Avoid unnecessary evaluations - comp.lang.haskell

Допис з розмови Avoid unnecessary evaluations

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From: Frank Poettgen <Frank.Poett...@Post.RxWyTxH-AyAzCyHzEyN.de>
Newsgroups: comp.lang.haskell
Subject: Re: Avoid unnecessary evaluations
Date: Thu, 28 May 2009 17:01:17 +0200
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Florian Kreidler wrote:

>> f :: Int -> Int
>> f     x  = fst (expensive x) + snd (expensive x)
>>
>> Will 'expensive x' be evaluated twice to compute 'f x'?
> 
> That will depend on your compiler. Some compilers do common
> subexpression elimination, others don't. The reason is that
> this transformation can sometimes introduce space leaks. In
> your case it wouldn't be dangerous, but you cannot rely on
> your compiler recognizing this.
> 
>> If so, how can I avoid it?
> 
> Rewrite the function by hand:
> 
>   f x = fst e + snd e where e = expensive
> 
> or, using pattern matching,
> 
>   f x = l + r where (l, r) = expensive x
> 
> or, more compact but less comprehensible,
> 
>   f = uncurry (+) . expensive

Thanks a lot!