Re: [Haskell-cafe] Parsec float

On Sat, May 30, 2009 at 1:12 PM, Jason Dusek wrote:

2009/05/30 Bartosz Wójcik :

...

...reading RWH I could not memorize what those liftM funtions meant.

 The basic one, `liftM`, means `fmap`, though specialized for  functors that are monads.

   Prelude Control.Monad> :t liftM    liftM :: forall a b (m :: * -> *). (Monad m) => (a -> b) -> m a -> m b    Prelude Control.Monad> :t fmap    fmap :: forall a b (f :: * -> *). (Functor f) => (a -> b) -> f a -> f b

 I think we have `liftM` either to help the inferencer or due  to the absence of a `(Functor m)` constraint in the definition  of the `Monad` typeclass.

It's the latter effectively. liftM doesn't make anything easier for the type checker. liftM simply has a different type than fmap, not a more specialized one, but even if Monad did have a Functor constraint, liftM would still never lead to any ambiguity being resolved.