On Friday 17 December 2010 18:04:20, 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
The fmap is relative to IO, your code is equivalent to do let lst = (return [1,2,3,4,5]) fmap (+1) lst ~> fmap (+1) (return [1,2,3,4,5]) ~> do lst <- return [1,2,3,4,5] return $ (+1) lst but there's no instance Num [Int] in scope You probably meant do let lst = f [1,2,3,4,5] fmap (map (+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