String Interpolation

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</code> </code>
-Although if we only construct this string to then immediately pass it off to a port, we can more efficiently: <code>+Although if we only construct this string to then immediately pass it off to a port, we can more efficiently:
 +<code>
2> BBBB = ["Today's date and time is ", httpd_util:rfc1123_date(), ".\n"]. 2> BBBB = ["Today's date and time is ", httpd_util:rfc1123_date(), ".\n"].
["Today's date and time is ", "Fri, 20 Aug 2004 22:03:13 GMT", ".\n"] ["Today's date and time is ", "Fri, 20 Aug 2004 22:03:13 GMT", ".\n"]
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-- see the Efficiency Guide for more information about this. -- see the Efficiency Guide for more information about this.
-Other cases require that you use the generic io_lib formatting functions to +Other cases require that you use the generic io_lib formatting functions to create strings with the data you want. For example, we could achive the same result as follows:
-create strings with the data you want. For example, we could achive the same +<code>
-result as follows: <code>+
3> io:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 2004]). 3> io:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 2004]).
Today's date is Fri, Aug 20 2004. Today's date is Fri, Aug 20 2004.
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</code> </code>
-Finally, this unoptimized module offers more traditional string +Finally, this unoptimized module offers more traditional string interpolation:
-interpolation: <code>+<code>
-module(template). -module(template).
-export([subst/2]). -export([subst/2]).
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6> Vars = [{patient,"Mary Becket"}, {diagnosis,"You are an octopus."}], 6> Vars = [{patient,"Mary Becket"}, {diagnosis,"You are an octopus."}],
6> template:subst("E-bloodwork autodoctor: Dear $patient, today's checkup 6> template:subst("E-bloodwork autodoctor: Dear $patient, today's checkup
-indicated a problem: $diagnosis.\n", Vars). "E-bloodwork autodiagnosis: Dear +indicated a problem: $diagnosis.\n", Vars). "E-bloodwork autodoctor: Dear
Mary Becket, today's checkup indicated a problem: You are an octopus.\n" Mary Becket, today's checkup indicated a problem: You are an octopus.\n"
</code> </code>
[[Category:CookBook]][[Category:StringRecipes]] [[Category:CookBook]][[Category:StringRecipes]]

Current revision

[edit] Problem

You want to create string using a mixture of literals and computed values. For example you want to display the current date is the string "Today's date is "

[edit] Solution

Make use of Erlang's generic display procedure and variable argument list. For an easy case (displaying date and time), we can make use of the httpd_util:rfc1123_date/0 function to get a string date, and the built-in concatenation (++) operator to construct the string we want: 

1> BBBB = "Today's date and time is " ++ httpd_util:rfc1123_date() ++ ".\n".
"Today's date and time is Fri, 20 Aug 2004 22:03:13 GMT.\n"

Although if we only construct this string to then immediately pass it off to a port, we can more efficiently: 

2> BBBB = ["Today's date and time is ", httpd_util:rfc1123_date(), ".\n"].
["Today's date and time is ", "Fri, 20 Aug 2004 22:03:13 GMT", ".\n"]

-- see the Efficiency Guide for more information about this.

Other cases require that you use the generic io_lib formatting functions to create strings with the data you want. For example, we could achive the same result as follows: 

3> io:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 2004]).
Today's date is Fri, Aug 20 2004.
ok
4> io_lib:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 
2004]). [84, 111, 100, 97, 121, 39, 115, 32, 100,    % abbreviated output.
97, 116, 101, 32, 105, 115, 32, "Fri", 44,
32, "Aug", 32, "20", 32, "2004", 46, 10]
5> lists:flatten(v(4)).
"Today's date is Fri, Aug 20 2004.\n"

Finally, this unoptimized module offers more traditional string interpolation: 

-module(template).
-export([subst/2]).

subst(Fmt,[]) -> lists:flatten(Fmt);
subst(Fmt,[{K,V}|T]) ->
    {ok,Fmt1,_} = regexp:gsub(Fmt,"\\$" ++ atom_to_list(K),[V]),
    subst(Fmt1,T).

6> Vars = [{patient,"Mary Becket"}, {diagnosis,"You are an octopus."}],
6> template:subst("E-bloodwork autodoctor: Dear $patient, today's checkup 
indicated a problem: $diagnosis.\n", Vars). "E-bloodwork autodoctor: Dear 
Mary Becket, today's checkup indicated a problem: You are an octopus.\n"
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