On 4 May 2007, at 08:43, Adrian Neumann wrote:
However I
don't understand the type signatures for bind and fmap. I'd say (and
ghci's type inference agrees) that bind and fmap have the type
bind:: (a->W b) -> W a -> W b
fmap:: (a->b) -> W a -> W b
They take a function f and something and return what f does to that. I
don't see why they should return a function.
I suggest you look up currying. In the mean time, I shall attempt an
explanation.
In Haskell (as in many other functional languages), function types
appear as if the function were curried. This means that a function
accepts one single argument, and returns one single result. The
important thing to realise though is that the result (or less
importantly the argument) may be a function.
Let's study a (slightly over constrained) variant on the (+)
function, with type (+) :: Int -> Int -> Int. This type signature
should be read to mean:
(+) takes an Int, and returns a new function of type (Int -> Int).
We can see an example of this (+ 5) -- (+) is given an argument (5),
and returns a function (that adds 5 to integers).
Another way to think about it is that the (->) type constructor is
right associative, so any type written as a -> b -> c -> d -> e, can
also be written as a -> (b -> (c -> (d -> e))).
I hope that helped a bit, and if it didn't I suggest going and
looking up currying in as many places as you can.
Bob
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