On Fri, Feb 22, 2013 at 7:27 PM, xiao Ling wrote:

h :: M Int -> M Int -> M Int h x y = bind ( \x-> g x y ) x

where g is

g :: Int -> W Int -> W Int g x y = y >>= (return . (+x))

for the monad:

data M a = M a deriving Show

Now I am a little confused, how can you put x in g if it takes an Int as first parameter but x is M Int?

Because it's a different "x". Lemme rewrite it slightly: h :: M Int -> M Int -> M Int h x y = bind ( \w -> g w y ) x All I did was replace the inner "x" with "w", to demonstrate that it has no relationship to the outer "x"; the \... -> syntax introduces new local bindings unrelated to any outside of it, in this case for "w" (or what he had "x", shadowing the original binding of "x" within the lambda). -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

On Sat, Feb 23, 2013 at 7:27 AM, xiao Ling wrote:

How do you define a function of signature h :: M Int -> M Int -> M Int so that h ( M x ) ( M y ) = M ( x + y ), but without unwrapping the value from the monad?

In addition to the fine points that Brent and Brandon have already made, I observe that there seems to be a reservation about "unwrapping the value from the monad", which I don't get.

Your code is equivalent to h = \mx my -> do { x <- mx; y <- my; return $ x+y; }, which, I suspect, doesn't go well with you because of the "unwrapping". Or are you aiming at syntactic compositionality, i.e. point-free style? Once you've hacked Haskell enough, you just reach for the liftM2 combinator and write h = liftM2 (+). -- Kim-Ee