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

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