Re: Proposal: Strict scanl, scanl1 and mapAccumL

On Mon, 12 Nov 2012, 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

Yes, this looks nice!

force :: WHNFData a => a -> a force x = x `seq` x

Does this function do something?

-- Doesn't type check unfortunately -- since the b in seq :: a -> b -> b is of kind * and not #: -- evaluate :: WHNFData a => a -> IO a -- evaluate x = IO (\s -> x `seq` (# s, x #))

Maybe it can be implemented in terms of the existing 'evaluate' function but without applying 'Prelude.seq' to the 'a' typed value?