[Haskell-cafe] Parsec float

[wren ng thornton]

Jason Dusek wrote:
> 2009/05/30 Bartosz Wójcik <bartek at sudety.it>:
>> ...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


Category theoretically, all the following are (or should be!) equal: 
fmap, (<$>), liftA, liftM.

Type theoretically, they differ in whether they require Functor, 
Applicative, or Monad. Unfortunately there's a clash between the current 
types and their CT backing. That is, Monad doesn't require Applicative 
(or Functor), so people will often use liftM to avoid extra type 
constraints.

Operationally, fmap and (<$>) are potentially more efficient. The liftA 
and liftM functions re-engineer fmap by using pure/(<*>) or return/ap, 
thanks to CT. The (<$>) function is just an alias for fmap. But the fmap 
function is part of a type class and so it may have a specific 
implementation which is more efficient than the generic one provided by CT.

-- 
Live well,
~wren