Try test' = square . (4 :: a -> (Integer,a)) Otherwise, how is the compiler to know that you want 4 to be of that type? S | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Greg | Buchholz | Sent: 26 October 2006 18:46 | To: haskell-cafe@haskell.org | Subject: [Haskell-cafe] function types as instances of Num | | | Let's say we've got a little stack language, where you compute | things by transformations of stacks, using compositions of functions | from stacks to stacks (represented here as nested tuples). (See also | Chris Okasaki's "Techniques for Embedding Postfix Languages in Haskell" | www.eecs.harvard.edu/~nr/ cs252r/archive/chris-okasaki/hw02.ps ) | | For example, the simple program below calculates the square of 4... | | > {-# OPTIONS -fglasgow-exts #-} | > | > main = print $ test () | > | > test = square . (lit 4) | > | > lit :: Integer -> a -> (Integer,a) | > lit val stack = (val, stack) | > | > dup (a, b) = (a, (a, b)) | > mult (a, (b, c)) = (b*a, c) | > square = mult . dup | | ...now let's say I find that using the function "lit" to annotation | numeric literals ugly. What I really want is something like... | | > test' = square . 4 | | ...Seems simple enough, I'll just make an appropriate instance of Num | and I'll be able to use fromInteger... | | > instance Eq (a -> (Integer, a)) | > instance Show (a -> (Integer, a)) | > instance Num (a -> (Integer, a)) where | > fromInteger = lit | | ...but now when I try it, GHC complains... | | No instance for (Num (a -> (Integer, t))) | arising from the literal `4' at final.hs:15:17 | Possible fix: | add an instance declaration for (Num (a -> (Integer, t))) | In the second argument of `(.)', namely `4' | In the expression: square . 4 | In the definition of `test'': test' = square . 4 | | ...so it seems that (a -> (Integer, t)) can't be unified with (a -> | (Integer, a)), or somesuch. Any thoughts on how to get this to work? | | | Thanks, | | Greg Buchholz | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe