[Haskell-cafe] Re: Request to review my attempt at understanding
uzytkownik2 at gmail.com
Tue Dec 29 06:41:37 EST 2009
On Tue, 2009-12-29 at 02:07 -0800, CK Kashyap wrote:
> Thanks Jason,
> > You should make a `Functor' instance since monads are all
> > functors (though the typeclass does not enforce this).
> What are the benefits of making it an instance of Functor?
1. For example to use function of type Functor f => f a -> f d.
2. Also you need Functor to have Applicative which is rather useful (f <
$> arg1 <*> arg2 <*> arg3 <*> ... as opposed to return f `ap` arg1 `ap`
arg2 `ap` arg3 ..., (*>), (<*) etc.)
3. Because it is functor ;). Every Monad is functor:
instance Functor MyMonad where
fmap = liftM
instance Applicative MyMonad where
pure = return
(<*>) = ap
4. If you use Control.Applicative you can find:
read <$> getLine
I find it much more readable then liftM read getLine (it looks nearly
like read $ getLine).
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