[Haskell-cafe] Re: about Haskell code written to be "too smart"

Ryan Ingram ryani.spam at gmail.com
Wed Mar 25 18:41:10 EDT 2009

On Wed, Mar 25, 2009 at 8:25 AM, Jonathan Cast
<jonathanccast at fastmail.fm> wrote:
> On Wed, 2009-03-25 at 15:09 +0000, Simon Marlow wrote:
>> the ordering that the state monad expects
>> (and I can never remember which way around they are in Control.Monad.State).
> Really?  I found it obvious once I figured out it how simple it made
> (>>=).  With the order from Control.Monad.State (with constructors
> ignored):
>    a >>= f = \ s -> case s a of
>       (x, s') -> f x s'
> Reversing the order of the components of the result gives you
>    a >>= f = \ s -> case s a of
>        (s', x) -> f x s'
> which just looks weird.

However, if you are used to thinking in terms of type composition, s
-> (s, a) makes more sense, because it is effectively

  (s ->) . (s,) . Identity

whose "functor-ness" is automatic via composition of functors:

newtype Identity a = Identity a
inIdentity f (Identity a) = Identity (f a)

instance Functor Identity where
    fmap f = inIdentity f

instance Functor ((,) a) where
    fmap f (a, x) = (a, f x)
instance Functor ((->) a) where
    fmap f k a = f (k a)

newtype O f g x = O (f (g x))
inO f (O a) = O (f a)
instance (Functor f, Functor g) => Functor (O f g) where
    fmap f = inO (fmap (fmap f))
    -- or fmap = inO . fmap . fmap

-- not valid haskell, but if there were sections at the type level it would be.
type State s = (s ->) `O` (s,) `O` Identity

  -- ryan

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