On Tuesday 12 December 2006 08:57, Nicola Paolucci wrote: > - How do I know - or how does the interpreter know - that the "m" of > this example is an instance of type ((->) e) ? > - Is it always like that for liftM2 ? Or is it like that only because > I used the function (-) ?
It's the snd that forces the interpreter to infer the ((->) e) monad. You can guess from the type of liftM2 that the (-) won't supply any more information/constraints about m because m is is only mentioned in the snd and fst parts. If you use different monadic values, instead of snd and fst, then the m will end up constrained to a different monad Try these commands in GHCi to see what happens if you use something in the Maybe monad: Prelude> :m + Control.Monad Prelude Control.Monad> :t liftM2 liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r Prelude Control.Monad> :t liftM2 (-) liftM2 (-) :: (Num a1, Monad m) => m a1 -> m a1 -> m a1 Prelude Control.Monad> :t liftM2 (-) (Just 5) liftM2 (-) (Just 5) :: (Num a1) => Maybe a1 -> Maybe a1 Prelude Control.Monad> :t liftM2 (-) (Just 5) Nothing liftM2 (-) (Just 5) Nothing :: (Num a1) => Maybe a1 Prelude Control.Monad> liftM2 (-) (Just 5) Nothing Nothing Daniel _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
