aboutsummaryrefslogtreecommitdiffstats
path: root/lib/dialyzer/src/dialyzer.erl
blob: d875c45c4a7241029e20e6cb010aa24137331264 (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
%% -*- erlang-indent-level: 2 -*-
%%-----------------------------------------------------------------------
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 2006-2012. All Rights Reserved.
%%
%% 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.
%%
%% %CopyrightEnd%
%%

%%%-------------------------------------------------------------------
%%% File        : dialyzer.erl
%%% Authors     : Tobias Lindahl <[email protected]>
%%%               Kostis Sagonas <[email protected]>
%%% Description : This is the interface for the Dialyzer tool.
%%%
%%% Created     : 27 Apr 2004 by Tobias Lindahl <[email protected]>
%%%-------------------------------------------------------------------

-module(dialyzer).

%%--------------------------------------------------------------------
%% NOTE: Only functions exported by this module are available to
%%       other applications.
%%--------------------------------------------------------------------
-export([plain_cl/0,
	 run/1,
	 gui/0,
	 gui/1,
	 plt_info/1,
	 format_warning/1,
	 format_warning/2]).

-include("dialyzer.hrl").

%%--------------------------------------------------------------------
%% Interfaces:
%%  - plain_cl/0 :      to be used ONLY by the dialyzer C program.
%%  - run/1:            Erlang interface for a command line-like analysis
%%  - gui/0/1:          Erlang interface for the gui.
%%  - format_warning/1: Get the string representation of a warning.
%%  - format_warning/1: Likewise, but with an option whether
%%			to display full path names or not
%%  - plt_info/1:       Get information of the specified plt.
%%--------------------------------------------------------------------

-spec plain_cl() -> no_return().

plain_cl() ->
  case dialyzer_cl_parse:start() of
    {check_init, Opts} ->
      cl_halt(cl_check_init(Opts), Opts);
    {plt_info, Opts} ->
      cl_halt(cl_print_plt_info(Opts), Opts);
    {{gui, Type}, Opts} ->
      try check_gui_options(Opts)
      catch throw:{dialyzer_error, Msg} -> cl_error(Msg)
      end,
      case Opts#options.check_plt of
	true ->
	  case cl_check_init(Opts#options{get_warnings = false}) of
	    {ok, _} -> gui_halt(internal_gui(Type, Opts), Opts);
	    {error, _} = Error -> cl_halt(Error, Opts)
	  end;
	false ->
	  gui_halt(internal_gui(Type, Opts), Opts)
      end;
    {cl, Opts} ->
      case Opts#options.check_plt of
	true ->
	  case cl_check_init(Opts#options{get_warnings = false}) of
	    {error, _} = Error -> cl_halt(Error, Opts);
	    {ok, _} -> cl_halt(cl(Opts), Opts)
	  end;
	false ->
	  cl_halt(cl(Opts), Opts)
      end;
    {error, Msg} ->
      cl_error(Msg)
  end.

cl_check_init(#options{analysis_type = AnalType} = Opts) ->
  case AnalType of
    plt_build ->  {ok, ?RET_NOTHING_SUSPICIOUS};
    plt_add ->    {ok, ?RET_NOTHING_SUSPICIOUS};
    plt_remove -> {ok, ?RET_NOTHING_SUSPICIOUS};
    Other when Other =:= succ_typings; Other =:= plt_check ->
      F = fun() ->
	      NewOpts = Opts#options{analysis_type = plt_check},
	      {Ret, _Warnings} = dialyzer_cl:start(NewOpts),
	      Ret
	  end,
      doit(F)
  end.

cl_print_plt_info(Opts) ->
  F = fun() ->
	  print_plt_info(Opts)
      end,
  doit(F).

print_plt_info(#options{init_plts = PLTs, output_file = OutputFile}) ->
  PLTInfo = get_plt_info(PLTs),
  do_print_plt_info(PLTInfo, OutputFile).

get_plt_info([PLT|PLTs]) ->
  String =
    case dialyzer_plt:included_files(PLT) of
      {ok, Files} ->
	io_lib:format("The PLT ~s includes the following files:\n~p\n\n",
		      [PLT, Files]);
      {error, read_error} ->
	Msg = io_lib:format("Could not read the PLT file ~p\n\n", [PLT]),
	throw({dialyzer_error, Msg});
      {error, no_such_file} ->
	Msg = io_lib:format("The PLT file ~p does not exist\n\n", [PLT]),
	throw({dialyzer_error, Msg})
    end,
  String ++ get_plt_info(PLTs);
get_plt_info([]) -> "".

do_print_plt_info(PLTInfo, OutputFile) ->
  case OutputFile =:= none of
    true ->
      io:format("~s", [PLTInfo]),
      ?RET_NOTHING_SUSPICIOUS;
    false ->
      case file:open(OutputFile, [write]) of
	{ok, FileDesc} ->
	  io:format(FileDesc, "~s", [PLTInfo]),
	  ok = file:close(FileDesc),
	  ?RET_NOTHING_SUSPICIOUS;
	{error, Reason} ->
	  Msg1 = io_lib:format("Could not open output file ~p, Reason: ~p\n",
			       [OutputFile, Reason]),
	  throw({dialyzer_error, Msg1})
      end
  end.

cl(Opts) ->
  F = fun() ->
	  {Ret, _Warnings} = dialyzer_cl:start(Opts),
	  Ret
      end,
  doit(F).

-spec run(dial_options()) -> [dial_warning()].

run(Opts) ->
  try dialyzer_options:build([{report_mode, quiet},
			      {erlang_mode, true}|Opts]) of
    {error, Msg} ->
      throw({dialyzer_error, Msg});
    OptsRecord ->
      case cl_check_init(OptsRecord) of
	{ok, ?RET_NOTHING_SUSPICIOUS} ->
	  case dialyzer_cl:start(OptsRecord) of
	    {?RET_DISCREPANCIES, Warnings} -> Warnings;
	    {?RET_NOTHING_SUSPICIOUS, []}  -> []
	  end;
	{error, ErrorMsg1} ->
	  throw({dialyzer_error, ErrorMsg1})
      end
  catch
    throw:{dialyzer_error, ErrorMsg} ->
      erlang:error({dialyzer_error, lists:flatten(ErrorMsg)})
  end.

internal_gui(Type, OptsRecord) ->
  F = fun() ->
	  case Type of
	    gs -> dialyzer_gui:start(OptsRecord);
	    wx -> dialyzer_gui_wx:start(OptsRecord)
	  end,
	  ?RET_NOTHING_SUSPICIOUS
      end,
  doit(F).

-spec gui() -> 'ok'.

gui() ->
  gui([]).

-spec gui(dial_options()) -> 'ok'.

gui(Opts) ->
  try dialyzer_options:build([{report_mode, quiet}|Opts]) of
    {error, Msg} ->
      throw({dialyzer_error, Msg});
    OptsRecord ->
      ok = check_gui_options(OptsRecord),
      case cl_check_init(OptsRecord) of
	{ok, ?RET_NOTHING_SUSPICIOUS} ->
	  F = fun() ->
		  dialyzer_gui:start(OptsRecord)
	      end,
	  case doit(F) of
	    {ok, _} -> ok;
	    {error, Msg} -> throw({dialyzer_error, Msg})
	  end;
	{error, ErrorMsg1} ->
	  throw({dialyzer_error, ErrorMsg1})
      end
  catch
    throw:{dialyzer_error, ErrorMsg} ->
      erlang:error({dialyzer_error, lists:flatten(ErrorMsg)})
  end.

check_gui_options(#options{analysis_type = succ_typings}) ->
  ok;
check_gui_options(#options{analysis_type = Mode}) ->
  Msg = io_lib:format("Analysis mode ~w is illegal in GUI mode", [Mode]),
  throw({dialyzer_error, Msg}).

-spec plt_info(file:filename()) ->
     {'ok', [{'files', [file:filename()]}]} | {'error', atom()}.

plt_info(Plt) ->
  case dialyzer_plt:included_files(Plt) of
    {ok, Files} -> {ok, [{files, Files}]};
    Error -> Error
  end.


%%-----------
%% Machinery
%%-----------

-type doit_ret() :: {'ok', dial_ret()} | {'error', string()}.

doit(F) ->
  try
    {ok, F()}
  catch
    throw:{dialyzer_error, Msg} ->
      {error, lists:flatten(Msg)}
  end.

-spec cl_error(string()) -> no_return().

cl_error(Msg) ->
  cl_halt({error, Msg}, #options{}).

-spec gui_halt(doit_ret(), #options{}) -> no_return().

gui_halt(R, Opts) ->
  cl_halt(R, Opts#options{report_mode = quiet}).

-spec cl_halt(doit_ret(), #options{}) -> no_return().

cl_halt({ok, R = ?RET_NOTHING_SUSPICIOUS}, #options{report_mode = quiet}) ->
  halt(R);
cl_halt({ok, R = ?RET_DISCREPANCIES}, #options{report_mode = quiet}) ->
  halt(R);
cl_halt({ok, R = ?RET_NOTHING_SUSPICIOUS}, #options{}) ->
  io:put_chars("done (passed successfully)\n"),
  halt(R);
cl_halt({ok, R = ?RET_DISCREPANCIES}, #options{output_file = Output}) ->
  io:put_chars("done (warnings were emitted)\n"),
  cl_check_log(Output),
  halt(R);
cl_halt({error, Msg1}, #options{output_file = Output}) ->
  %% Msg2 = "dialyzer: Internal problems were encountered in the analysis",
  io:format("\ndialyzer: ~s\n", [Msg1]),
  cl_check_log(Output),
  halt(?RET_INTERNAL_ERROR).

-spec cl_check_log('none' | file:filename()) -> 'ok'.

cl_check_log(none) ->
  ok;
cl_check_log(Output) ->
  io:format("  Check output file `~s' for details\n", [Output]).

-spec format_warning(dial_warning()) -> string().

format_warning(W) ->
  format_warning(W, basename).

-spec format_warning(dial_warning(), fopt()) -> string().

format_warning({_Tag, {File, Line}, Msg}, FOpt) when is_list(File),
						     is_integer(Line) ->
  F = case FOpt of
	fullpath -> File;
	basename -> filename:basename(File)
      end,
  String = lists:flatten(message_to_string(Msg)),
  lists:flatten(io_lib:format("~s:~w: ~s", [F, Line, String])).


%%-----------------------------------------------------------------------------
%% Message classification and pretty-printing below. Messages appear in
%% categories and in more or less alphabetical ordering within each category.
%%-----------------------------------------------------------------------------

%%----- Warnings for general discrepancies ----------------
message_to_string({apply, [Args, ArgNs, FailReason,
			   SigArgs, SigRet, Contract]}) ->
  io_lib:format("Fun application with arguments ~s ", [Args]) ++
    call_or_apply_to_string(ArgNs, FailReason, SigArgs, SigRet, Contract);
message_to_string({app_call, [M, F, Args, Culprit, ExpectedType, FoundType]}) ->
  io_lib:format("The call ~s:~s~s requires that ~s is of type ~s not ~s\n",
		[M, F, Args, Culprit, ExpectedType, FoundType]);
message_to_string({bin_construction, [Culprit, Size, Seg, Type]}) ->
  io_lib:format("Binary construction will fail since the ~s field ~s in"
		" segment ~s has type ~s\n", [Culprit, Size, Seg, Type]);
message_to_string({call, [M, F, Args, ArgNs, FailReason,
			  SigArgs, SigRet, Contract]}) ->
  io_lib:format("The call ~w:~w~s ", [M, F, Args]) ++
    call_or_apply_to_string(ArgNs, FailReason, SigArgs, SigRet, Contract);
message_to_string({call_to_missing, [M, F, A]}) ->
  io_lib:format("Call to missing or unexported function ~w:~w/~w\n", [M, F, A]);
message_to_string({exact_eq, [Type1, Op, Type2]}) ->
  io_lib:format("The test ~s ~s ~s can never evaluate to 'true'\n",
		[Type1, Op, Type2]);
message_to_string({fun_app_args, [Args, Type]}) ->
  io_lib:format("Fun application with arguments ~s will fail"
		" since the function has type ~s\n", [Args, Type]);
message_to_string({fun_app_no_fun, [Op, Type, Arity]}) ->
  io_lib:format("Fun application will fail since ~s :: ~s"
		" is not a function of arity ~w\n", [Op, Type, Arity]);
message_to_string({guard_fail, []}) ->
  "Clause guard cannot succeed.\n";
message_to_string({guard_fail, [Arg1, Infix, Arg2]}) ->
  io_lib:format("Guard test ~s ~s ~s can never succeed\n", [Arg1, Infix, Arg2]);
message_to_string({neg_guard_fail, [Arg1, Infix, Arg2]}) ->
  io_lib:format("Guard test not(~s ~s ~s) can never succeed\n",
		[Arg1, Infix, Arg2]);
message_to_string({guard_fail, [Guard, Args]}) ->
  io_lib:format("Guard test ~w~s can never succeed\n", [Guard, Args]);
message_to_string({neg_guard_fail, [Guard, Args]}) ->
  io_lib:format("Guard test not(~w~s) can never succeed\n", [Guard, Args]);
message_to_string({guard_fail_pat, [Pat, Type]}) ->
  io_lib:format("Clause guard cannot succeed. The ~s was matched"
		" against the type ~s\n", [Pat, Type]);
message_to_string({improper_list_constr, [TlType]}) ->
  io_lib:format("Cons will produce an improper list"
		" since its 2nd argument is ~s\n", [TlType]);
message_to_string({no_return, [Type|Name]}) ->
  NameString =
    case Name of
      [] -> "The created fun ";
      [F, A] -> io_lib:format("Function ~w/~w ", [F, A])
    end,
  case Type of
    no_match -> NameString ++ "has no clauses that will ever match\n";
    only_explicit -> NameString ++ "only terminates with explicit exception\n";
    only_normal -> NameString ++ "has no local return\n";
    both -> NameString ++ "has no local return\n"
  end;
message_to_string({record_constr, [RecConstr, FieldDiffs]}) ->
  io_lib:format("Record construction ~s violates the"
		" declared type of field ~s\n", [RecConstr, FieldDiffs]);
message_to_string({record_constr, [Name, Field, Type]}) ->
  io_lib:format("Record construction violates the declared type for #~w{}"
		" since ~s cannot be of type ~s\n", [Name, Field, Type]);
message_to_string({record_matching, [String, Name]}) ->
  io_lib:format("The ~s violates the"
		" declared type for #~w{}\n", [String, Name]);
message_to_string({record_match, [Pat, Type]}) ->
  io_lib:format("Matching of ~s tagged with a record name violates the declared"
		" type of ~s\n", [Pat, Type]);
message_to_string({pattern_match, [Pat, Type]}) ->
  io_lib:format("The ~s can never match the type ~s\n", [Pat, Type]);
message_to_string({pattern_match_cov, [Pat, Type]}) ->
  io_lib:format("The ~s can never match since previous"
		" clauses completely covered the type ~s\n",
		[Pat, Type]);
message_to_string({unmatched_return, [Type]}) ->
  io_lib:format("Expression produces a value of type ~s,"
		" but this value is unmatched\n", [Type]);
message_to_string({unused_fun, [F, A]}) ->
  io_lib:format("Function ~w/~w will never be called\n", [F, A]);
%%----- Warnings for specs and contracts -------------------
message_to_string({contract_diff, [M, F, _A, Contract, Sig]}) ->
  io_lib:format("Type specification ~w:~w~s"
		" is not equal to the success typing: ~w:~w~s\n",
		[M, F, Contract, M, F, Sig]);
message_to_string({contract_subtype, [M, F, _A, Contract, Sig]}) ->
  io_lib:format("Type specification ~w:~w~s"
		" is a subtype of the success typing: ~w:~w~s\n",
		[M, F, Contract, M, F, Sig]);
message_to_string({contract_supertype, [M, F, _A, Contract, Sig]}) ->
  io_lib:format("Type specification ~w:~w~s"
		" is a supertype of the success typing: ~w:~w~s\n",
		[M, F, Contract, M, F, Sig]);
message_to_string({contract_range, [Contract, M, F, ArgStrings, Line, CRet]}) ->
  io_lib:format("The contract ~w:~w~s cannot be right because the inferred"
		" return for ~w~s on line ~w is ~s\n",
		[M, F, Contract, F, ArgStrings, Line, CRet]);
message_to_string({invalid_contract, [M, F, A, Sig]}) ->
  io_lib:format("Invalid type specification for function ~w:~w/~w."
		" The success typing is ~s\n", [M, F, A, Sig]);
message_to_string({extra_range, [M, F, A, ExtraRanges, SigRange]}) ->
  io_lib:format("The specification for ~w:~w/~w states that the function"
		" might also return ~s but the inferred return is ~s\n",
		[M, F, A, ExtraRanges, SigRange]);
message_to_string({overlapping_contract, []}) ->
  "Overloaded contract has overlapping domains;"
    " such contracts are currently unsupported and are simply ignored\n";
message_to_string({spec_missing_fun, [M, F, A]}) ->
  io_lib:format("Contract for function that does not exist: ~w:~w/~w\n",
		[M, F, A]);
%%----- Warnings for opaque type violations -------------------
message_to_string({call_with_opaque, [M, F, Args, ArgNs, ExpArgs]}) ->
  io_lib:format("The call ~w:~w~s contains ~s when ~s\n",
		[M, F, Args, form_positions(ArgNs), form_expected(ExpArgs)]);
message_to_string({call_without_opaque, [M, F, Args, ExpectedTriples]}) ->
  io_lib:format("The call ~w:~w~s does not have ~s\n",
		[M, F, Args, form_expected_without_opaque(ExpectedTriples)]);
message_to_string({opaque_eq, [Type, _Op, OpaqueType]}) ->
  io_lib:format("Attempt to test for equality between a term of type ~s"
		" and a term of opaque type ~s\n", [Type, OpaqueType]);
message_to_string({opaque_guard, [Guard, Args]}) ->
  io_lib:format("Guard test ~w~s breaks the opaqueness of its argument\n",
		[Guard, Args]);
message_to_string({opaque_match, [Pat, OpaqueType, OpaqueTerm]}) ->
  Term = if OpaqueType =:= OpaqueTerm -> "the term";
	    true -> OpaqueTerm
	 end,
  io_lib:format("The attempt to match a term of type ~s against the ~s"
		" breaks the opaqueness of ~s\n", [OpaqueType, Pat, Term]);
message_to_string({opaque_neq, [Type, _Op, OpaqueType]}) ->
  io_lib:format("Attempt to test for inequality between a term of type ~s"
		" and a term of opaque type ~s\n", [Type, OpaqueType]);
message_to_string({opaque_type_test, [Fun, Opaque]}) ->
  io_lib:format("The type test ~s(~s) breaks the opaqueness of the term ~s\n", [Fun, Opaque, Opaque]);
%%----- Warnings for concurrency errors --------------------
message_to_string({race_condition, [M, F, Args, Reason]}) ->
  io_lib:format("The call ~w:~w~s ~s\n", [M, F, Args, Reason]);
%%----- Warnings for behaviour errors --------------------
message_to_string({callback_type_mismatch, [B, F, A, ST, CT]}) ->
  io_lib:format("The inferred return type of ~w/~w (~s) has nothing in common"
		" with ~s, which is the expected return type for the callback of"
		" ~w behaviour\n", [F, A, ST, CT, B]);
message_to_string({callback_arg_type_mismatch, [B, F, A, N, ST, CT]}) ->
  io_lib:format("The inferred type for the ~s argument of ~w/~w (~s) is"
		" not a supertype of ~s, which is expected type for this"
		" argument in the callback of the ~w behaviour\n",
		[ordinal(N), F, A, ST, CT, B]);
message_to_string({callback_spec_type_mismatch, [B, F, A, ST, CT]}) ->
  io_lib:format("The return type ~s in the specification of ~w/~w is not a"
		" subtype of ~s, which is the expected return type for the"
		" callback of ~w behaviour\n", [ST, F, A, CT, B]);
message_to_string({callback_spec_arg_type_mismatch, [B, F, A, N, ST, CT]}) ->
  io_lib:format("The specified type for the ~s argument of ~w/~w (~s) is"
		" not a supertype of ~s, which is expected type for this"
		" argument in the callback of the ~w behaviour\n",
		[ordinal(N), F, A, ST, CT, B]);
message_to_string({callback_missing, [B, F, A]}) ->
  io_lib:format("Undefined callback function ~w/~w (behaviour '~w')\n",
		[F, A, B]);
message_to_string({callback_info_missing, [B]}) ->
  io_lib:format("Callback info about the ~w behaviour is not available\n", [B]).

%%-----------------------------------------------------------------------------
%% Auxiliary functions below
%%-----------------------------------------------------------------------------

call_or_apply_to_string(ArgNs, FailReason, SigArgs, SigRet,
			{IsOverloaded, Contract}) ->
  PositionString = form_position_string(ArgNs),
  case FailReason of
    only_sig ->
      case ArgNs =:= [] of
	true ->
	  %% We do not know which argument(s) caused the failure
	  io_lib:format("will never return since the success typing arguments"
			" are ~s\n", [SigArgs]);
        false ->
	  io_lib:format("will never return since it differs in the ~s argument"
			" from the success typing arguments: ~s\n",
			[PositionString, SigArgs])
      end;
    only_contract ->
      case (ArgNs =:= []) orelse IsOverloaded of
	true ->
	  %% We do not know which arguments caused the failure
	  io_lib:format("breaks the contract ~s\n", [Contract]);
	false ->
	  io_lib:format("breaks the contract ~s in the ~s argument\n",
			[Contract, PositionString])
      end;
    both ->
      io_lib:format("will never return since the success typing is ~s -> ~s"
		    " and the contract is ~s\n", [SigArgs, SigRet, Contract])
  end.

form_positions(ArgNs) ->
  case ArgNs of
    [_] -> "an opaque term as ";
    [_,_|_] -> "opaque terms as "
 end ++ form_position_string(ArgNs) ++
  case ArgNs of
    [_] -> " argument";
    [_,_|_] -> " arguments"
  end.

%% We know which positions N are to blame;
%% the list of triples will never be empty.
form_expected_without_opaque([{N, T, TStr}]) ->
  case erl_types:t_is_opaque(T) of
    true  ->
      io_lib:format("an opaque term of type ~s as ", [TStr]);
    false ->
      io_lib:format("a term of type ~s (with opaque subterms) as ", [TStr])
  end ++ form_position_string([N]) ++ " argument";
form_expected_without_opaque(ExpectedTriples) -> %% TODO: can do much better here
  {ArgNs, _Ts, _TStrs} = lists:unzip3(ExpectedTriples),
  "opaque terms as " ++ form_position_string(ArgNs) ++ " arguments".

form_expected(ExpectedArgs) ->
  case ExpectedArgs of
    [T] ->
      TS = erl_types:t_to_string(T),
      case erl_types:t_is_opaque(T) of
	true  -> io_lib:format("an opaque term of type ~s is expected", [TS]);
	false -> io_lib:format("a structured term of type ~s is expected", [TS])
      end;
    [_,_|_] -> "terms of different types are expected in these positions"
  end.

form_position_string(ArgNs) ->
  case ArgNs of
    [] -> "";
    [N1] -> ordinal(N1);
    [_,_|_] ->
      [Last|Prevs] = lists:reverse(ArgNs),
      ", " ++ Head = lists:flatten([io_lib:format(", ~s",[ordinal(N)]) ||
				     N <- lists:reverse(Prevs)]),
      Head ++ " and " ++ ordinal(Last)
  end.

ordinal(1) -> "1st";
ordinal(2) -> "2nd";
ordinal(3) -> "3rd";
ordinal(N) when is_integer(N) -> io_lib:format("~wth",[N]).