On 5 Dec 2008, at 16:54, Ross Paterson wrote:
On Fri, Dec 05, 2008 at 04:35:51PM +0100, Martijn van Steenbergen wrote:
How do I implement the following?
instance Applicative f => Applicative (StateT s f)
The implementation of pure is trivial, but I can't figure out an implementation for <*>. Is it possible at all, or do I need to require f to be a monad?
Yes, because you need part of the value generated by the first computation, namely the state (inside the f), to construct the second one. You can do that in a Monad, but not in an Applicative.
I don't think that's true, although I'm yet to decide if Applicative for State is possible. someState <*> someOtherState should take the value out of the first state, take the value out of the second state, apply one to the other, and return a new stateful value as the result. At no point in that description do I make mention of the previous state of one of these values. Bob