String Interpolation

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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.
 +ok
4> io_lib:fwrite("Today's date is ~s, ~s ~w ~w.\n", ["Fri", "Aug", 20, 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. 2004]). [84, 111, 100, 97, 121, 39, 115, 32, 100, % abbreviated output.

Revision as of 04:54, 31 October 2006

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 "

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 autodiagnosis: Dear 
Mary Becket, today's checkup indicated a problem: You are an octopus.\n"
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