[erlang-questions] Pattern matching in function calls

[Robert Raschke]

That code makes me wonder if it could be rewritten using gen_fsm. Never used
gen_fsm myself, so I'm not really sure.

Robby

On Thu, Apr 14, 2011 at 11:48 AM, Dave Challis <dsc@REDACTED> wrote:

> Hi Robby,
> Yup, I agree completely :)
>
> My question was mostly prompted by a blog post (
> http://ppolv.wordpress.com/2008/02/25/parsing-csv-in-erlang/) on parsing
> CSV in erlang.
>
> The do_parse function there has a dozen items which search binaries and
> check state, which made me wonder whether swapping the argument order round
> would make any difference at all.
>
> Cheers,
> Dave
>
>
> On 13/04/11 19:44, Robert Raschke wrote:
>
>> 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
>>> _______________________________________________
>>> erlang-questions mailing list
>>> erlang-questions@REDACTED
>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>
>>>
>
> --
> Dave Challis
> dsc@REDACTED
>
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