On Fri, Dec 17, 2010 at 11:04 AM, michael rice nowg...@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
===
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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Excuse any inaccuracies, I'm somewhat new at Haskell myself, but what it
looks like is happening is that at the point in main where you've bound
lst, it will have type of IO [Int]. The signature for fmap is:
fmap :: (Functor f) = (a - b) - f a - f b
if you call fmap (+1) the next argument that fmap expects is something
that is in just one functor, for example, this
fmap (+1) [1,2,3,4,5]
works fine, but, something that is IO [Int] won't. You can compose two
'fmap's to solve this:
:t (fmap.fmap)
(fmap.fmap)
:: (Functor f, Functor f1) = (a - b) - f (f1 a) - f (f1 b)
which means that 'main' looks like:
main = do let lst = f [1, 2, 3, 4, 5]
(fmap.fmap) (+1) lst
--
Chris Wilson christopher.j.wil...@gmail.com
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