[erlang-questions] Pattern matching in function calls

[Robert Raschke]

Hi Dave,

probably not quite the answer you were looking for, but aim for
readability first (that's usually already hard enough :-). Only
compromise that if something is measurably too slow.

In the example you give, it doesn't appear to make any big difference
for readability, but I assume your real code is a bit more
"interesting"?

Robby


On 4/13/11, Dave Challis <dsc@REDACTED> wrote:
> 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
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