On Oct 9, 2010, at 4:17 PM, André Batista Martins wrote:
If i want compose f and f1, i need to do a correct input to f1 from the output of f. So i want one function to convert the output of "f" to input off "f!". In this case, we do f1 fst (snd (t,(t1,t2))) snd (snd (t, (t1,t2))) But i want do this automaticaly, for type of any two function. I search for the "glue".
You have to write the glue. There is no way to do it automatically, for all cases. If there was a way to derive the glue, they wouldn't be separate cases. Type classes are a common approach for writing glue functions. The Functor and Monad type classes are good examples. (And there is a nice algebra of functors in the small category of functors, too) I wrote a weird little functorial type class yesterday, specifically to handle newtypes as functors over their underlying datatype/category. class Transform obj functor where transform :: (obj -> obj) -> functor -> functor instance Functor f => Transform obj (f obj) where transform = fmap