[Haskell-cafe] Why is Haskell flagging this?

[Darrin Chandler]

On Fri, Dec 17, 2010 at 09:04:20AM -0800, michael rice 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' operates on '[Int]', but '(+1)' operates on 'Int'... Does the
following do what you want?

main = do let lst = f [1,2,3,4,5] in
                fmap (map (+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> 
> 
> 
> 
>       

> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe


-- 
You've been warned.