My two favorite functions in Haskell are "unsafeInterleaveIO" and
"unsafeInterleaveST". I'm surprised there isn't a type class for
them. I just fixed this by adding the "monad-interleave" package to
hackage:
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monad-interleave
The package adds a "Control.Monad.Interleave" module. Here is the
entirety of the module:
\begin{code}
-- | Monads that have an operation like 'unsafeInterleaveIO'.
class Monad m => MonadInterleave m where
-- | Get the baton from the monad without doing any computation.
unsafeInterleave :: m a -> m a
instance MonadInterleave IO where
unsafeInterleave = unsafeInterleaveIO
{-# INLINE unsafeInterleave #-}
instance MonadInterleave (ST s) where
unsafeInterleave = unsafeInterleaveST
{-# INLINE unsafeInterleave #-}
instance MonadInterleave (Lazy.ST s) where
unsafeInterleave = Lazy.unsafeInterleaveST
{-# INLINE unsafeInterleave #-}
\end{code}
Use it if you need it.
Patrick
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