On Friday 28 September 2007, David Benbennick wrote:
> On 9/28/07, Ross Paterson <[EMAIL PROTECTED]> wrote:
> > However one can define
> >
> > reversor :: Traversable f => f a -> f a
> >
> > which returns something of the same shape, but with the contents
> > reversed.
>
> How? Is it possible to define a version of foldl for Traversable?
At the very least, you can do this:
{-# LANGUAGE FlexibleContexts #-}
import Prelude hiding (mapM)
import Control.Monad hiding (mapM)
import Control.Monad.State hiding (mapM)
import Data.Foldable (toList)
import Data.Traversable (mapM, Traversable(..))
reversor :: Traversable t => t a -> t a
reversor t = evalState (mapM (const pick) t) (reverse $ toList t)
pick :: MonadState [a] m => m a
pick = do (h:t) <- get ; put t ; return h
There may be something nicer out there, though.
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