p :: (forall o. M o -> M o) -> ...
Отправлено с iPad
19.08.2011, в 16:06, Anupam Jain
Hi all,
Suppose I have a compound data type -
data M o = M (String,o)
Now, I can define a function that works for ALL M irrespective of o. For example -
f :: M o -> M o f (M (s,o)) = M (s++"!", o)
I can also use this function in an expression, applying it to different types without problem -
p = (m1',m2') where m1 = M ("1", ()) m2 = M ("2", True) m1' = f m1 m2' = f m2
Main*> p (M ("1!",()),M ("2!",True))
However, if I try to parameterise over the function 'f' it does not work! -
p f = (m1',m2') where m1 = M ("1", ()) m2 = M ("2", True) m1' = f m1 m2' = f m2
It doesn't even typecheck, producing the error - "Couldn't match expected type 'Bool' with actual type '()'"
Is there a particular reason for this? How can I define a function like 'p' within Haskell?
Thanks, Anupam Jain _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe