Hi All, Hi Cale, Can you tell me if I understood things right ? Please see below ...
On 12/11/06, Cale Gibbard <[EMAIL PROTECTED]> wrote:
The monad instance which is being used here is the instance for ((->) e) -- that is, functions from a fixed type e form a monad. So in this case: liftM2 :: (a1 -> a2 -> r) -> (e -> a1) -> (e -> a2) -> (e -> r)
I bet you can guess what this does just by contemplating the type. (If it's not automatic, then it's good exercise) Now, why does it do that?
So the way I have to reason on the output I get from ghci is: Prelude> :t liftM2 liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r The m stands for ((->) e), that is like writing (e -> a1): a function which will take an argument of type e and will return an argument of type a1. And so the above line has a signature that reads something like: liftM2 will takes 3 arguments: - a function (-) that takes two arguments and returns one result of type r. - a function (fst) that takes one argument and returns one result. - a function (snd) that takes one argument and returns one result. - the result will be a certain function that will return the same type r of the (-) function. - Overall to this liftM2 I will actually pass two values of type a1 and a2 and will get a result of type r.
From the type signature - correct me if I am wrong - I cannot actually
tell that liftM2 will apply (-) to the rest of the expression, I can only make a guess. I mean I know it now that you showed me:
liftM2 f x y = do u <- x v <- y return (f u v)
If this is correct and it all makes sense, my next question is: - How do I know - or how does the interpreter know - that the "m" of this example is an instance of type ((->) e) ? - Is it always like that for liftM2 ? Or is it like that only because I used the function (-) ? I am trying to understand this bit by bit I am sorry if this is either very basic and easy stuff, or if all I wrote is completely wrong and I did not understand anything. :D Feedback welcome. Thanks again, Regards, Nick _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe