[Haskell-cafe] Strict version of Data.Map.map

[Edsko de Vries]

> I guess so. Maybe using mapAccum helps:
>
>  import qualified Data.Map as M
>
>  strictMap :: (a -> b) -> M.Map k a -> M.Map k b
>  strictMap f m = case M.mapAccum f' () m of
>                    ((), m') -> m'
>      where f' () x = x' `seq` ((), x') where x' = f x
>
>  testStrictness mapper = m `seq` "Not strict."
>      where m = mapper (const e) (M.singleton () ())
>            e = error "Strict!" :: ()

Very clever. I had tried to use mapAccum but I couldn't figure out  
what to put in the accumulator. I didn't realize it didn't make a  
difference (unit will do) as long as it is evaluated when the Map is.

Seq wrecks my head ;)

Thanks!

Edsko