[Haskell-cafe] benchmarking pure code

[Bas van Dijk]

On Wed, Mar 31, 2010 at 11:06 AM, Paul Brauner <paul.brauner at loria.fr> wrote:
> data Term = Lam Term | App Term Term | Var Int
>
> instance NFData where
>  rnf (Lam t)     = rnf t
>  rnf (App t1 t2) = rnf t1 `seq` rnf t2
>  rnf (Var x)     = rnf x
>
> the actual datatype doesn't have fancy stuff like higher-order
> types for constructors, it's really similar. The only difference
> is that it is a GADT, but this souldn't change anything right?
>
> Did I make some mistake in instancing NFData ?

No, your NFData instance is correct. You first pattern match on the
term followed by recursively calling rnf on the sub-terms. This will
correctly force the entire term.

Maybe you could try using criterion[1] for your benchmark and see if
that makes any difference. Something like:

{-# LANGUAGE BangPatterns #-}

import Criterion.Main

main :: IO ()
main = let !t = genterm in defaultMain [bench "subst" $ nf (subst u) t]

regards,

Bas

[1] http://hackage.haskell.org/package/criterion