[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


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


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


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


(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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