Hi Michael The type of lst is "IO [Int]" and therefore "fmap (+1)" applies (+1) to the hole lists of integers, and not to each member of the list. That is:
fmap (+1) lst <=> fmap (+1) (return [1,2,3,4,5]) <=> return ([1,2,3,4,5] + 1) and you cannot say [1,2,3,4,5] + 1. Does that make sense? Maybe you want to say: main = do let lst = [1,2,3,4,5] print $ map (+1) lst /Mads On Fri, 2010-12-17 at 09:04 -0800, michael rice wrote: > I don't understand this error message. Haskell appears not to > understand that 1 is a Num. > > Prelude> :t 1 > 1 :: (Num t) => t > Prelude> :t [1,2,3,4,5] > [1,2,3,4,5] :: (Num t) => [t] > Prelude> > > Michael > > =================== > > f :: [Int] -> IO [Int] > f lst = do return lst > > main = do let lst = f [1,2,3,4,5] > fmap (+1) lst > > =============================== > > Prelude> :l test > [1 of 1] Compiling Main ( test.hs, interpreted ) > > test.hs:5:17: > No instance for (Num [Int]) > arising from the literal `1' at test.hs:5:17 > Possible fix: add an instance declaration for (Num [Int]) > In the second argument of `(+)', namely `1' > In the first argument of `fmap', namely `(+ 1)' > In the expression: fmap (+ 1) lst > Failed, modules loaded: none. > Prelude> > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe