On Fri, Dec 17, 2010 at 9:04 AM, michael rice
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
f takes [Int] and returns IO [Int] fmap is fmap :: (Functor f) => (a -> b) -> f a -> f b That is it takes a function of a's to b's, a functor of a, and returns you a functor of b. So when you fmap (+1) to an IO [Int], it's trying to add 1 to a [Int], and [Int] is not an instance of Num, so the + does not work. Luckily you can use function composition here (fmap . fmap) (+1) $ f [1..10] [2,3,4,5,6,7,8,9,10,11] fmap . fmap is the type I think you wanted: Prelude> :t fmap . fmap fmap . fmap :: (Functor f, Functor f1) => (a -> b) -> f (f1 a) -> f (f1 b) With IO as the f Functor, and [] as the f1 Functor.
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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>
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