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
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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
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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>