[erlang-questions] Wildcard matching a list of strings vs atoms/binaries

Olav Frengstad <>
Wed Apr 17 23:43:32 CEST 2013


Thanks for the explanation Richard, that answers my question.

lists:prefix/2 seems the obvious way then.


2013/4/17 Richard Carlsson <>

> On 2013-04-17 22:26, Olav Frengstad wrote:
>
>> Hey,
>>
>> I'm getting some unexpected behavior when trying to pattern match the
>> head of a list.
>>
>> When matching against a list of integers I can successfully do a
>> wildcard match:
>> 27> ("ab ++ _)  = "abcd".
>>
>> But when trying to do the same with a list of atoms i get the error: *
>> 1: illegal pattern:
>> 28>  ([a,b,c] ++ _) = [a,b,c,d].
>> * 1: illegal pattern
>>
>> So just making sure there's no magic in string matching i also try with
>> an actual list of integers:
>> 29> ([0,1,2] ++ _) = [0,1,2,3,4].
>> [0,1,2,3,4]
>>
>> Why can't I pattern match a lists with other values integers?
>>
>
> The Erlang Reference Manual (http://www.erlang.org/doc/**
> reference_manual/expressions.**html#id76648<http://www.erlang.org/doc/reference_manual/expressions.html#id76648>)
> says the following:
>
>   When matching strings, the following is a valid pattern:
>
>     f("prefix" ++ Str) -> ...
>
>   This is syntactic sugar for the equivalent, but harder to read
>
>     f([$p,$r,$e,$f,$i,$x | Str]) -> ...
>
> Since strings are really just lists of integers, and the double-quoted
> string syntax has been expanded to actual lists by the time the compiler
> gets the code, it actually works on lists of any integers, as in your
> example:
>
>
>   ([0,1,2] ++ _) = [0,1,2,3,4].
>
> There's no particular reason that this syntax couldn't be allowed for
> lists of arbitrary elements, but apparently, that hasn't been done. In
> fact, it seems that the compiler (in the module sys_pre_expand) doesn't
> care about what the elements are, but the erl_lint module that checks the
> code after parsing explicitly disallows this syntax for other things than
> strings/lists of integers.
>
> For now, you'll just have to use the normal list pattern syntax for
> matching prefixes:
>
>    [a,b,c | _] = [a,b,c,d].
>
> In any case, the (Prefix++Tail) pattern can only be used when Prefix is a
> fixed-length list literal (so it can be rewritten to the last form at
> compile time). Prefix can never be a variable.
>
>     /Richard
>
>
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-- 
Med Vennlig Hilsen
Olav Frengstad

Systemutvikler // FWT
+47 920 42 090
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