So you flipped the elements of the pair in your definition of return from (x,st) to (st,x). But look at your newtype:s->(a,s) becomes a->(a,s). /Everything else/ stays the same, including the order of your (State s a) parameters. Does that help? Another hint: what do you think the (type!) variables "a" and "s" stand for? -- Kim-Ee On Fri, Dec 7, 2012 at 7:06 AM, Ezequiel Hernan Di Giorgi < hernan.digiorgi@gmail.com> wrote:

this code compiles correcly:

newtype State s a = State { runState :: *s* -> (a,s) }

instance Monad (State s) where --return :: a -> State s a return x = State (\st -> (*x,st*))

but this one doesn't compile:

newtype State s a = State { runState :: *a* -> (a,s) }

instance Monad (State s) where --return :: a -> State s a return x = State (\st -> (*st,x*))

Why is this happening?

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