[Haskell-cafe] [Q] A typing problem

[Didier Verna]

        Hi !

I'm trying to write a function that combines folding and mapping. This
function would take two arguments (a folding function and a mapping function),
and would return a function of one argument (a list of 'a''s) returning a 'a'.

The idea is to write something like sumsquare = foldmap (+) square

Here's what I write:

foldmap :: (c -> c -> c) -> (c -> c) -> (c -> c -> c)
foldmap f m = foldr1 f (map m)

But this gives a typing error:

ERROR "src/ssq4.hs":2 - Type error in application
*** Expression     : foldr1 f (map m)
*** Term           : map m
*** Type           : [b] -> [b]
*** Does not match : [a]


What can I do to get what I want ?

Thanks.

--
Didier Verna, didier at lrde.epita.fr, http://www.lrde.epita.fr/~didier

EPITA / LRDE, 14-16 rue Voltaire   Tel.+33 (1) 44 08 01 85
94276 Le Kremlin-Bicêtre, France   Fax.+33 (1) 53 14 59 22   didier at xemacs.org