[Haskell-cafe] Why is Haskell flagging this?
leimy2k at gmail.com
Fri Dec 17 22:56:37 CET 2010
On Fri, Dec 17, 2010 at 9:04 AM, michael rice <nowgate at yahoo.com> 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]
> 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 :: (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]
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.
> Prelude> :l test
> [1 of 1] Compiling Main ( test.hs, interpreted )
> 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.
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