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: Avoid unnecessary evaluations
Date: Thu, 28 May 2009 15:36:12 +0200
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Assume that the evaluation of a function 'expensive' takes a lot of 
time. Hence I want to avoid unnecessary evaluations of 'expensive'.

expensive :: Int -> (Int,Int)
expensive     x  =  (x,x)

f :: Int -> Int
f     x  = fst (expensive x) + snd (expensive x)

Will 'expensive x' be evaluated twice to compute 'f x'?
If so, how can I avoid it?