[erlang-questions] Dict size in guards

[Raphael Korsoski]

Hi,

To my understanding, Haskell guards are very different from Erlang guards.  Haskell compiles guards to case statements (afaik), while Erlang evaluates them for selection at runtime.

In Haskell we can write:

f x | my_length(x)  > 3 = "longer than 3"
    | otherwise = "shorter than or equal to 3"
    where my_length = λa →  <some function, e.g. length(a)>

which does contain a user-defined non-monadic funtion. 

In GHC it's also possible to do binding within guards, like so:

g x | s ←  my_length(x)
    , s > 3 = "longer than 3"
    | otherwise = "shorter than or equal to 3"
    where my_length = λa →  <some funtion>

Hope that answers something of your question.

BR,
Raphael K  

-----Original Message-----
From: erlang-questions@REDACTED [mailto:erlang-questions@REDACTED] On Behalf Of David Mercer
Sent: den 11 februari 2011 15:35
To: 'Richard O'Keefe'
Cc: 'Erlang Questions'
Subject: RE: [erlang-questions] Dict size in guards

On Thursday, February 10, 2011, Richard O'Keefe wrote:

> Guards cannot and should not call functions.
                ^^^^^^^^^^^^^^

Is the "should not" because of side-effects in Erlang functions?  If Erlang functions were pure, would you remove the "and should not" from your statement?  I realize there may be issues of performance if they are allowed, but that would be up to the programmer, I suppose.  Does Haskell permit user-defined (non-monadic) functions in guards?

Just asking from a language theory point of view, since I know you're an expert in this area.  Thanks, ROK.

Cheers,

DBM


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