On Tue, May 13, 2008 at 12:48 PM, Paul Johnson <[EMAIL PROTECTED]> wrote: > > $ pointfree "\xs -> foldl' (+) 0 xs / fromIntegral (length xs)" > > ap ((/) . foldl' (+) 0) (fromIntegral . length) > > But when I try this in GHCi 6.8.2 I get: > > > Prelude Data.List Control.Monad> let mean2 = ap ((/) . foldl' (+) 0) > (fromIntegral . length) > > > > <interactive>:1:12: > > No instance for (Monad ((->) [b])) > > arising from a use of `ap' at <interactive>:1:12-58 > > Possible fix: add an instance declaration for (Monad ((->) [b])) > > In the expression: ap ((/) . foldl' (+) 0) (fromIntegral . length) > > In the definition of `mean2': > > mean2 = ap ((/) . foldl' (+) 0) (fromIntegral . length)
It's using the Monad ((->) r) instance, which doesn't exist by default. import Control.Monad.Instances to get it. Luke _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
