[erlang-questions] View patterns

Ulf Wiger (TN/EAB) <>
Thu Jul 26 10:04:37 CEST 2007


Hmph! Some parentheses were also missing in
my transcription. Luckily, I received the Erlang
book yesterday, so I can read up how to do basic
Erlang programming...

I've attached my parse transform. Compiling with
'P' gives a good idea of what transformation is 
done. It only handles patterns in function clauses,
and doesn't attempt to reduce repeated work.
It's just intended as a starting point for some
experimentation.

== Example

-module(test)
-compile({parse_transform, vp_expand}).
-export([factorial/1]).


np(K, X) when integer(X), integer(K), X >= K ->
    {X-K};
np(_, _) ->
    false.

factorial(0) -> 1;
factorial(_:_(np,[1], {N})) ->
    (N+1)*factorial(N).

==

The nice thing about the _:_(...) pattern
is that it's syntactically valid, but wholly
illegal in all contexts. If you forget to do
the parse transform, the code will not compile. (:

BR,
Ulf W


> -----Original Message-----
> From:  
> [mailto:] On Behalf Of ok
> Sent: den 26 juli 2007 05:13
> To: Erlang Questions
> Subject: Re: [erlang-questions] View patterns
> 
> I made a mistake.
> 
>      np(K, X) when integer(X), integer(K), X >= K ->
>          {X};              %%%% HERE
>      np(_, _) ->
>          false.
> 
> should of course be
> 
>      np(K, X) when integer(X), integer(K), X >= K ->
>          {X - K};
>      np(_, _) ->
>          false.
> 
> Fix that and all is well.
> 
> The real trick with view patterns is to compile them so that 
> you do not do too much repeated work.
> 
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> erlang-questions mailing list
> 
> http://www.erlang.org/mailman/listinfo/erlang-questions
> 
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