Re: Proposal: Strict scanl, scanl1 and mapAccumL

On Mon, Nov 12, 2012 at 2:12 PM, Bas van Dijk wrote:

On 12 November 2012 13:25, Henning Thielemann wrote:

...

I don't think of changing it. We could provide a package that exports "the right 'seq'" and then encourage people to use this instead of Prelude.seq.

I think I like this idea. So the package would export something like:

module Control.Seq where

import Prelude hiding (seq) import GHC.Base hiding (seq)

class WHNFData a where rwhnf :: a -> ()

instance WHNFData [a] where rwhnf [] = () rwhnf (_:_) = ()

-- and all the others...

seq :: WHNFData a => a -> b -> b seq a b = case rwhnf a of () -> b

($!) :: WHNFData a => (a -> b) -> a -> b f $! x = x `seq` f x

For reference, older versions of Haskell defined the following class:

class Eval a where strict :: (a -> b) -> a -> b seq :: a -> b -> b strict f x = x `seq` f x The function `strict` is quite useful for making strict versions of lazy functions. Each type was automagically an instance of this class (with the notable exception of functions, there was no way to force function values). Cheers, Josef