (See RFC-3629 or the Unicode standard.)</p>

    <p>A segment of type <c>utf16</c> may match 2 or 4 bytes in the binary.
    The match will fail if the binary at the match position does not contain
    a legal UTF-16 encoding of a Unicode code point. (See RFC-2781 or
    the Unicode standard.)</p>

    <p>A segment of type <c>utf32</c> may match 4 bytes in the binary in the
    same way as an <c>integer</c> segment matching 32 bits.
    The match will fail if the resulting integer is outside the legal ranges
    mentioned above.</p>

    <p>Examples:</p>
    <pre>
1> <input>Bin1 = &lt;&lt;1,17,42&gt;&gt;.</input>
&lt;&lt;1,17,42&gt;&gt;
2> <input>Bin2 = &lt;&lt;"abc"&gt;&gt;.</input>
&lt;&lt;97,98,99&gt;&gt;
3> <input>Bin3 = &lt;&lt;1,17,42:16&gt;&gt;.</input>
&lt;&lt;1,17,0,42&gt;&gt;
4> <input>&lt;&lt;A,B,C:16&gt;&gt; = &lt;&lt;1,17,42:16&gt;&gt;.</input>
&lt;&lt;1,17,0,42&gt;&gt;
5> <input>C.</input>
42
6> <input>&lt;&lt;D:16,E,F&gt;&gt; = &lt;&lt;1,17,42:16&gt;&gt;.</input>
&lt;&lt;1,17,0,42&gt;&gt;
7> <input>D.</input>
273
8> <input>F.</input>
42
9> <input>&lt;&lt;G,H/binary&gt;&gt; = &lt;&lt;1,17,42:16&gt;&gt;.</input>
&lt;&lt;1,17,0,42&gt;&gt;
10> <input>H.</input>
&lt;&lt;17,0,42&gt;&gt;
11> <input>&lt;&lt;G,H/bitstring&gt;&gt; = &lt;&lt;1,17,42:12&gt;&gt;.</input>
&lt;&lt;1,17,1,10:4&gt;&gt;
12> <input>H.</input>
&lt;&lt;17,1,10:4&gt;&gt;
13> <input>&lt;&lt;1024/utf8&gt;&gt;.</input>
&lt;&lt;208,128&gt;&gt;
</pre>
    <p>Note that bit string patterns cannot be nested.</p>
    <p>Note also that "<c><![CDATA[B=<<1>>]]></c>" is interpreted as
      "<c><![CDATA[B =<<1>>]]></c>" which is a syntax error. The correct way is
      to write a space after '=': "<c><![CDATA[B= <<1>>]]></c>.</p>
    <p>More examples can be found in <em>Programming Examples</em>.</p>
  </section>

  <section>
    <marker id="funs"></marker>
    <title>Fun Expressions</title>
    <pre>
fun
    [Name](Pattern11,...,Pattern1N) [when GuardSeq1] ->
              Body1;
    ...;
    [Name](PatternK1,...,PatternKN) [when GuardSeqK] ->
              BodyK
end</pre>
    <p>A fun expression begins with the keyword <c>fun</c> and ends
      with the keyword <c>end</c>. Between them should be a function
      declaration, similar to a
      <seealso marker="functions#syntax">regular function declaration</seealso>,
      except that the function name is optional and should be a variable if
      any.</p>
    <p>Variables in a fun head shadow the function name and both shadow
      variables in the function clause surrounding the fun expression, and
      variables bound in a fun body are local to the fun body.</p>
    <p>The return value of the expression is the resulting fun.</p>
    <p>Examples:</p>
    <pre>
1> <input>Fun1 = fun (X) -> X+1 end.</input>
#Fun&lt;erl_eval.6.39074546&gt;
2> <input>Fun1(2).</input>
3
3> <input>Fun2 = fun (X) when X>=5 -> gt; (X) -> lt end.</input>
#Fun&lt;erl_eval.6.39074546&gt;
4> <input>Fun2(7).</input>
gt
5> <input>Fun3 = fun Fact(1) -> 1; Fact(X) when X > 1 -> X * Fact(X - 1) end.</input>
#Fun&lt;erl_eval.6.39074546&gt;
6> <input>Fun3(4).</input>
24</pre>
    <p>The following fun expressions are also allowed:</p>
    <pre>
fun Name/Arity
fun Module:Name/Arity</pre>
    <p>In <c>Name/Arity</c>, <c>Name</c> is an atom and <c>Arity</c> is an integer.
      <c>Name/Arity</c> must specify an existing local function. The expression is
      syntactic sugar for:</p>
    <pre>
fun (Arg1,...,ArgN) -> Name(Arg1,...,ArgN) end</pre>
    <p>In <c>Module:Name/Arity</c>, <c>Module</c> and <c>Name</c> are atoms
      and <c>Arity</c> is an integer. Starting from the R15 release,
      <c>Module</c>, <c>Name</c>, and <c>Arity</c> may also be variables.
      A fun defined in this way will refer to the function <c>Name</c>
      with arity <c>Arity</c> in the <em>latest</em> version of module
      <c>Module</c>. A fun defined in this way will not be dependent on
      the code for module in which it is defined.
      </p>
    <p>More examples can be found in <em>Programming Examples</em>.</p>
  </section>

  <section>
    <marker id="catch"></marker>
    <title>Catch and Throw</title>
    <code type="none">
catch Expr</code>
    <p>Returns the value of <c>Expr</c> unless an exception
      occurs during the evaluation. In that case, the exception is
      caught. For exceptions of class <c>error</c>, 
      that is run-time errors: <c>{'EXIT',{Reason,Stack}}</c> 
      is returned. For exceptions of class <c>exit</c>, that is
      the code called <c>exit(Term)</c>: <c>{'EXIT',Term}</c> is returned. 
      For exceptions of class <c>throw</c>, that is
      the code called <c>throw(Term)</c>: <c>Term</c> is returned.</p>
    <p><c>Reason</c> depends on the type of error that occurred, and
      <c>Stack</c> is the stack of recent function calls, see
      <seealso marker="errors#exit_reasons">Errors and Error Handling</seealso>.</p>
    <p>Examples:</p>
    <p></p>
    <pre>
1> <input>catch 1+2.</input>
3
2> <input>catch 1+a.</input>
{'EXIT',{badarith,[...]}}</pre>
    <p>Note that <c>catch</c> has low precedence and catch
      subexpressions often needs to be enclosed in a block
      expression or in parenthesis:</p>
    <pre>
3> <input>A = catch 1+2.</input>
** 1: syntax error before: 'catch' **
4> <input>A = (catch 1+2).</input>
3</pre>
    <p>The BIF <c>throw(Any)</c> can be used for non-local return from
      a function. It must be evaluated within a <c>catch</c>, which will
      return the value <c>Any</c>. Example:</p>
    <pre>
5> <input>catch throw(hello).</input>
hello</pre>
    <p>If <c>throw/1</c> is not evaluated within a catch, a
      <c>nocatch</c> run-time error will occur.</p>
  </section>

  <section>
    <marker id="try"></marker>
    <title>Try</title>
    <code type="none">
try Exprs
catch
    [Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
        ExceptionBody1;
    [ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
        ExceptionBodyN
end</code>
    <p>This is an enhancement of
      <seealso marker="#catch">catch</seealso> that appeared in
      Erlang 5.4/OTP-R10B. It gives the possibility do distinguish
      between different exception classes, and to choose to handle only
      the desired ones, passing the others on to an enclosing
      <c>try</c> or <c>catch</c> or to default error handling.</p>
    <p>Note that although the keyword <c>catch</c> is used in
      the <c>try</c> expression, there is not a <c>catch</c> expression
      within the <c>try</c> expression.</p>
    <p>Returns the value of <c>Exprs</c> (a sequence of expressions
      <c>Expr1, ..., ExprN</c>) unless an exception occurs during
      the evaluation. In that case the exception is caught and
      the patterns <c>ExceptionPattern</c> with the right exception
      class <c>Class</c> are sequentially matched against the caught
      exception. An omitted <c>Class</c> is shorthand for <c>throw</c>.
      If a match succeeds and the optional guard sequence
      <c>ExceptionGuardSeq</c> is true, the corresponding
      <c>ExceptionBody</c> is evaluated to become the return value.</p>
    <p>If an exception occurs during evaluation of <c>Exprs</c> but
      there is no matching <c>ExceptionPattern</c> of the right
      <c>Class</c> with a true guard sequence, the exception is passed
      on as if <c>Exprs</c> had not been enclosed in a <c>try</c>
      expression.</p>
    <p>If an exception occurs during evaluation of <c>ExceptionBody</c>
      it is not caught.</p>
    <p>The <c>try</c> expression can have an <c>of</c>
      section:
      </p>
    <code type="none">
try Exprs of
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
catch
    [Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
        ExceptionBody1;
    ...;
    [ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
        ExceptionBodyN
end</code>
    <p>If the evaluation of <c>Exprs</c> succeeds without an exception,
      the patterns <c>Pattern</c> are sequentially matched against
      the result in the same way as for a
      <seealso marker="#case">case</seealso> expression, except that if
      the matching fails, a <c>try_clause</c> run-time error will occur.</p>
    <p>An exception occurring during the evaluation of <c>Body</c> is
      not caught.</p>
    <p>The <c>try</c> expression can also be augmented with an
      <c>after</c> section, intended to be used for cleanup with side
      effects:</p>
    <code type="none">
try Exprs of
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
catch
    [Class1:]ExceptionPattern1 [when ExceptionGuardSeq1] ->
        ExceptionBody1;
    ...;
    [ClassN:]ExceptionPatternN [when ExceptionGuardSeqN] ->
        ExceptionBodyN
after
    AfterBody
end</code>
    <p><c>AfterBody</c> is evaluated after either <c>Body</c> or
      <c>ExceptionBody</c> no matter which one. The evaluated value of
      <c>AfterBody</c> is lost; the return value of the <c>try</c>
      expression is the same with an <c>after</c> section as without.</p>
    <p>Even if an exception occurs during evaluation of <c>Body</c> or
      <c>ExceptionBody</c>, <c>AfterBody</c> is evaluated. In this case
      the exception is passed on after <c>AfterBody</c> has been
      evaluated, so the exception from the <c>try</c> expression is
      the same with an <c>after</c> section as without.</p>
    <p>If an exception occurs during evaluation of <c>AfterBody</c>
      itself it is not caught, so if <c>AfterBody</c> is evaluated after
      an exception in <c>Exprs</c>, <c>Body</c> or <c>ExceptionBody</c>,
      that exception is lost and masked by the exception in
      <c>AfterBody</c>.</p>
    <p>The <c>of</c>, <c>catch</c> and <c>after</c> sections are all
      optional, as long as there is at least a <c>catch</c> or an
      <c>after</c> section, so the following are valid <c>try</c>
      expressions:</p>
    <code type="none">
try Exprs of 
    Pattern when GuardSeq -> 
        Body 
after 
    AfterBody 
end

try Exprs
catch 
    ExpressionPattern -> 
        ExpressionBody
after
    AfterBody
end

try Exprs after AfterBody end</code>
    <p>Example of using <c>after</c>, this code will close the file
      even in the event of exceptions in <c>file:read/2</c> or in
      <c>binary_to_term/1</c>, and exceptions will be the same as
      without the <c>try</c>...<c>after</c>...<c>end</c> expression:</p>
    <code type="none">
termize_file(Name) ->
    {ok,F} = file:open(Name, [read,binary]),
    try
        {ok,Bin} = file:read(F, 1024*1024),
        binary_to_term(Bin)
    after
        file:close(F)
    end.</code>
    <p>Example: Using <c>try</c> to emulate <c>catch Expr</c>.</p>
    <code type="none">
try Expr
catch
    throw:Term -> Term;
    exit:Reason -> {'EXIT',Reason}
    error:Reason -> {'EXIT',{Reason,erlang:get_stacktrace()}}
end</code>
  </section>

  <section>
    <title>Parenthesized Expressions</title>
    <pre>
(Expr)</pre>
    <p>Parenthesized expressions are useful to override
      <seealso marker="#prec">operator precedences</seealso>,
      for example in arithmetic expressions:</p>
    <pre>
1> <input>1 + 2 * 3.</input>
7
2> <input>(1 + 2) * 3.</input>
9</pre>
  </section>

  <section>
    <title>Block Expressions</title>
    <pre>
begin
   Expr1,
   ...,
   ExprN
end</pre>
    <p>Block expressions provide a way to group a sequence of
      expressions, similar to a clause body. The return value is
      the value of the last expression <c>ExprN</c>.</p>
  </section>

  <section>
    <marker id="lcs"></marker>
    <title>List Comprehensions</title>
    <p>List comprehensions are a feature of many modern functional
      programming languages. Subject to certain rules, they provide a
      succinct notation for generating elements in a list.</p>
    <p>List comprehensions are analogous to set comprehensions in
      Zermelo-Frankel set theory and are called ZF expressions in
      Miranda. They are analogous to the <c>setof</c> and
      <c>findall</c> predicates in Prolog.</p>
    <p>List comprehensions are written with the following syntax:</p>
    <pre>
[Expr || Qualifier1,...,QualifierN]</pre>
    <p><c>Expr</c> is an arbitrary expression, and each
      <c>Qualifier</c> is either a generator or a filter.</p>
    <list type="bulleted">
      <item>A <em>generator</em> is written as:      <br></br>

       &nbsp;&nbsp;<c><![CDATA[Pattern <- ListExpr]]></c>.      <br></br>
<c>ListExpr</c> must be an expression which evaluates to a
       list of terms.</item>
<item>A <em>bit string generator</em> is written as:      <br></br>

       &nbsp;&nbsp;<c><![CDATA[BitstringPattern <= BitStringExpr]]></c>.      <br></br>
<c>BitStringExpr</c> must be an expression which evaluates to a
       bitstring.</item>
      <item>A <em>filter</em> is an expression which evaluates to
      <c>true</c> or <c>false</c>.</item>
    </list>
    <p>The variables in the generator patterns shadow variables in the function
    clause surrounding the list comprehensions.</p> <p>A list comprehension
    returns a list, where the elements are the result of evaluating <c>Expr</c>
    for each combination of generator list elements and bit string generator
    elements for which all filters are true.</p> <p></p> <p>Example:</p>
    <pre>
1> <input>[X*2 || X &lt;- [1,2,3]].</input>
[2,4,6]</pre>
    <p>More examples can be found in <em>Programming Examples</em>.</p>
  

  </section>

<section>
    <title>Bit String Comprehensions</title> 
   
    <p>Bit string comprehensions are
    analogous to List Comprehensions. They are used to generate bit strings
    efficiently and succinctly.</p> 
    <p>Bit string comprehensions are written with
    the following syntax:</p>
    <pre>
&lt;&lt; BitString || Qualifier1,...,QualifierN &gt;&gt;</pre>
    <p><c>BitString</c> is a bit string expression, and each
      <c>Qualifier</c> is either a generator, a bit string generator or a filter.</p>
    <list type="bulleted">
 <item>A <em>generator</em> is written as:      <br></br>
      &nbsp;&nbsp;<c><![CDATA[Pattern <- ListExpr]]></c>.      <br></br>
       <c>ListExpr</c> must be an expression which evaluates to a
       list of terms.</item>
      <item>A <em>bit string generator</em> is written as:      <br></br>

       &nbsp;&nbsp;<c><![CDATA[BitstringPattern <= BitStringExpr]]></c>.      <br></br>
<c>BitStringExpr</c> must be an expression which evaluates to a
       bitstring.</item>
      <item>A <em>filter</em> is an expression which evaluates to
      <c>true</c> or <c>false</c>.</item>
    </list>
    <p>The variables in the generator patterns shadow variables in
      the function clause surrounding the bit string comprehensions.</p>
    <p>A bit string comprehension returns a bit string, which is 
      created by concatenating the results of evaluating <c>BitString</c> 
      for each combination of bit string generator elements for which all 
      filters are true.</p>
    <p></p>
    <p>Example:</p>
    <pre>
1> <input>&lt;&lt; &lt;&lt; (X*2) &gt;&gt; || 
&lt;&lt;X&gt;&gt; &lt;= &lt;&lt; 1,2,3 &gt;&gt; &gt;&gt;.</input>
&lt;&lt;2,4,6&gt;&gt;</pre>
    <p>More examples can be found in <em>Programming Examples</em>.</p>
  </section>

  <section>
    <marker id="guards"></marker>
    <title>Guard Sequences</title>

    <p>A <em>guard sequence</em> is a sequence of guards, separated
      by semicolon (;). The guard sequence is true if at least one of
      the guards is true. (The remaining guards, if any, will not be
      evaluated.)<br></br>
<c>Guard1;...;GuardK</c></p>
    <p>A <em>guard</em> is a sequence of guard expressions, separated
      by comma (,). The guard is true if all guard expressions
      evaluate to <c>true</c>.<br></br>
<c>GuardExpr1,...,GuardExprN</c></p>
    <p>The set of valid <em>guard expressions</em> (sometimes called
      guard tests) is a subset of the set of valid Erlang expressions.
      The reason for restricting the set of valid expressions is that
      evaluation of a guard expression must be guaranteed to be free
      of side effects. Valid guard expressions are:</p>
    <list type="bulleted">
      <item>the atom <c>true</c>,</item>
      <item>other constants (terms and bound variables), all regarded
       as false,</item>
      <item>calls to the BIFs specified below,</item>
      <item>term comparisons,</item>
      <item>arithmetic expressions,</item>
      <item>boolean expressions, and</item>
      <item>short-circuit expressions (<c>andalso</c>/<c>orelse</c>).</item>
    </list>
    <table>
      <row>
        <cell align="left" valign="middle"><c>is_atom/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_binary/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_bitstring/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_boolean/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_float/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_function/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_function/2</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_integer/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_list/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_map/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_number/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_pid/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_port/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_record/2</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_record/3</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_reference/1</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>is_tuple/1</c></cell>
      </row>
      <tcaption>Type Test BIFs.</tcaption>
    </table>
    <p>Note that most type test BIFs have older equivalents, without
      the <c>is_</c> prefix. These old BIFs are retained for backwards
      compatibility only and should not be used in new code. They are
      also only allowed at top level. For example, they are not allowed
      in boolean expressions in guards.</p>
    <table>
      <row>
        <cell align="left" valign="middle"><c>abs(Number)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>bit_size(Bitstring)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>byte_size(Bitstring)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>element(N, Tuple)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>float(Term)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>hd(List)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>length(List)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>map_size(Map)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>node()</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>node(Pid|Ref|Port)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>round(Number)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>self()</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>size(Tuple|Bitstring)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>tl(List)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>trunc(Number)</c></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><c>tuple_size(Tuple)</c></cell>
      </row>
      <tcaption>Other BIFs Allowed in Guard Expressions.</tcaption>
    </table>

    <p>If an arithmetic expression, a boolean expression, a
    short-circuit expression, or a call to a guard BIF fails (because
    of invalid arguments), the entire guard fails. If the guard was
    part of a guard sequence, the next guard in the sequence (that is,
    the guard following the next semicolon) will be evaluated.</p>

  </section>

  <section>
    <marker id="prec"></marker>
    <title>Operator Precedence</title>
    <p>Operator precedence in falling priority:</p>
    <table>
      <row>
        <cell align="left" valign="middle">:</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">#</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">Unary + - bnot not</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">/ * div rem band and</cell>
        <cell align="left" valign="middle">Left associative</cell>
      </row>
      <row>
        <cell align="left" valign="middle">+ - bor bxor bsl bsr or xor</cell>
        <cell align="left" valign="middle">Left associative</cell>
      </row>
      <row>
        <cell align="left" valign="middle">++ --</cell>
        <cell align="left" valign="middle">Right associative</cell>
      </row>
      <row>
        <cell align="left" valign="middle">== /= =&lt; &lt; >= > =:= =/=</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">andalso</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">orelse</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <row>
        <cell align="left" valign="middle">= !</cell>
        <cell align="left" valign="middle">Right associative</cell>
      </row>
      <row>
        <cell align="left" valign="middle">catch</cell>
        <cell align="left" valign="middle">&nbsp;</cell>
      </row>
      <tcaption>Operator Precedence.</tcaption>
    </table>
    <p>When evaluating an expression, the operator with the highest
      priority is evaluated first. Operators with the same priority
      are evaluated according to their associativity. Example:
      The left associative arithmetic operators are evaluated left to
      right:</p>
    <pre>
<input>6 + 5 * 4 - 3 / 2</input> evaluates to
<input>6 + 20 - 1.5</input> evaluates to
<input>26 - 1.5</input> evaluates to
<input>24.5</input></pre>
  </section>
</chapter>