[Haskell-cafe] Why is Haskell flagging this?
aslatter at gmail.com
Fri Dec 17 22:12:54 CET 2010
This is a bit tricky.
The type of 'f' is '[Int] -> IO [Int]', which means that the type of 'lst'
is 'IO [Int]'.
So fmap (+1) tries to add one to the [Int] underneath the 'IO' type
You can either use two 'fmap's, the first to lift up to IO and the second to
lift into the list, or you can use monad notation:
> lst <- f [1,2,3,4]
> return $ fmap (+1) lst
Does that make sense?
On Fri, Dec 17, 2010 at 11:04 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
> 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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