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diff --git a/system/doc/programming_examples/funs.xmlsrc b/system/doc/programming_examples/funs.xmlsrc index 7bfac9db8c..d4c32bc854 100644 --- a/system/doc/programming_examples/funs.xmlsrc +++ b/system/doc/programming_examples/funs.xmlsrc @@ -30,146 +30,138 @@ </header> <section> - <title>Example 1 - map</title> - <p>If we want to double every element in a list, we could write a - function named <c>double</c>:</p> + <title>map</title> + <p>The following function, <c>double</c>, doubles every element in a list:</p> <code type="none"> double([H|T]) -> [2*H|double(T)]; double([]) -> [].</code> - <p>This function obviously doubles the argument entered as input - as follows:</p> + <p>Hence, the argument entered as input is doubled as follows:</p> <pre> > <input>double([1,2,3,4]).</input> [2,4,6,8]</pre> - <p>We now add the function <c>add_one</c>, which adds one to every + <p>The following function, <c>add_one</c>, adds one to every element in a list:</p> <code type="none"> add_one([H|T]) -> [H+1|add_one(T)]; add_one([]) -> [].</code> - <p>These functions, <c>double</c> and <c>add_one</c>, have a very - similar structure. We can exploit this fact and write a function - <c>map</c> which expresses this similarity:</p> + <p>The functions <c>double</c> and <c>add_one</c> have a + similar structure. This can be used by writing a function + <c>map</c> that expresses this similarity:</p> <codeinclude file="funs1.erl" tag="%1" type="erl"></codeinclude> - <p>We can now express the functions <c>double</c> and - <c>add_one</c> in terms of <c>map</c> as follows:</p> + <p>The functions <c>double</c> and <c>add_one</c> can now be expressed + in terms of <c>map</c> as follows:</p> <code type="none"> double(L) -> map(fun(X) -> 2*X end, L). add_one(L) -> map(fun(X) -> 1 + X end, L).</code> - <p><c>map(F, List)</c> is a function which takes a function - <c>F</c> and a list <c>L</c> as arguments and returns the new - list which is obtained by applying <c>F</c> to each of + <p><c>map(F, List)</c> is a function that takes a function + <c>F</c> and a list <c>L</c> as arguments and returns a new + list, obtained by applying <c>F</c> to each of the elements in <c>L</c>.</p> <p>The process of abstracting out the common features of a number - of different programs is called procedural abstraction. - Procedural abstraction can be used in order to write several - different functions which have a similar structure, but differ - only in some minor detail. This is done as follows:</p> + of different programs is called <em>procedural abstraction</em>. + Procedural abstraction can be used to write several + different functions that have a similar structure, but differ + in some minor detail. This is done as follows:</p> <list type="ordered"> - <item>write one function which represents the common features of - these functions</item> - <item>parameterize the difference in terms of functions which + <item><em>Step 1.</em> Write one function that represents the common features of + these functions.</item> + <item><em>Step 2.</em> Parameterize the difference in terms of functions that are passed as arguments to the common function.</item> </list> </section> <section> - <title>Example 2 - foreach</title> - <p>This example illustrates procedural abstraction. Initially, we - show the following two examples written as conventional - functions:</p> - <list type="ordered"> - <item>all elements of a list are printed onto a stream</item> - <item>a message is broadcast to a list of processes.</item> - </list> + <title>foreach</title> + <p>This section illustrates procedural abstraction. Initially, + the following two examples are written as conventional + functions.</p> + <p>This function prints all elements of a list onto a stream:</p> <code type="none"> print_list(Stream, [H|T]) -> io:format(Stream, "~p~n", [H]), print_list(Stream, T); print_list(Stream, []) -> true.</code> + <p>This function broadcasts a message to a list of processes:</p> <code type="none"> broadcast(Msg, [Pid|Pids]) -> Pid ! Msg, broadcast(Msg, Pids); broadcast(_, []) -> true.</code> - <p>Both these functions have a very similar structure. They both - iterate over a list doing something to each element in the list. - The "something" has to be carried round as an extra argument to - the function which does this.</p> + <p>These two functions have a similar structure. They both + iterate over a list and do something to each element in the list. + The "something" is passed on as an extra argument to + the function that does this.</p> <p>The function <c>foreach</c> expresses this similarity:</p> <codeinclude file="funs1.erl" tag="%2" type="erl"></codeinclude> - <p>Using <c>foreach</c>, <c>print_list</c> becomes:</p> + <p>Using the function <c>foreach</c>, the function <c>print_list</c> becomes:</p> <code type="none"> foreach(fun(H) -> io:format(S, "~p~n",[H]) end, L)</code> - <p><c>broadcast</c> becomes:</p> + <p>Using the function <c>foreach</c>, the function <c>broadcast</c> becomes:</p> <code type="none"> foreach(fun(Pid) -> Pid ! M end, L)</code> <p><c>foreach</c> is evaluated for its side-effect and not its value. <c>foreach(Fun ,L)</c> calls <c>Fun(X)</c> for each element <c>X</c> in <c>L</c> and the processing occurs in - the order in which the elements were defined in <c>L</c>. + the order that the elements were defined in <c>L</c>. <c>map</c> does not define the order in which its elements are processed.</p> </section> <section> - <title>The Syntax of Funs</title> - <p>Funs are written with the syntax:</p> + <title>Syntax of Funs</title> + <p>Funs are written with the following syntax:</p> <code type="none"> F = fun (Arg1, Arg2, ... ArgN) -> ... end</code> <p>This creates an anonymous function of <c>N</c> arguments and binds it to the variable <c>F</c>.</p> - <p>If we have already written a function in the same module and - wish to pass this function as an argument, we can use - the following syntax:</p> + <p>Another function, <c>FunctionName</c>, written in the same module, + can be passed as an argument, using the following syntax:</p> <code type="none"> F = fun FunctionName/Arity</code> - <p>With this form of function reference, the function which is + <p>With this form of function reference, the function that is referred to does not need to be exported from the module.</p> - <p>We can also refer to a function defined in a different module + <p>It is also possible to refer to a function defined in a different module, with the following syntax:</p> <code type="none"> -F = {Module, FunctionName}</code> +F = fun Module:FunctionName/Arity</code> <p>In this case, the function must be exported from the module in question.</p> - <p>The follow program illustrates the different ways of creating + <p>The following program illustrates the different ways of creating funs:</p> <codeinclude file="fun_test.erl" tag="%1" type="erl"></codeinclude> - <p>We can evaluate the fun <c>F</c> with the syntax:</p> + <p>The fun <c>F</c> can be evaluated with the following syntax:</p> <code type="none"> F(Arg1, Arg2, ..., Argn)</code> <p>To check whether a term is a fun, use the test - <c>is_function/1</c> in a guard. Example:</p> + <c>is_function/1</c> in a guard.</p> + <p><em>Example:</em></p> <code type="none"> f(F, Args) when is_function(F) -> apply(F, Args); f(N, _) when is_integer(N) -> N.</code> - <p>Funs are a distinct type. The BIFs erlang:fun_info/1,2 can + <p>Funs are a distinct type. The BIFs <c>erlang:fun_info/1,2</c> can be used to retrieve information about a fun, and the BIF - erlang:fun_to_list/1 returns a textual representation of a fun. - The check_process_code/2 BIF returns true if the process + <c>erlang:fun_to_list/1</c> returns a textual representation of a fun. + The <c>check_process_code/2</c> BIF returns <c>true</c> if the process contains funs that depend on the old version of a module.</p> - <note> - <p>In OTP R5 and earlier releases, funs were represented using - tuples.</p> - </note> </section> <section> <title>Variable Bindings Within a Fun</title> - <p>The scope rules for variables which occur in funs are as + <p>The scope rules for variables that occur in funs are as follows:</p> <list type="bulleted"> - <item>All variables which occur in the head of a fun are assumed + <item>All variables that occur in the head of a fun are assumed to be "fresh" variables.</item> - <item>Variables which are defined before the fun, and which + <item>Variables that are defined before the fun, and that occur in function calls or guard tests within the fun, have the values they had outside the fun.</item> - <item>No variables may be exported from a fun.</item> + <item>Variables cannot be exported from a fun.</item> </list> <p>The following examples illustrate these rules:</p> <code type="none"> @@ -177,12 +169,13 @@ print_list(File, List) -> {ok, Stream} = file:open(File, write), foreach(fun(X) -> io:format(Stream,"~p~n",[X]) end, List), file:close(Stream).</code> - <p>In the above example, the variable <c>X</c> which is defined in - the head of the fun is a new variable. The value of the variable - <c>Stream</c> which is used within within the fun gets its value + <p>Here, the variable <c>X</c>, defined in + the head of the fun, is a new variable. The variable + <c>Stream</c>, which is used within the fun, gets its value from the <c>file:open</c> line.</p> - <p>Since any variable which occurs in the head of a fun is - considered a new variable it would be equally valid to write:</p> + <p>As any variable that occurs in the head of a fun is + considered a new variable, it is equally valid to write + as follows:</p> <code type="none"> print_list(File, List) -> {ok, Stream} = file:open(File, write), @@ -190,21 +183,21 @@ print_list(File, List) -> io:format(Stream,"~p~n",[File]) end, List), file:close(Stream).</code> - <p>In this example, <c>File</c> is used as the new variable - instead of <c>X</c>. This is rather silly since code in the body - of the fun cannot refer to the variable <c>File</c> which is - defined outside the fun. Compiling this example will yield - the diagnostic:</p> + <p>Here, <c>File</c> is used as the new variable + instead of <c>X</c>. This is not so wise because code in the fun + body cannot refer to the variable <c>File</c>, which is + defined outside of the fun. Compiling this example gives + the following diagnostic:</p> <code type="none"> ./FileName.erl:Line: Warning: variable 'File' - shadowed in 'lambda head'</code> - <p>This reminds us that the variable <c>File</c> which is defined - inside the fun collides with the variable <c>File</c> which is + shadowed in 'fun'</code> + <p>This indicates that the variable <c>File</c>, which is defined + inside the fun, collides with the variable <c>File</c>, which is defined outside the fun.</p> <p>The rules for importing variables into a fun has the consequence - that certain pattern matching operations have to be moved into + that certain pattern matching operations must be moved into guard expressions and cannot be written in the head of the fun. - For example, we might write the following code if we intend + For example, you might write the following code if you intend the first clause of <c>F</c> to be evaluated when the value of its argument is <c>Y</c>:</p> <code type="none"> @@ -216,7 +209,7 @@ f(...) -> ... end, ...) ...</code> - <p>instead of</p> + <p>instead of writng the following code:</p> <code type="none"> f(...) -> Y = ... @@ -229,35 +222,37 @@ f(...) -> </section> <section> - <title>Funs and the Module Lists</title> + <title>Funs and Module Lists</title> <p>The following examples show a dialogue with the Erlang shell. All the higher order functions discussed are exported from the module <c>lists</c>.</p> <section> <title>map</title> + <p><c>map</c> takes a function of one argument and a list of terms:</p> <codeinclude file="funs1.erl" tag="%1" type="erl"></codeinclude> - <p><c>map</c> takes a function of one argument and a list of - terms. It returns the list obtained by applying the function + <p>It returns the list obtained by applying the function to every argument in the list.</p> + <p>When a new fun is defined in the shell, the value of the fun + is printed as <c><![CDATA[Fun#<erl_eval>]]></c>:</p> <pre> > <input>Double = fun(X) -> 2 * X end.</input> #Fun<erl_eval.6.72228031> > <input>lists:map(Double, [1,2,3,4,5]).</input> [2,4,6,8,10]</pre> - <p>When a new fun is defined in the shell, the value of the Fun - is printed as <c><![CDATA[Fun#<erl_eval>]]></c>.</p> + </section> <section> <title>any</title> - <codeinclude file="funs1.erl" tag="%4" type="erl"></codeinclude> <p><c>any</c> takes a predicate <c>P</c> of one argument and a - list of terms. A predicate is a function which returns - <c>true</c> or <c>false</c>. <c>any</c> is true if there is a - term <c>X</c> in the list such that <c>P(X)</c> is <c>true</c>.</p> - <p>We define a predicate <c>Big(X)</c> which is <c>true</c> if - its argument is greater that 10.</p> + list of terms:</p> + <codeinclude file="funs1.erl" tag="%4" type="erl"></codeinclude> + <p>A predicate is a function that returns <c>true</c> or <c>false</c>. + <c>any</c> is <c>true</c> if there is a term <c>X</c> in the list such that + <c>P(X)</c> is <c>true</c>.</p> + <p>A predicate <c>Big(X)</c> is defined, which is <c>true</c> if + its argument is greater that 10:</p> <pre> > <input>Big = fun(X) -> if X > 10 -> true; true -> false end end.</input> #Fun<erl_eval.6.72228031> @@ -269,9 +264,10 @@ true</pre> <section> <title>all</title> + <p><c>all</c> has the same arguments as <c>any</c>:</p> <codeinclude file="funs1.erl" tag="%3" type="erl"></codeinclude> - <p><c>all</c> has the same arguments as <c>any</c>. It is true - if the predicate applied to all elements in the list is true.</p> + <p>It is <c>true</c> + if the predicate applied to all elements in the list is <c>true</c>.</p> <pre> > <input>lists:all(Big, [1,2,3,4,12,6]).</input> false @@ -281,11 +277,12 @@ true</pre> <section> <title>foreach</title> - <codeinclude file="funs1.erl" tag="%2" type="erl"></codeinclude> <p><c>foreach</c> takes a function of one argument and a list of - terms. The function is applied to each argument in the list. - <c>foreach</c> returns <c>ok</c>. It is used for its - side-effect only.</p> + terms:</p> + <codeinclude file="funs1.erl" tag="%2" type="erl"></codeinclude> + <p>The function is applied to each argument in the list. + <c>foreach</c> returns <c>ok</c>. It is only used for its + side-effect:</p> <pre> > <input>lists:foreach(fun(X) -> io:format("~w~n",[X]) end, [1,2,3,4]).</input> 1 @@ -297,15 +294,16 @@ ok</pre> <section> <title>foldl</title> - <codeinclude file="funs1.erl" tag="%8" type="erl"></codeinclude> <p><c>foldl</c> takes a function of two arguments, an - accumulator and a list. The function is called with two + accumulator and a list:</p> + <codeinclude file="funs1.erl" tag="%8" type="erl"></codeinclude> + <p>The function is called with two arguments. The first argument is the successive elements in - the list, the second argument is the accumulator. The function - must return a new accumulator which is used the next time + the list. The second argument is the accumulator. The function + must return a new accumulator, which is used the next time the function is called.</p> - <p>If we have a list of lists <c>L = ["I","like","Erlang"]</c>, - then we can sum the lengths of all the strings in <c>L</c> as + <p>If you have a list of lists <c>L = ["I","like","Erlang"]</c>, + then you can sum the lengths of all the strings in <c>L</c> as follows:</p> <pre> > <input>L = ["I","like","Erlang"].</input> @@ -325,11 +323,11 @@ end</code> <section> <title>mapfoldl</title> + <p><c>mapfoldl</c> simultaneously maps and folds over a list:</p> <codeinclude file="funs1.erl" tag="%10" type="erl"></codeinclude> - <p><c>mapfoldl</c> simultaneously maps and folds over a list. - The following example shows how to change all letters in - <c>L</c> to upper case and count them.</p> - <p>First upcase:</p> + <p>The following example shows how to change all letters in + <c>L</c> to upper case and then count them.</p> + <p>First the change to upper case:</p> <pre> > <input>Upcase = fun(X) when $a =< X, X =< $z -> X + $A - $a;</input> <input>(X) -> X</input> @@ -344,7 +342,7 @@ end</code> "ERLANG" > <input>lists:map(Upcase_word, L).</input> ["I","LIKE","ERLANG"]</pre> - <p>Now we can do the fold and the map at the same time:</p> + <p>Now, the fold and the map can be done at the same time:</p> <pre> > <input>lists:mapfoldl(fun(Word, Sum) -></input> <input>{Upcase_word(Word), Sum + length(Word)}</input> @@ -354,23 +352,24 @@ end</code> <section> <title>filter</title> - <codeinclude file="funs1.erl" tag="%9" type="erl"></codeinclude> <p><c>filter</c> takes a predicate of one argument and a list - and returns all element in the list which satisfy - the predicate.</p> + and returns all elements in the list that satisfy + the predicate:</p> + <codeinclude file="funs1.erl" tag="%9" type="erl"></codeinclude> <pre> > <input>lists:filter(Big, [500,12,2,45,6,7]).</input> [500,12,45]</pre> - <p>When we combine maps and filters we can write very succinct - code. For example, suppose we want to define a set difference - function. We want to define <c>diff(L1, L2)</c> to be - the difference between the lists <c>L1</c> and <c>L2</c>. - This is the list of all elements in L1 which are not contained - in L2. This code can be written as follows:</p> + <p>Combining maps and filters enables writing of very succinct + code. For example, to define a set difference + function <c>diff(L1, L2)</c> to be + the difference between the lists <c>L1</c> and <c>L2</c>, + the code can be written as follows:</p> <code type="none"> diff(L1, L2) -> filter(fun(X) -> not member(X, L2) end, L1).</code> - <p>The AND intersection of the list <c>L1</c> and <c>L2</c> is + <p>This gives the list of all elements in L1 that are not contained + in L2.</p> + <p> The AND intersection of the list <c>L1</c> and <c>L2</c> is also easily defined:</p> <code type="none"> intersection(L1,L2) -> filter(fun(X) -> member(X,L1) end, L2).</code> @@ -378,9 +377,9 @@ intersection(L1,L2) -> filter(fun(X) -> member(X,L1) end, L2).</code> <section> <title>takewhile</title> - <codeinclude file="funs1.erl" tag="%5" type="erl"></codeinclude> <p><c>takewhile(P, L)</c> takes elements <c>X</c> from a list - <c>L</c> as long as the predicate <c>P(X)</c> is true.</p> + <c>L</c> as long as the predicate <c>P(X)</c> is true:</p> + <codeinclude file="funs1.erl" tag="%5" type="erl"></codeinclude> <pre> > <input>lists:takewhile(Big, [200,500,45,5,3,45,6]).</input> [200,500,45]</pre> @@ -388,8 +387,8 @@ intersection(L1,L2) -> filter(fun(X) -> member(X,L1) end, L2).</code> <section> <title>dropwhile</title> + <p><c>dropwhile</c> is the complement of <c>takewhile</c>:</p> <codeinclude file="funs1.erl" tag="%6" type="erl"></codeinclude> - <p><c>dropwhile</c> is the complement of <c>takewhile</c>.</p> <pre> > <input>lists:dropwhile(Big, [200,500,45,5,3,45,6]).</input> [5,3,45,6]</pre> @@ -397,10 +396,10 @@ intersection(L1,L2) -> filter(fun(X) -> member(X,L1) end, L2).</code> <section> <title>splitwith</title> - <codeinclude file="funs1.erl" tag="%7" type="erl"></codeinclude> <p><c>splitwith(P, L)</c> splits the list <c>L</c> into the two - sub-lists <c>{L1, L2}</c>, where <c>L = takewhile(P, L)</c> - and <c>L2 = dropwhile(P, L)</c>.</p> + sublists <c>{L1, L2}</c>, where <c>L = takewhile(P, L)</c> + and <c>L2 = dropwhile(P, L)</c>:</p> + <codeinclude file="funs1.erl" tag="%7" type="erl"></codeinclude> <pre> > <input>lists:splitwith(Big, [200,500,45,5,3,45,6]).</input> {[200,500,45],[5,3,45,6]}</pre> @@ -408,17 +407,17 @@ intersection(L1,L2) -> filter(fun(X) -> member(X,L1) end, L2).</code> </section> <section> - <title>Funs Which Return Funs</title> - <p>So far, this section has only described functions which take - funs as arguments. It is also possible to write more powerful - functions which themselves return funs. The following examples - illustrate these type of functions.</p> + <title>Funs Returning Funs</title> + <p>So far, only functions that take + funs as arguments have been described. More powerful + functions, that themselves return funs, can also be written. The following + examples illustrate these type of functions.</p> <section> <title>Simple Higher Order Functions</title> - <p><c>Adder(X)</c> is a function which, given <c>X</c>, returns + <p><c>Adder(X)</c> is a function that given <c>X</c>, returns a new function <c>G</c> such that <c>G(K)</c> returns - <c>K + X</c>.</p> + <c>K + X</c>:</p> <pre> > <input>Adder = fun(X) -> fun(Y) -> X + Y end end.</input> #Fun<erl_eval.6.72228031> @@ -438,7 +437,7 @@ ints_from(N) -> fun() -> [N|ints_from(N+1)] end.</code> - <p>Then we can proceed as follows:</p> + <p>Then proceed as follows:</p> <pre> > <input>XX = lazy:ints_from(1).</input> #Fun<lazy.0.29874839> @@ -450,7 +449,7 @@ ints_from(N) -> #Fun<lazy.0.29874839> > <input>hd(Y()).</input> 2</pre> - <p>etc. - this is an example of "lazy embedding".</p> + <p>And so on. This is an example of "lazy embedding".</p> </section> <section> @@ -459,17 +458,21 @@ ints_from(N) -> <pre> Parser(Toks) -> {ok, Tree, Toks1} | fail</pre> <p><c>Toks</c> is the list of tokens to be parsed. A successful - parse returns <c>{ok, Tree, Toks1}</c>, where <c>Tree</c> is a - parse tree and <c>Toks1</c> is a tail of <c>Tree</c> which - contains symbols encountered after the structure which was - correctly parsed. Otherwise <c>fail</c> is returned.</p> - <p>The example which follows illustrates a simple, functional - parser which parses the grammar:</p> + parse returns <c>{ok, Tree, Toks1}</c>.</p> + <list type="bulleted"> + <item><c>Tree</c> is a parse tree.</item> + <item><c>Toks1</c> is a tail of <c>Tree</c> that + contains symbols encountered after the structure that was + correctly parsed.</item> + </list> + <p>An unsuccessful parse returns <c>fail</c>.</p> + <p>The following example illustrates a simple, functional + parser that parses the grammar:</p> <pre> (a | b) & (c | d)</pre> <p>The following code defines a function <c>pconst(X)</c> in - the module <c>funparse</c>, which returns a fun which parses a - list of tokens.</p> + the module <c>funparse</c>, which returns a fun that parses a + list of tokens:</p> <codeinclude file="funparse.erl" tag="%14" type="erl"></codeinclude> <p>This function can be used as follows:</p> <pre> @@ -479,17 +482,18 @@ Parser(Toks) -> {ok, Tree, Toks1} | fail</pre> {ok,{const,a},[b,c]} > <input>P1([x,y,z]).</input> fail</pre> - <p>Next, we define the two higher order functions <c>pand</c> - and <c>por</c> which combine primitive parsers to produce more - complex parsers. Firstly <c>pand</c>:</p> + <p>Next, the two higher order functions <c>pand</c> + and <c>por</c> are defined. They combine primitive parsers to produce more + complex parsers.</p> + <p>First <c>pand</c>:</p> <codeinclude file="funparse.erl" tag="%16" type="erl"></codeinclude> <p>Given a parser <c>P1</c> for grammar <c>G1</c>, and a parser <c>P2</c> for grammar <c>G2</c>, <c>pand(P1, P2)</c> returns a - parser for the grammar which consists of sequences of tokens - which satisfy <c>G1</c> followed by sequences of tokens which + parser for the grammar, which consists of sequences of tokens + that satisfy <c>G1</c>, followed by sequences of tokens that satisfy <c>G2</c>.</p> <p><c>por(P1, P2)</c> returns a parser for the language - described by the grammar <c>G1</c> or <c>G2</c>.</p> + described by the grammar <c>G1</c> or <c>G2</c>:</p> <codeinclude file="funparse.erl" tag="%15" type="erl"></codeinclude> <p>The original problem was to parse the grammar <c><![CDATA[(a | b) & (c | d)]]></c>. The following code addresses this @@ -497,7 +501,7 @@ fail</pre> <codeinclude file="funparse.erl" tag="%13" type="erl"></codeinclude> <p>The following code adds a parser interface to the grammar:</p> <codeinclude file="funparse.erl" tag="%12" type="erl"></codeinclude> - <p>We can test this parser as follows:</p> + <p>The parser can be tested as follows:</p> <pre> > <input>funparse:parse([a,c]).</input> {ok,{'and',{'or',1,{const,a}},{'or',1,{const,c}}}} |