[erlang-questions] Binary match in function head doesn't compile

Erik Pearson <>
Sun Oct 28 19:08:44 CET 2012


Is there a reference for the resolution of patterns in function/clause head
similar to

http://www.erlang.org/doc/apps/erts/match_spec.html

but for regular Erlang? The docs on the abstract format is useful

http://www.erlang.org/doc/apps/erts/absform.html

There are a few places in the docs that refer to the process of matching
the function clause head against arguments

e.g. the function overview

http://www.erlang.org/doc/reference_manual/functions.html#id74558

but it would be really useful to have those references link to
documentation which describes this process.

Thanks,
Erik.

(ps - I'm happy with your answer, Björn-Egil, but hoping this thread can
include some solid leads for others researching similar issues.)

(pps - there is a definitive response to my original post from
Björn-Egil Dahlberg
below -- he inadvertently sent it directly to me)

On Fri, Oct 26, 2012 at 11:59 AM, Björn-Egil Dahlberg <
> wrote:

>
>
> 2012/10/26 Erik Pearson <>
>
>> Hi,
>>
>> I'm wondering why this
>>
>> test(<<Field:Len/binary, Rest/binary>>, Len) ->
>>     {Len, Field, Rest}.
>>
>> does not compile, complaining that "variable 'Len' is unbound", while this
>>
>> test(<<Field:2/binary, Rest/binary>>, Field) ->
>>     {Field, Rest}.
>>
>> does. For some reason the compiler doesn't see the Len from the match
>> spec in the arguments, but it does see Field. Is that by design?
>>
>
> Yes and a limitation that is being adressed.
>
>
>
>> BTW  supplying a variable for Len does work in this case:
>>
>> test(Bin, Len) ->
>>   <<Field:Len/binary, Rest/binary>> = Bin,
>>    {Len, Field, Rest}.
>>
>
> The difference here is that Len is bound when entering the function body
> as opposed when it is in the function head.
>
> We have had fierce debates on, among other things, matching behaviors for
> Maps (extended frames/hashes) which also have led to redesigning parts how
> binary matching is done in function heads. This is currently in the
> prototyping stages and it is to early to say to which release this will be
> ready.
>
> // Björn-Egil
>
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