On Fri, Dec 17, 2010 at 9:04 AM, michael rice
<nowgate@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]
Prelude>
Michael
===================
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]
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 )
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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