Re: [Haskell-cafe] lengthOP rewrite rules

2008/12/18 Luke Palmer :

On Thu, Dec 18, 2008 at 1:53 AM, Cetin Sert wrote:

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Hi,

I tested the following, why does the rewrite rules not fire when using tuples also in testRewrite2, testRewriteReverse2?

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testRewrite2 :: a → (Bool,Bool) testRewrite2 x = (pf,pi) where f = replicate 2000 x i = repeat x lf = length f li = length i pf = lf > 300 pi = li > 300

Why would you expect it to? The compiler is free to inline lf and li to discover that the rule applies, but it is also free not to. Applying all applicable rules while maintaining the ability to abstract is undecidable (big surprise). Thus the dependency on compiler cleverness I mentioned...

I'm agreeing with Luke here. It's possible that the compiler decided to inline f and i, and length, and determined that lf == 2000 and li == _|_ Or it could have decided not to inline at all. Or some other possibility. If you specify {-# INLINE lf #-}, do the results change? I suspect they might. -- ryan