On Tue, 2007-03-13 at 00:47 -0400, Albert Y. C. Lai wrote: > Hans van Thiel wrote: > > sequence :: Monad m => [m a] -> m [a] > > > > You write: > > The >>= used by sequence is the same >>= in the MyState monad, > > since you instantiate m to MyState String. Therefore, sequence > > performs all the state transformations correctly, since >>= is > > correct. > > > > So the m becomes MyState String, and therefore the list elements > > have type (MyState String Int), or the other way around. I > > understand, from your explanation, how this works from there on, > > but I'm still confused about what the Monad is. Is it MyState > > which takes two types, or (MyState String) which takes one type? > > Or neither? Does it involve some 'sort of currying' of type > > parameters? > > > > The monad is (MyState String) or generally (MyState a) which takes one > type. So this is some sort of currying of type parameters. > > With this in mind, the rest is straightforward: > > > data MyState a b = MyStateC ([a] -> ([a], b)) > > > > This defines an algebraic data type (...why is it called algebraic?) > > with two type variables and a unary constructor. > > > > instance Monad (MyState a) where > > return x = MyStateC (\tb -> (tb, x)) > > (MyStateC st) >>= f = > > MyStateC (\tb -> let > > (newtb, y) = st tb > > (MyStateC trans) = f y > > in trans newtb ) > > > > Now, if the instantiated a has type String, Int or whatever, I > > would understand, but the type of the Monad seems to be built up > > from two types. > > > The general type of return is > > return :: (Monad m) => b -> m b > > Tailor-made to our case, m = MyState a, > > return :: b -> MyState a b > > Similarly, the general type of >>= is > > (>>=) :: (Monad m) => m b -> (b -> m c) -> m c > > Tailor-made to our case, m = MyState a, > > (>>=) :: MyState a b -> (b -> MyState a c) -> MyState a c > > If we now consider > > foo >>= toMyState > > then a=String, b=String, c=Int. Note again that the part (MyState a), or > now (MyState String), is the monad.
Got it! Many thanks. > > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
