path: root/system/doc/getting_started/seq_prog.xml

<?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>Sequential Programming</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>seq_prog.xml</file>
  </header>

  <section>
    <title>The Erlang Shell</title>
    <p>Most operating systems have a command interpreter or shell- Unix
      and Linux have many, while Windows has the Command Prompt. Erlang has
      its own shell where you can directly write bits of Erlang code
      and evaluate (run) them to see what happens (see
      <seealso marker="stdlib:shell">shell(3)</seealso>). Start
      the Erlang shell (in Linux or UNIX) by starting a shell or
      command interpreter in your operating system and typing
      <c>erl</c>. You will see something like this.</p>
    <pre>
% <input>erl</input>
Erlang R15B (erts-5.9.1) [source] [smp:8:8] [rq:8] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1></pre>
    <p>Now type in "2 + 5." as shown below.</p>
    <pre>
1> <input>2 + 5.</input>
7
2></pre>
    <p>In Windows, the shell is started by double-clicking on the Erlang
      shell icon.</p>
    <p>You'll notice that the Erlang shell has numbered the lines that
      can be entered, (as 1&gt; 2&gt;) and that it has correctly told you
      that 2 + 5 is 7! Also notice that you have to tell it you are
      done entering code by finishing with a full stop "." and a
      carriage return. If you make mistakes writing things in the shell,
      you can delete things by using the backspace key as in most
      shells. There are many more editing commands in the shell
      (See the chapter <seealso marker="erts:tty">"tty - A command line interface"</seealso> in ERTS User's Guide).</p>
    <p>(Note: you will find a lot of line numbers given by the shell
      out of sequence in this tutorial as it was written and the code
      tested in several sessions.)</p>
    <p>Now let's try a more complex calculation.</p>
    <pre>
2> <input>(42 + 77) * 66 / 3.</input>
2618.0</pre>
    <p>Here you can see the use of brackets and the multiplication
      operator "*" and division operator "/", just as in normal
      arithmetic (see the chapter
      <seealso marker="doc/reference_manual:expressions">"Arithmetic Expressions"</seealso> in the Erlang Reference Manual).</p>
    <p>To shutdown the Erlang system and the Erlang shell type
      Control-C. You will see the following output:</p>
    <pre>
BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded
       (v)ersion (k)ill (D)b-tables (d)istribution
<input>a</input>
%</pre>
    <p>Type "a" to leave the Erlang system.</p>
    <p>Another way to shutdown the Erlang system is by entering
      <c>halt()</c>:</p>
    <pre>
3> <input>halt().</input>
% </pre>
  </section>

  <section>
    <title>Modules and Functions</title>
    <p>A programming language isn't much use if you can just run code
      from the shell. So here is a small Erlang program. Enter it into
      a file called <c>tut.erl</c> (the file name <c>tut.erl</c> is
      important, also make sure that it is in the same directory as
      the one where you started <c>erl</c>) using a suitable
      text editor. If you are lucky your editor will have an Erlang
      mode which will make it easier for you to enter and format your
      code nicely (see the chapter
      <seealso marker="tools:erlang_mode_chapter">"The Erlang mode for Emacs"</seealso> in Tools User's Guide), but you can manage
      perfectly well without. Here's the code to enter:</p>
    <code type="none">
-module(tut).
-export([double/1]).

double(X) ->
    2 * X.</code>
    <p>It's not hard to guess that this "program" doubles the value of
      numbers. I'll get back to the first two lines later. Let's compile
      the program. This can be done in your Erlang shell as shown below:</p>
    <pre>
3> <input>c(tut).</input>
{ok,tut}</pre>
    <p>The <c>{ok,tut}</c> tells you that the compilation was OK. If it
      said "error" instead, you have made some mistake in the text you
      entered and there will also be error messages to give you some
      idea as to what has gone wrong so you can change what you have
      written and try again.</p>
    <p>Now let's run the program.</p>
    <pre>
4> <input>tut:double(10).</input>
20</pre>
    <p>As expected double of 10 is 20.</p>
    <p>Now let's get back to the first two lines. Erlang programs are
      written in files. Each file contains what we call an Erlang
      <em>module</em>. The first line of code in the module tells us
      the name of the module (see the chapter
      <seealso marker="doc/reference_manual:modules">"Modules"</seealso>
      in the Erlang Reference Manual).</p>
    <code type="none">
-module(tut).</code>
    <p>This tells us that the module is called <em>tut</em>. Note
      the "." at the end of the line. The files which are used to store
      the module must have the same name as the module but with
      the extension ".erl". In our case the file name is <c>tut.erl</c>.
      When we use a function in another module, we use the syntax,
      <c>module_name:function_name(arguments)</c>. So</p>
    <pre>
4> <input>tut:double(10).</input></pre>
    <p>means call function <c>double</c> in module <c>tut</c> with
      argument "10".</p>
    <p>The second line:</p>
    <code type="none">
-export([double/1]).</code>
    <p>says that the module <c>tut</c> contains a function called
      <c>double</c> which takes one argument (<c>X</c> in our example)
      and that this function can be called from outside the module
      <c>tut</c>. More about this later. Again note the "." at the end
      of the line.</p>
    <p>Now for a more complicated example, the factorial of a number
      (e.g. factorial of 4 is 4 * 3 * 2 * 1). Enter the following code
      in a file called <c>tut1.erl</c>.</p>
    <code type="none">
-module(tut1).
-export([fac/1]).

fac(1) ->
    1;
fac(N) ->
    N * fac(N - 1).</code>
    <p>Compile the file</p>
    <pre>
5> <input>c(tut1).</input>
{ok,tut1}</pre>
    <p>And now calculate the factorial of 4.</p>
    <pre>
6> <input>tut1:fac(4).</input>
24</pre>
    <p>The first part:</p>
    <code type="none">
fac(1) ->
    1;</code>
    <p>says that the factorial of 1 is 1. Note that we end this part
      with a ";" which indicates that there is more of this function to
      come. The second part:</p>
    <code type="none">
fac(N) ->
    N * fac(N - 1).</code>
    <p>says that the factorial of N is N multiplied by the factorial of
      N - 1. Note that this part ends with a "." saying that there are
      no more parts of this function.</p>
    <p>A function can have many arguments. Let's expand the module
      <c>tut1</c> with the rather stupid function to multiply two
      numbers:</p>
    <code type="none">
-module(tut1).
-export([fac/1, mult/2]).

fac(1) ->
    1;
fac(N) ->
    N * fac(N - 1).

mult(X, Y) ->
    X * Y.</code>
    <p>Note that we have also had to expand the <c>-export</c> line
      with the information that there is another function <c>mult</c>
      with two arguments.</p>
    <p>Compile:</p>
    <pre>
7> <input>c(tut1).</input>
{ok,tut1}</pre>
    <p>and try it out:</p>
    <pre>
8> <input>tut1:mult(3,4).</input>
12</pre>
    <p>In the example above the numbers are integers and the arguments
      in the functions in the code, <c>N</c>, <c>X</c>, <c>Y</c> are
      called variables. Variables must start with a capital letter
      (see the chapter
      <seealso marker="doc/reference_manual:expressions">"Variables"</seealso>
      in the Erlang Reference Manual). Examples of variable could be
      <c>Number</c>, <c>ShoeSize</c>, <c>Age</c> etc.</p>
  </section>

  <section>
    <title>Atoms</title>
    <p>Atoms are another data type in Erlang. Atoms start with a small
      letter ((see the chapter
      <seealso marker="doc/reference_manual:data_types">"Atom"</seealso>
      in the Erlang Reference Manual)), for example: <c>charles</c>,
      <c>centimeter</c>, <c>inch</c>. Atoms are simply names, nothing
      else. They are not like variables which can have a value.</p>
    <p>Enter the next program (file: <c>tut2.erl</c>) which could be
      useful for converting from inches to centimeters and vice versa:</p>
    <code type="none">
-module(tut2).
-export([convert/2]).

convert(M, inch) ->
    M / 2.54;

convert(N, centimeter) ->
    N * 2.54.</code>
    <p>Compile and test:</p>
    <pre>
9> <input>c(tut2).</input>
{ok,tut2}
10> <input>tut2:convert(3, inch).</input>
1.1811023622047243
11> <input>tut2:convert(7, centimeter).</input>
17.78</pre>
    <p>Notice that I have introduced decimals (floating point numbers)
      without any explanation, but I guess you can cope with that.</p>
    <p>See what happens if I enter something other than centimeter or
      inch in the convert function:</p>
    <pre>
12> <input>tut2:convert(3, miles).</input>
** exception error: no function clause matching tut2:convert(3,miles) (tut2.erl, line 4)</pre>
    <p>The two parts of the <c>convert</c> function are called its
      clauses. Here we see that "miles" is not part of either of
      the clauses. The Erlang system can't <em>match</em> either of
      the clauses so we get an error message <c>function_clause</c>.
      The shell formats the error message nicely, but the error tuple 
      is saved in the shell's history list and can be output by the shell 
      command <c>v/1</c>:</p>
    <pre>
13> <input>v(12).</input>
{'EXIT',{function_clause,[{tut2,convert,
                                [3,miles],
                                [{file,"tut2.erl"},{line,4}]},
                          {erl_eval,do_apply,5,[{file,"erl_eval.erl"},{line,482}]},
                          {shell,exprs,7,[{file,"shell.erl"},{line,666}]},
                          {shell,eval_exprs,7,[{file,"shell.erl"},{line,621}]},
                          {shell,eval_loop,3,[{file,"shell.erl"},{line,606}]}]}}</pre>

  </section>

  <section>
    <title>Tuples</title>
    <p>Now the <c>tut2</c> program is hardly good programming style.
      Consider:</p>
    <code type="none">
tut2:convert(3, inch).</code>
    <p>Does this mean that 3 is in inches? or that 3 is in centimeters
      and we want to convert it to inches? So Erlang has a way to group
      things together to make things more understandable. We call these
      <em>tuples</em>. Tuples are surrounded by "{" and "}".</p>
    <p>So we can write <c>{inch,3}</c> to denote 3 inches and
      <c>{centimeter,5}</c> to denote 5 centimeters. Now let's write a
      new program which converts centimeters to inches and vice versa.
      (file <c>tut3.erl</c>).</p>
    <code type="none">
-module(tut3).
-export([convert_length/1]).

convert_length({centimeter, X}) ->
    {inch, X / 2.54};
convert_length({inch, Y}) ->
    {centimeter, Y * 2.54}.</code>
    <p>Compile and test:</p>
    <pre>
14> <input>c(tut3).</input>
{ok,tut3}
15> <input>tut3:convert_length({inch, 5}).</input>
{centimeter,12.7}
16> <input>tut3:convert_length(tut3:convert_length({inch, 5})).</input>
{inch,5.0}</pre>
    <p>Note on line 16 we convert 5 inches to centimeters and back
      again and reassuringly get back to the original value. I.e
      the argument to a function can be the result of another function.
      Pause for a moment and consider how line 16 (above) works.
      The argument we have given the function <c>{inch,5}</c> is first
      matched against the first head clause of <c>convert_length</c>
      i.e. <c>convert_length({centimeter,X})</c> where it can be seen
      that <c>{centimeter,X}</c> does not match <c>{inch,5}</c>
      (the head is the bit before the "-&gt;"). This having failed, we try
      the head of the next clause i.e. <c>convert_length({inch,Y})</c>,
      this matches and <c>Y</c> get the value 5.</p>
    <p>We have shown tuples with two parts above, but tuples can have
      as many parts as we want and contain any valid Erlang
      <em>term</em>. For example, to represent the temperature of
      various cities of the world we could write:</p>
    <code type="none">
{moscow, {c, -10}}
{cape_town, {f, 70}}
{paris, {f, 28}}</code>
    <p>Tuples have a fixed number of things in them. We call each thing
      in a tuple an element. So in the tuple <c>{moscow,{c,-10}}</c>,
      element 1 is <c>moscow</c> and element 2 is <c>{c,-10}</c>. I
      have chosen <c>c</c> meaning Centigrade (or Celsius) and <c>f</c>
      meaning Fahrenheit.</p>
  </section>

  <section>
    <title>Lists</title>
    <p>Whereas tuples group things together, we also want to be able to
      represent lists of things. Lists in Erlang are surrounded by "["
      and "]". For example a list of the temperatures of various cities
      in the world could be:</p>
    <code type="none">
[{moscow, {c, -10}}, {cape_town, {f, 70}}, {stockholm, {c, -4}},
 {paris, {f, 28}}, {london, {f, 36}}]</code>
    <p>Note that this list was so long that it didn't fit on one line.
      This doesn't matter, Erlang allows line breaks at all "sensible
      places" but not, for example, in the middle of atoms, integers
      etc.</p>
    <p>A very useful way of looking at parts of lists, is by using "|".
      This is best explained by an example using the shell.</p>
    <pre>
17> <input>[First |TheRest] = [1,2,3,4,5].</input>
[1,2,3,4,5]
18> <input>First.</input>
1
19> <input>TheRest.</input>
[2,3,4,5]</pre>
    <p>We use | to separate the first elements of the list from
      the rest of the list. (<c>First</c> has got value 1 and
      <c>TheRest</c> value [2,3,4,5].)</p>
    <p>Another example:</p>
    <pre>
20> <input>[E1, E2 | R] = [1,2,3,4,5,6,7].</input>
[1,2,3,4,5,6,7]
21> <input>E1.</input>
1
22> <input>E2.</input>
2
23> <input>R.</input>
[3,4,5,6,7]</pre>
    <p>Here we see the use of | to get the first two elements from
      the list. Of course if we try to get more elements from the list
      than there are elements in the list we will get an error. Note
      also the special case of the list with no elements [].</p>
    <pre>
24> <input>[A, B | C] = [1, 2].</input>
[1,2]
25> <input>A.</input>
1
26> <input>B.</input>
2
27> <input>C.</input>
[]</pre>
    <p>In all the examples above, I have been using new variable names,
      not reusing the old ones: <c>First</c>, <c>TheRest</c>, <c>E1</c>,
      <c>E2</c>, <c>R</c>, <c>A</c>, <c>B</c>, <c>C</c>. The reason
      for this is that a variable can only be given a value once in its
      context (scope). I'll get back to this later, it isn't so
      peculiar as it sounds!</p>
    <p>The following example shows how we find the length of a list:</p>
    <code type="none">
-module(tut4).

-export([list_length/1]).

list_length([]) ->
    0;    
list_length([First | Rest]) ->
    1 + list_length(Rest).</code>
    <p>Compile (file <c>tut4.erl</c>) and test:</p>
    <pre>
28> <input>c(tut4).</input>
{ok,tut4}
29> <input>tut4:list_length([1,2,3,4,5,6,7]).</input>
7</pre>
    <p>Explanation:</p>
    <code type="none">
list_length([]) ->
    0;</code>
    <p>The length of an empty list is obviously 0.</p>
    <code type="none">
list_length([First | Rest]) ->
    1 + list_length(Rest).</code>
    <p>The length of a list with the first element <c>First</c> and
      the remaining elements <c>Rest</c> is 1 + the length of
      <c>Rest</c>.</p>
    <p>(Advanced readers only: This is not tail recursive, there is a
      better way to write this function.)</p>
    <p>In general we can say we use tuples where we would use "records"
      or "structs" in other languages and we use lists when we want to
      represent things which have varying sizes, (i.e. where we would
      use linked lists in other languages).</p>
    <p>Erlang does not have a string data type, instead strings can be
      represented by lists of ASCII characters. So the list
      <c>[97,98,99]</c> is equivalent to "abc". The Erlang shell is
      "clever" and guesses the what sort of list we mean and outputs it
      in what it thinks is the most appropriate form, for example:</p>
    <pre>
30> <input>[97,98,99].</input>
"abc"</pre>
  </section>

  <section>
    <title>Maps</title>
	<p>Maps are a set of key to value associations. These associations
		are encapsulated with "#{" and "}". To create an association from
		"key" to value 42, we write:</p>
<code type="none">
> #{ "key" => 42 }.
#{"key" => 42}</code>
    <p>We will jump straight into the deep end with an example using some
		interesting features.</p>
	<p>The following example shows how we calculate alpha blending using
		maps to reference color and alpha channels:</p>
    <code type="none">
-module(color).

-export([new/4, blend/2]).

-define(is_channel(V), (is_float(V) andalso V &gt;= 0.0 andalso V =&lt; 1.0)).

new(R,G,B,A) when ?is_channel(R), ?is_channel(G),
                  ?is_channel(B), ?is_channel(A) ->
    #{red =&gt; R, green =&gt; G, blue =&gt; B, alpha =&gt; A}.

blend(Src,Dst) ->
    blend(Src,Dst,alpha(Src,Dst)).

blend(Src,Dst,Alpha) when Alpha > 0.0 ->
    Dst#{
        red   := red(Src,Dst) / Alpha,
        green := green(Src,Dst) / Alpha,
        blue  := blue(Src,Dst) / Alpha,
        alpha := Alpha
    };
blend(_,Dst,_) ->
    Dst#{
        red   := 0.0,
        green := 0.0,
        blue  := 0.0,
        alpha := 0.0
    }.

alpha(#{alpha := SA}, #{alpha := DA}) ->
    SA + DA*(1.0 - SA).

red(#{red := SV, alpha := SA}, #{red := DV, alpha := DA}) ->
    SV*SA + DV*DA*(1.0 - SA).
green(#{green := SV, alpha := SA}, #{green := DV, alpha := DA}) ->
    SV*SA + DV*DA*(1.0 - SA).
blue(#{blue := SV, alpha := SA}, #{blue := DV, alpha := DA}) ->
    SV*SA + DV*DA*(1.0 - SA).</code>
    <p>Compile (file <c>color.erl</c>) and test:</p>
    <pre>
> <input>c(color).</input>
{ok,color}
> <input>C1 = color:new(0.3,0.4,0.5,1.0).</input>
#{alpha => 1.0,blue => 0.5,green => 0.4,red => 0.3}
> <input>C2 = color:new(1.0,0.8,0.1,0.3).</input>
#{alpha => 0.3,blue => 0.1,green => 0.8,red => 1.0}
> <input>color:blend(C1,C2).</input>
#{alpha => 1.0,blue => 0.5,green => 0.4,red => 0.3}
> <input>color:blend(C2,C1).</input>
#{alpha => 1.0,blue => 0.38,green => 0.52,red => 0.51}
</pre>
    <p>This example warrants some explanation:</p>
    <code type="none">
-define(is_channel(V), (is_float(V) andalso V &gt;= 0.0 andalso V =&lt; 1.0)).</code>
    <p>
		First we define a macro <c>is_channel</c> to help with our guard tests.
		This is only here for convenience and to reduce syntax cluttering.

		You can read more about <seealso marker="doc/reference_manual:macros">Macros</seealso>
		in the Erlang Reference Manual.
	</p>
    <code type="none">
new(R,G,B,A) when ?is_channel(R), ?is_channel(G),
                  ?is_channel(B), ?is_channel(A) ->
    #{red =&gt; R, green =&gt; G, blue =&gt; B, alpha =&gt; A}.</code>
    <p>
		The function <c>new/4</c> creates a new map term with and lets the keys
		<c>red</c>, <c>green</c>, <c>blue</c> and <c>alpha</c> be associated
		with an initial value. In this case we only allow for float
		values between and including 0.0 and 1.0 as ensured by the <c>?is_channel/1</c> macro
		for each argument. Only the <c>=></c> operator is allowed when creating a new map.
    </p>
	<p>
		By calling <c>blend/2</c> on any color term created by <c>new/4</c> we can calculate
		the resulting color as determined by the two maps terms.
	</p>
	<p>
		The first thing <c>blend/2</c> does is to calculate the resulting alpha channel.
	</p>
    <code type="none">
alpha(#{alpha := SA}, #{alpha := DA}) ->
    SA + DA*(1.0 - SA).</code>
    <p>
		We fetch the value associated with key <c>alpha</c> for both arguments using
		the <c>:=</c> operator. Any other keys
		in the map are ignored, only the key <c>alpha</c> is required and checked for.
	</p>
	<p>This is also the case for functions <c>red/2</c>, <c>blue/2</c> and <c>green/2</c>.</p>
    <code type="none">
red(#{red := SV, alpha := SA}, #{red := DV, alpha := DA}) ->
    SV*SA + DV*DA*(1.0 - SA).</code>
    <p>
		The difference here is that we check for two keys in each map argument. The other keys
		are ignored.
	</p>
    <p>
		Finally we return the resulting color in <c>blend/3</c>.
	</p>
    <code type="none">
blend(Src,Dst,Alpha) when Alpha > 0.0 ->
    Dst#{
        red   := red(Src,Dst) / Alpha,
        green := green(Src,Dst) / Alpha,
        blue  := blue(Src,Dst) / Alpha,
        alpha := Alpha
    };</code>
    <p>
		We update the <c>Dst</c> map with new channel values. The syntax for updating an existing key with a new value is done with <c>:=</c> operator.
	</p>
  </section>

  <section>
    <title>Standard Modules and Manual Pages</title>
    <p>Erlang has a lot of standard modules to help you do things. For
      example, the module <c>io</c> contains a lot of functions to help
      you do formatted input/output. To look up information about
      standard modules, the command <c>erl -man</c> can be used at
      the operating shell or command prompt (i.e. at the same place as
      that where you started <c>erl</c>). Try the operating system
      shell command:</p>
    <pre>
% <input>erl -man io</input>
ERLANG MODULE DEFINITION                                    io(3)

MODULE
     io - Standard I/O Server Interface Functions

DESCRIPTION
     This module provides an  interface  to  standard  Erlang  IO
     servers. The output functions all return ok if they are suc-
     ...</pre>
    <p>If this doesn't work on your system, the documentation is
      included as HTML in the Erlang/OTP release, or you can read
      the documentation as HTML or download it as PDF from either of
      the sites www.erlang.se (commercial Erlang) or www.erlang.org
      (open source), for example for release R9B:</p>
    <code type="none">
http://www.erlang.org/doc/r9b/doc/index.html</code>
  </section>

  <section>
    <title>Writing Output to a Terminal</title>
    <p>It's nice to be able to do formatted output in these example, so
      the next example shows a simple way to use to use
      the <c>io:format</c> function. Of course, just like all other
      exported functions, you can test the <c>io:format</c> function in
      the shell:</p>
    <pre>
31> <input>io:format("hello world~n", []).</input>
hello world
ok
32> <input>io:format("this outputs one Erlang term: ~w~n", [hello]).</input>
this outputs one Erlang term: hello
ok
33> <input>io:format("this outputs two Erlang terms: ~w~w~n", [hello, world]).</input>
this outputs two Erlang terms: helloworld
ok
34> <input>io:format("this outputs two Erlang terms: ~w ~w~n", [hello, world]).</input>
this outputs two Erlang terms: hello world
ok</pre>
    <p>The function <c>format/2</c> (i.e. <c>format</c> with two
      arguments) takes two lists. The first one is nearly always a list
      written between " ". This list is printed out as it stands,
      except that each ~w is replaced by a term taken in order from
      the second list. Each ~n is replaced by a new line.
      The <c>io:format/2</c> function itself returns the atom <c>ok</c>
      if everything goes as planned. Like other functions in Erlang, it
      crashes if an error occurs. This is not a fault in Erlang, it is
      a deliberate policy. Erlang has sophisticated mechanisms to
      handle errors which we will show later. As an exercise, try to
      make <c>io:format</c> crash, it shouldn't be difficult. But
      notice that although <c>io:format</c> crashes, the Erlang shell
      itself does not crash.</p>
  </section>

  <section>
    <title>A Larger Example</title>
    <p>Now for a larger example to consolidate what we have learnt so
      far. Assume we have a list of temperature readings from a number
      of cities in the world. Some of them are in Celsius (Centigrade)
      and some in Fahrenheit (as in the previous list). First let's
      convert them all to Celsius, then let's print out the data neatly.</p>
    <code type="none">
%% This module is in file tut5.erl

-module(tut5).
-export([format_temps/1]).

%% Only this function is exported
format_temps([])->                        % No output for an empty list
    ok;
format_temps([City | Rest]) ->
    print_temp(convert_to_celsius(City)),
    format_temps(Rest).

convert_to_celsius({Name, {c, Temp}}) ->  % No conversion needed
    {Name, {c, Temp}};
convert_to_celsius({Name, {f, Temp}}) ->  % Do the conversion
    {Name, {c, (Temp - 32) * 5 / 9}}.

print_temp({Name, {c, Temp}}) ->
    io:format("~-15w ~w c~n", [Name, Temp]).</code>
    <pre>
35> <input>c(tut5).</input>
{ok,tut5}
36> <input>tut5:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},</input>
<input>{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).</input>
moscow          -10 c
cape_town       21.11111111111111 c
stockholm       -4 c
paris           -2.2222222222222223 c
london          2.2222222222222223 c
ok</pre>
    <p>Before we look at how this program works, notice that we have
      added a few comments to the code. A comment starts with a %
      character and goes on to the end of the line. Note as well that
      the <c>-export([format_temps/1]).</c> line only includes
      the function <c>format_temps/1</c>, the other functions are
      <em>local</em> functions, i.e. they are not visible from outside
      the module <c>tut5</c>.</p>
    <p>Note as well that when testing the program from the shell, I had
      to spread the input over two lines as the line was too long.</p>
    <p>When we call <c>format_temps</c> the first time, <c>City</c>
      gets the value <c>{moscow,{c,-10}}</c> and <c>Rest</c> is
      the rest of the list. So we call the function
      <c>print_temp(convert_to_celsius({moscow,{c,-10}}))</c>.</p>
    <p>Here we see a function call as
      <c>convert_to_celsius({moscow,{c,-10}})</c> as the argument to
      the function <c>print_temp</c>. When we <em>nest</em> function
      calls like this we execute (evaluate) them from the inside out.
      I.e. we first evaluate <c>convert_to_celsius({moscow,{c,-10}})</c>
      which gives the value <c>{moscow,{c,-10}}</c> as the temperature
      is already in Celsius and then we evaluate
      <c>print_temp({moscow,{c,-10}})</c>. The function
      <c>convert_to_celsius</c> works in a similar way to
      the <c>convert_length</c> function in the previous example.</p>
    <p><c>print_temp</c> simply calls <c>io:format</c> in a similar way
      to what has been described above. Note that ~-15w says to print
      the "term" with a field length (width) of 15 and left justify it.
      (<seealso marker="stdlib:io#fwrite/1">io(3)</seealso>).</p>
    <p>Now we call <c>format_temps(Rest)</c> with the rest of the list
      as an argument. This way of doing things is similar to the loop
      constructs in other languages. (Yes, this is recursion, but don't
      let that worry you.) So the same <c>format_temps</c> function is
      called again, this time <c>City</c> gets the value
      <c>{cape_town,{f,70}}</c> and we repeat the same procedure as
      before. We go on doing this until the list becomes empty, i.e. [],
      which causes the first clause <c>format_temps([])</c> to match.
      This simply returns (results in) the atom <c>ok</c>, so
      the program ends.</p>
  </section>

  <section>
    <title>Matching, Guards and Scope of Variables</title>
    <p>It could be useful to find the maximum and minimum temperature
      in lists like this. Before extending the program to do this,
      let's look at functions for finding the maximum value of
      the elements in a list:</p>
    <code type="none">
-module(tut6).
-export([list_max/1]).

list_max([Head|Rest]) ->
   list_max(Rest, Head).

list_max([], Res) ->
    Res;
list_max([Head|Rest], Result_so_far) when Head > Result_so_far ->
    list_max(Rest, Head);
list_max([Head|Rest], Result_so_far)  ->
    list_max(Rest, Result_so_far).</code>
    <pre>
37> <input>c(tut6).</input>
{ok,tut6}
38> <input>tut6:list_max([1,2,3,4,5,7,4,3,2,1]).</input>
7</pre>
    <p>First note that we have two functions here with the same name
      <c>list_max</c>. However each of these takes a different number
      of arguments (parameters). In Erlang these are regarded as
      completely different functions. Where we need to distinguish
      between these functions we write <c>name/arity</c>, where
      <c>name</c> is the name of the function and <c>arity</c> is
      the number of arguments, in this case <c>list_max/1</c> and
      <c>list_max/2</c>.</p>
    <p>This is an example where we walk through a list "carrying" a
      value with us, in this case <c>Result_so_far</c>.
      <c>list_max/1</c> simply assumes that the max value of the list
      is the head of the list and calls <c>list_max/2</c> with the rest
      of the list and the value of the head of the list, in the above
      this would be <c>list_max([2,3,4,5,7,4,3,2,1],1)</c>. If we tried
      to use <c>list_max/1</c> with an empty list or tried to use it
      with something which isn't a list at all, we would cause an error.
      Note that the Erlang philosophy is not to handle errors of this
      type in the function they occur, but to do so elsewhere. More
      about this later.</p>
    <p>In <c>list_max/2</c> we walk down the list and use <c>Head</c>
      instead of <c>Result_so_far</c> when <c>Head</c> &gt;
      <c>Result_so_far</c>. <c>when</c> is a special word we use before
      the -&gt; in the function to say that we should only use this part
      of the function if the test which follows is true. We call tests
      of this type a <em>guard</em>. If the guard isn't true (we say
      the guard fails), we try the next part of the function. In this
      case if <c>Head</c> isn't greater than <c>Result_so_far</c> then
      it must be smaller or equal to is, so we don't need a guard on
      the next part of the function.</p>
    <p>Some useful operators in guards are, &lt; less than, &gt;
      greater than, == equal, &gt;= greater or equal, =&lt; less or
      equal, /= not equal. (see the chapter
      <seealso marker="doc/reference_manual:expressions">"Guard Sequences"</seealso> in the Erlang Reference Manual.)</p>
    <p>To change the above program to one which works out the minimum
      value of the element in a list, all we would need to do is to
      write &lt; instead of &gt;. (But it would be wise to change
      the name of the function to <c>list_min</c> :-).)</p>
    <p>Remember that I mentioned earlier that a variable could only be
      given a value once in its scope? In the above we see, for example,
      that <c>Result_so_far</c> has been given several values. This is
      OK since every time we call <c>list_max/2</c> we create a new
      scope and one can regard the <c>Result_so_far</c> as a completely
      different variable in each scope.</p>
    <p>Another way of creating and giving a variable a value is by using
      the match operator = . So if I write <c>M = 5</c>, a variable
      called <c>M</c> will be created and given the value 5. If, in
      the same scope I then write <c>M = 6</c>, I'll get an error. Try
      this out in the shell:</p>
    <pre>
39> <input>M = 5.</input>
5
40> <input>M = 6.</input>
** exception error: no match of right hand side value 6
41> <input>M = M + 1.</input>
** exception error: no match of right hand side value 6
42> <input>N = M + 1.</input>
6</pre>
    <p>The use of the match operator is particularly useful for pulling
      apart Erlang terms and creating new ones.</p>
    <pre>
43> <input>{X, Y} = {paris, {f, 28}}.</input>
{paris,{f,28}}
44> <input>X.</input>
paris
45> <input>Y.</input>
{f,28}</pre>
    <p>Here we see that <c>X</c> gets the value <c>paris</c> and
      <c>Y</c><c>{f,28}</c>.</p>
    <p>Of course if we try to do the same again with another city, we
      get an error:</p>
    <pre>
46> <input>{X, Y} = {london, {f, 36}}.</input>
** exception error: no match of right hand side value {london,{f,36}}</pre>
    <p>Variables can also be used to improve the readability of
      programs, for example, in the <c>list_max/2</c> function above,
      we could write:</p>
    <code type="none">
list_max([Head|Rest], Result_so_far) when Head > Result_so_far ->
    New_result_far = Head,
    list_max(Rest, New_result_far);</code>
    <p>which is possibly a little clearer.</p>
  </section>

  <section>
    <title>More About Lists</title>
    <p>Remember that the | operator can be used to get the head of a
      list:</p>
    <pre>
47> <input>[M1|T1] = [paris, london, rome].</input>
[paris,london,rome]
48> <input>M1.</input>
paris
49> <input>T1.</input>
[london,rome]</pre>
    <p>The | operator can also be used to add a head to a list:</p>
    <pre>
50> <input>L1 = [madrid | T1].</input>
[madrid,london,rome]
51> <input>L1.</input>
[madrid,london,rome]</pre>
    <p>Now an example of this when working with lists - reversing
      the order of a list:</p>
    <code type="none">
-module(tut8).

-export([reverse/1]).

reverse(List) ->
    reverse(List, []).

reverse([Head | Rest], Reversed_List) ->
    reverse(Rest, [Head | Reversed_List]);
reverse([], Reversed_List) ->
    Reversed_List.</code>
    <pre>
52> <input>c(tut8).</input>
{ok,tut8}
53> <input>tut8:reverse([1,2,3]).</input>
[3,2,1]</pre>
    <p>Consider how <c>Reversed_List</c> is built. It starts as [], we
      then successively take off the heads of the list to be reversed
      and add them to the the <c>Reversed_List</c>, as shown in
      the following:</p>
    <code type="none">
reverse([1|2,3], []) =>
    reverse([2,3], [1|[]])

reverse([2|3], [1]) =>
    reverse([3], [2|[1])

reverse([3|[]], [2,1]) =>
    reverse([], [3|[2,1]])

reverse([], [3,2,1]) =>
    [3,2,1]</code>
    <p>The module <c>lists</c> contains a lot of functions for
      manipulating lists, for example for reversing them, so before you
      write a list manipulating function it is a good idea to check
      that one isn't already written for you. (see
      <seealso marker="stdlib:lists">lists(3)</seealso>).</p>
    <p>Now let's get back to the cities and temperatures, but take a more
      structured approach this time. First let's convert the whole list
      to Celsius as follows and test the function:</p>
    <code type="none">
-module(tut7).
-export([format_temps/1]).

format_temps(List_of_cities) ->
    convert_list_to_c(List_of_cities).

convert_list_to_c([{Name, {f, F}} | Rest]) ->
    Converted_City = {Name, {c, (F -32)* 5 / 9}},
    [Converted_City | convert_list_to_c(Rest)];

convert_list_to_c([City | Rest]) ->
    [City | convert_list_to_c(Rest)];

convert_list_to_c([]) ->
    [].</code>
    <pre>
54> <input>c(tut7).</input>
{ok, tut7}.
55> <input>tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},</input>
<input>{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).</input>
[{moscow,{c,-10}},
 {cape_town,{c,21.11111111111111}},
 {stockholm,{c,-4}},
 {paris,{c,-2.2222222222222223}},
 {london,{c,2.2222222222222223}}]</pre>
    <p>Looking at this bit by bit:</p>
    <code type="none">
format_temps(List_of_cities) ->
    convert_list_to_c(List_of_cities).</code>
    <p>Here we see that <c>format_temps/1</c> calls
      <c>convert_list_to_c/1</c>. <c>convert_list_to_c/1</c> takes off
      the head of the <c>List_of_cities</c>, converts it to Celsius if
      needed. The | operator is used to add the (maybe) converted
      to the converted rest of the list:</p>
    <code type="none">
[Converted_City | convert_list_to_c(Rest)];</code>
    <p>or</p>
    <code type="none">
[City | convert_list_to_c(Rest)];</code>
    <p>We go on doing this until we get to the end of the list (i.e.
      the list is empty:</p>
    <code type="none">
convert_list_to_c([]) ->
    [].</code>
    <p>Now we have converted the list, we add a function to print it:</p>
    <code type="none">
-module(tut7).
-export([format_temps/1]).

format_temps(List_of_cities) ->
    Converted_List = convert_list_to_c(List_of_cities),
    print_temp(Converted_List).

convert_list_to_c([{Name, {f, F}} | Rest]) ->
    Converted_City = {Name, {c, (F -32)* 5 / 9}},
    [Converted_City | convert_list_to_c(Rest)];

convert_list_to_c([City | Rest]) ->
    [City | convert_list_to_c(Rest)];

convert_list_to_c([]) ->
    [].

print_temp([{Name, {c, Temp}} | Rest]) ->
    io:format("~-15w ~w c~n", [Name, Temp]),
    print_temp(Rest);
print_temp([]) ->
    ok.</code>
    <pre>
56> <input>c(tut7).</input>
{ok,tut7}
57> <input>tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},</input>
<input>{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).</input>
moscow          -10 c
cape_town       21.11111111111111 c
stockholm       -4 c
paris           -2.2222222222222223 c
london          2.2222222222222223 c
ok</pre>
    <p>We now have to add a function to find the cities with
      the maximum and minimum temperatures. The program below isn't
      the most efficient way of doing this as we walk through the list
      of cities four times. But it is better to first strive for
      clarity and correctness and to make programs efficient only if
      really needed.</p>
    <code type="none"><![CDATA[
-module(tut7).
-export([format_temps/1]).

format_temps(List_of_cities) ->
    Converted_List = convert_list_to_c(List_of_cities),
    print_temp(Converted_List),
    {Max_city, Min_city} = find_max_and_min(Converted_List),
    print_max_and_min(Max_city, Min_city).

convert_list_to_c([{Name, {f, Temp}} | Rest]) ->
    Converted_City = {Name, {c, (Temp -32)* 5 / 9}},
    [Converted_City | convert_list_to_c(Rest)];

convert_list_to_c([City | Rest]) ->
    [City | convert_list_to_c(Rest)];

convert_list_to_c([]) ->
    [].

print_temp([{Name, {c, Temp}} | Rest]) ->
    io:format("~-15w ~w c~n", [Name, Temp]),
    print_temp(Rest);
print_temp([]) ->
    ok.

find_max_and_min([City | Rest]) ->
    find_max_and_min(Rest, City, City).

find_max_and_min([{Name, {c, Temp}} | Rest], 
         {Max_Name, {c, Max_Temp}}, 
         {Min_Name, {c, Min_Temp}}) ->
    if 
        Temp > Max_Temp ->
            Max_City = {Name, {c, Temp}};           % Change
        true -> 
            Max_City = {Max_Name, {c, Max_Temp}} % Unchanged
    end,
    if
         Temp < Min_Temp ->
            Min_City = {Name, {c, Temp}};           % Change
        true -> 
            Min_City = {Min_Name, {c, Min_Temp}} % Unchanged
    end,
    find_max_and_min(Rest, Max_City, Min_City);

find_max_and_min([], Max_City, Min_City) ->
    {Max_City, Min_City}.

print_max_and_min({Max_name, {c, Max_temp}}, {Min_name, {c, Min_temp}}) ->
    io:format("Max temperature was ~w c in ~w~n", [Max_temp, Max_name]),
    io:format("Min temperature was ~w c in ~w~n", [Min_temp, Min_name]).]]></code><pre>
58> <input>c(tut7).</input>
{ok, tut7}
59> <input>tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},</input>
<input>{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).</input>
moscow          -10 c
cape_town       21.11111111111111 c
stockholm       -4 c
paris           -2.2222222222222223 c
london          2.2222222222222223 c
Max temperature was 21.11111111111111 c in cape_town
Min temperature was -10 c in moscow
ok</pre>
  </section>

  <section>
    <title>If and Case</title>
    <p>The function <c>find_max_and_min</c> works out the maximum and
      minimum temperature. We have introduced a new construct here
      <c>if</c>. If works as follows:</p>
    <code type="none">
if
    Condition 1 ->
        Action 1;
    Condition 2 ->
        Action 2;
    Condition 3 ->
        Action 3;
    Condition 4 ->
        Action 4
end</code>
    <p>Note there is no ";" before <c>end</c>! Conditions are the same
      as guards, tests which succeed or fail. Erlang starts at the top
      until it finds a condition which succeeds and then it evaluates
      (performs) the action following the condition and ignores all
      other conditions and action before the <c>end</c>. If no
      condition matches, there will be a run-time failure. A condition
      which always is succeeds is the atom, <c>true</c> and this is
      often used last in an <c>if</c> meaning do the action following
      the <c>true</c> if all other conditions have failed.</p>
    <p>The following is a short program to show the workings of
      <c>if</c>.</p>
    <code type="none">
-module(tut9).
-export([test_if/2]).

test_if(A, B) ->
    if 
        A == 5 ->
            io:format("A == 5~n", []),
            a_equals_5;
        B == 6 ->
            io:format("B == 6~n", []),
            b_equals_6;
        A == 2, B == 3 ->                      %i.e. A equals 2 and B equals 3
            io:format("A == 2, B == 3~n", []),
            a_equals_2_b_equals_3;
        A == 1 ; B == 7 ->                     %i.e. A equals 1 or B equals 7
            io:format("A == 1 ; B == 7~n", []),
            a_equals_1_or_b_equals_7
    end.</code>
    <p>Testing this program gives:</p>
    <pre>
60> <input>c(tut9).</input>
{ok,tut9}
61> <input>tut9:test_if(5,33).</input>
A == 5
a_equals_5
62> <input>tut9:test_if(33,6).</input>
B == 6
b_equals_6
63> <input>tut9:test_if(2, 3).</input>
A == 2, B == 3
a_equals_2_b_equals_3
64> <input>tut9:test_if(1, 33).</input>
A == 1 ; B == 7
a_equals_1_or_b_equals_7
65> <input>tut9:test_if(33, 7).</input>
A == 1 ; B == 7
a_equals_1_or_b_equals_7
66> <input>tut9:test_if(33, 33).</input>
** exception error: no true branch found when evaluating an if expression
     in function  tut9:test_if/2 (tut9.erl, line 5)</pre>
    <p>Notice that <c>tut9:test_if(33,33)</c> did not cause any
      condition to succeed so we got the run time error
      <c>if_clause</c>, here nicely formatted by the shell. See the chapter
      <seealso marker="doc/reference_manual:expressions">"Guard Sequences"</seealso> in the Erlang Reference Manual for details
      of the many guard tests available. <c>case</c> is another
      construct in Erlang. Recall that we wrote the
      <c>convert_length</c> function as:</p>
    <code type="none">
convert_length({centimeter, X}) ->
    {inch, X / 2.54};
convert_length({inch, Y}) ->
    {centimeter, Y * 2.54}.</code>
    <p>We could also write the same program as:</p>
    <code type="none">
-module(tut10).
-export([convert_length/1]).

convert_length(Length) ->
    case Length of
        {centimeter, X} ->
            {inch, X / 2.54};
        {inch, Y} ->
            {centimeter, Y * 2.54}
    end.</code>
    <pre>
67> <input>c(tut10).</input>
{ok,tut10}
68> <input>tut10:convert_length({inch, 6}).</input>
{centimeter,15.24}
69> <input>tut10:convert_length({centimeter, 2.5}).</input>
{inch,0.984251968503937}</pre>
    <p>Notice that both <c>case</c> and <c>if</c> have <em>return values</em>, i.e. in the above example <c>case</c> returned
      either <c>{inch,X/2.54}</c> or <c>{centimeter,Y*2.54}</c>.
      The behaviour of <c>case</c> can also be modified by using guards.
      An example should hopefully clarify this. The following example
      tells us the length of a month, given the year. We need to know
      the year of course, since February has 29 days in a leap year.</p>
    <code type="none">
-module(tut11).
-export([month_length/2]).

month_length(Year, Month) ->
    %% All years divisible by 400 are leap
    %% Years divisible by 100 are not leap (except the 400 rule above)
    %% Years divisible by 4 are leap (except the 100 rule above)
    Leap = if
        trunc(Year / 400) * 400 == Year ->
            leap;
        trunc(Year / 100) * 100 == Year ->
            not_leap;
        trunc(Year / 4) * 4 == Year ->
            leap;
        true ->
            not_leap
    end,  
    case Month of
        sep -> 30;
        apr -> 30;
        jun -> 30;
        nov -> 30;
        feb when Leap == leap -> 29;
        feb -> 28;
        jan -> 31;
        mar -> 31;
        may -> 31;
        jul -> 31;
        aug -> 31;
        oct -> 31;
        dec -> 31
    end.</code>
    <pre>
70> <input>c(tut11).</input>
{ok,tut11}
71> <input>tut11:month_length(2004, feb).</input>
29
72> <input>tut11:month_length(2003, feb).</input>
28
73> <input>tut11:month_length(1947, aug).</input>
31</pre>
  </section>

  <section>
    <title>Built In Functions (BIFs)</title>
    <p>Built in functions (BIFs) are functions which for some reason are
      built in to the Erlang virtual machine. BIFs often implement 
      functionality that is impossible to implement in Erlang or is too
      inefficient to implement in Erlang. Some BIFs can be called 
      by use of the function name only, but they by default belong
      to the erlang module. So for example, the call to the BIF <c>trunc</c>
      below is equivalent to a call to <c>erlang:trunc</c>.</p>
    <p>As you can see, we first find out if a year is leap or not. If a
      year is divisible by 400, it is a leap year. To find this out we
      first divide the year by 400 and use the built in function
      <c>trunc</c> (more later) to cut off any decimals. We then
      multiply by 400 again and see if we get back the same value. For
      example, year 2004:</p>
    <code type="none">
2004 / 400 = 5.01
trunc(5.01) = 5
5 * 400 = 2000</code>
    <p>and we can see that we got back 2000 which is not the same as
      2004, so 2004 isn't divisible by 400. Year 2000:</p>
    <code type="none">
2000 / 400 = 5.0
trunc(5.0) = 5
5 * 400 = 2000</code>
    <p>so we have a leap year. The next two tests, which check if the year is
      divisible by 100 or 4, are done in the same way. The first
      <c>if</c> returns <c>leap</c> or <c>not_leap</c> which ends up
      in the variable <c>Leap</c>. We use this variable in the guard
      for <c>feb</c> in the following <c>case</c> which tells us how
      long the month is.</p>
    <p>This example showed the use of <c>trunc</c>. An easier way would
      be to use the Erlang operator <c>rem</c>, which gives the remainder
      after division. For example:</p>
    <pre>
74> <input>2004 rem 400.</input>
4</pre>
    <p>so instead of writing:</p>
    <code type="none">
trunc(Year / 400) * 400 == Year ->
    leap;</code>
    <p>we could write:</p>
    <code type="none">
Year rem 400 == 0 ->
    leap;</code>
    <p>There are many other built in functions (BIF) such as
      <c>trunc</c>. Only a few built in functions can be used in guards,
      and you cannot use functions you have defined yourself in guards.
      (see the chapter
      <seealso marker="doc/reference_manual:expressions">"Guard Sequences"</seealso> in the Erlang Reference Manual) (Aside for
      advanced readers: This is to ensure that guards don't have side
      effects.) Let's play with a few of these functions in the shell:</p>
    <pre>
75> <input>trunc(5.6).</input>
5
76> <input>round(5.6).</input>
6
77> <input>length([a,b,c,d]).</input>
4
78> <input>float(5).</input>
5.0
79> <input>is_atom(hello).</input>
true
80> <input>is_atom("hello").</input>
false
81> <input>is_tuple({paris, {c, 30}}).</input>
true
82> <input>is_tuple([paris, {c, 30}]).</input>
false</pre>
    <p>All the above can be used in guards. Now for some which can't be
      used in guards:</p>
    <pre>
83> <input>atom_to_list(hello).</input>
"hello"
84> <input>list_to_atom("goodbye").</input>
goodbye
85> <input>integer_to_list(22).</input>
"22"</pre>
    <p>The 3 BIFs above do conversions which would be difficult (or
      impossible) to do in Erlang.</p>
  </section>

  <section>
    <title>Higher Order Functions (Funs)</title>
    <p>Erlang, like most modern functional programming languages, has
      higher order functions. We start with an example using the shell:</p>
    <pre>
86> <input>Xf = fun(X) -> X * 2 end.</input>
#Fun&lt;erl_eval.5.123085357&gt;
87> <input>Xf(5).</input>
10</pre>
    <p>What we have done here is to define a function which doubles
      the value of number and assign this function to a variable. Thus
      <c>Xf(5)</c> returned the value 10. Two useful functions when
      working with lists are <c>foreach</c> and <c>map</c>, which are
      defined as follows:</p>
    <code type="none">
foreach(Fun, [First|Rest]) ->
    Fun(First),
    foreach(Fun, Rest);
foreach(Fun, []) ->
    ok.

map(Fun, [First|Rest]) -> 
    [Fun(First)|map(Fun,Rest)];
map(Fun, []) -> 
    [].</code>
    <p>These two functions are provided in the standard module
      <c>lists</c>. <c>foreach</c> takes a list and applies a fun to
      every element in the list, <c>map</c> creates a new list by
      applying a fun to every element in a list. Going back to
      the shell, we start by using <c>map</c> and a fun to add 3 to
      every element of a list:</p>
    <pre>
88> <input>Add_3 = fun(X) -> X + 3 end.</input>
#Fun&lt;erl_eval.5.123085357&gt;
89> <input>lists:map(Add_3, [1,2,3]).</input>
[4,5,6]</pre>
    <p>Now let's print out the temperatures in a list of cities (yet
      again):</p>
    <pre>
90> <input>Print_City = fun({City, {X, Temp}}) -> io:format("~-15w ~w ~w~n",</input>
<input>[City, X, Temp]) end.</input>
#Fun&lt;erl_eval.5.123085357&gt;
91> <input>lists:foreach(Print_City, [{moscow, {c, -10}}, {cape_town, {f, 70}},</input>
<input>{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).</input>
moscow          c -10
cape_town       f 70
stockholm       c -4
paris           f 28
london          f 36
ok</pre>
    <p>We will now define a fun which can be used to go through a list
      of cities and temperatures and transform them all to Celsius.</p>
    <code type="none">
-module(tut13).

-export([convert_list_to_c/1]).

convert_to_c({Name, {f, Temp}}) ->
    {Name, {c, trunc((Temp - 32) * 5 / 9)}};
convert_to_c({Name, {c, Temp}}) ->
    {Name, {c, Temp}}.

convert_list_to_c(List) ->
    lists:map(fun convert_to_c/1, List).</code>
    <pre>
92> <input>tut13:convert_list_to_c([{moscow, {c, -10}}, {cape_town, {f, 70}},</input>
<input>{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).</input>
[{moscow,{c,-10}},
 {cape_town,{c,21}},
 {stockholm,{c,-4}},
 {paris,{c,-2}},
 {london,{c,2}}]</pre>
    <p>The <c>convert_to_c</c> function is the same as before, but we
      use it as a fun:</p>
    <code type="none">
lists:map(fun convert_to_c/1, List)</code>
    <p>When we use a function defined elsewhere as a fun we can refer
      to it as <c>Function/Arity</c> (remember that <c>Arity</c> =
      number of arguments). So in the <c>map</c> call we write
      <c>lists:map(fun convert_to_c/1, List)</c>. As you can see
      <c>convert_list_to_c</c> becomes much shorter and easier to
      understand.</p>
    <p>The standard module <c>lists</c> also contains a function
      <c>sort(Fun, List)</c> where <c>Fun</c> is a fun with two
      arguments. This fun should return <c>true</c> if the the first
      argument is less than the second argument, or else <c>false</c>.
      We add sorting to the <c>convert_list_to_c</c>:</p>
    <code type="none"><![CDATA[
-module(tut13).

-export([convert_list_to_c/1]).

convert_to_c({Name, {f, Temp}}) ->
    {Name, {c, trunc((Temp - 32) * 5 / 9)}};
convert_to_c({Name, {c, Temp}}) ->
    {Name, {c, Temp}}.

convert_list_to_c(List) ->
    New_list = lists:map(fun convert_to_c/1, List),
    lists:sort(fun({_, {c, Temp1}}, {_, {c, Temp2}}) ->
                       Temp1 < Temp2 end, New_list).]]></code>
    <pre>
93> <input>c(tut13).</input>
{ok,tut13}
94> <input>tut13:convert_list_to_c([{moscow, {c, -10}}, {cape_town, {f, 70}},</input>
<input>{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).</input>
[{moscow,{c,-10}},
 {stockholm,{c,-4}},
 {paris,{c,-2}},
 {london,{c,2}},
 {cape_town,{c,21}}]</pre>
    <p>In <c>sort</c> we use the fun:</p>
    <code type="none"><![CDATA[
fun({_, {c, Temp1}}, {_, {c, Temp2}}) -> Temp1 < Temp2 end,]]></code>
    <p>Here we introduce the concept of an <em>anonymous variable</em>
      "_". This is simply shorthand for a variable which is going to
      get a value, but we will ignore the value. This can be used
      anywhere suitable, not just in fun's. <c><![CDATA[Temp1 < Temp2]]></c>
      returns <c>true</c> if <c>Temp1</c> is less than <c>Temp2</c>.</p>
  </section>
</chapter>