On Fri, Nov 29, 2013 at 12:03:43PM -0500, Carter Schonwald wrote:
could someone explain to me why this <$!> would be for monads rather being more generally also for functors or applicatives?
It's not clear whether such a thing can be implemented for a functor or applicative. It seemingly needs to exploit the fact that the next action in a bind can depend on the "value returned by" the previous action. Still, the semantics depend very much on the laziness properties of the monad in question. f <$!> m = do a <- m return $! f a data R x = R x data S x = S x data T x = T x instance Monad T where return = T m >>= f = T (case m of T m' -> case f m' of T y -> y) instance Monad S where return = S m >>= f = case m of S m' -> S (case f m' of S y -> y) -- Equivalent implementation -- S m' >>= f = S (case f m' of S y -> y) instance Monad R where return = R m >>= f = case m of R m' -> case f m' of R y -> R y -- Equivalent implementations: -- m >>= f = case m of R m' -> f m' -- R m' >>= f = f m' try :: Monad m => m Int -> () try l = (+1) <$!> l `seq` () *Main> try (undefined :: T Int) () *Main> try (T undefined :: T Int) () *Main> try (undefined :: S Int) *** Exception: Prelude.undefined *Main> try (S undefined :: S Int) () *Main> try (undefined :: R Int) *** Exception: Prelude.undefined *Main> try (R undefined :: R Int) *** Exception: Prelude.undefined Tom