Proposal: Strict scanl, scanl1 and mapAccumL

[Henning Thielemann]

On Mon, 12 Nov 2012, Bas van Dijk wrote:

> On 12 November 2012 13:25, Henning Thielemann <lemming at henning-thielemann.de> 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?