[erlang-questions] Pattern matching in function calls

[Dave Challis]

On 13/04/11 13:46, Antoine Koener wrote:
>
> On Apr 13, 2011, at 13:44 , Dave Challis wrote:
>
>> Does the order of patterns in a function call matter? Are they all
>> tested, or will erlang stop trying to match them once a mismatch has
>> been found?
>>
>> As an example, if I've got:
>>
>> foo(<<SomeLargeBinary>>, a) -> a;
>> foo(<<SomeLargeBinary>>, b) -> b.
>>
>> will the above be any slower than defining:
>>
>> foo(a, <<SomeLargeBinary>>) -> a;
>> foo(b, <<SomeLargeBinary>>) -> b.
>>
>> Will the <<SomeLargeBinary>> attempt to be matched in every case? Is
>> it something worth thinking about, or will the compiler optimise this?
>
> If the content of <<SomeLargeBinary>> is irrelevant, you can use the '_'
> notation:
>
> foo(a, _Bin) -> a;
> foo(b, _Bin) -> b;
>
> Then this is explicit for the reader that you don't want to check _Bin.

Sorry, my example wasn't great, I was more wondering about the cases 
when the binary data is relevant.

So a better example might be:

foo(<<$x,_Rest/binary>>, a) -> a1;
foo(<<$y,_Rest/binary>>, a) -> a2;
foo(<<$z,_Rest/binary>>, a) -> a3;
foo(<<$x,_Rest/binary>>, b) -> b.

If I then called:
foo(<<"x">>, b).

Would erlang perform 4 binary pattern matches until it got to the last 
clause?

Or would it optimise things and only perform a single binary match by 
matching on the atoms first?

Or will it always test all arguments in all cases?


-- 
Dave Challis
dsc@REDACTED