[Haskell-cafe] (+) on two lists ?

[David McBride]

sum' [] = []  -- returns a list of something the way you intended
sum' (x:xs) = x + xum' xs -- you intended it not to return a list but it
could if you think about it.

The compiler says I think returns a list based on what I see so far, well
if you can add these together then the only way you could get a list from
that is if you were adding two lists together ie (+ l1 l2) :: [a] -> [a] ->
[a].  That works if we assume that sum' must have accepted [[a]] and
returned [a].

But in order for that to be the case [a] must be an instance of Num,
otherwise they couldn't be added together like that, so tack on a Num [a]
requirement on.

But having a typeclass of the form [a] that requires an extension,
FlexibleContexts, which you can read about here:
http://www.haskell.org/ghc/docs/7.0.2/html/users_guide/other-type-extensions.html#flexible-contexts

On Fri, Feb 15, 2013 at 2:33 AM, sheng chen <kkchensheng at gmail.com> wrote:

> Hi,
>
> I was puzzled by the following little program.
>
> sum' [] = []
> sum' (x:xs) = x + sum' xs
>
> I thought the GHC type checker will report a type error. However, the type
> checker accepts this program and gives the type
>
> Num [a] => [[a]] -> [a]
>
> When I add type annotation to the program
>
> sum' :: Num [a] => [[a]] -> [a]
> sum' [] = []
> sum' (x:xs) = x + sum' xs
>
> The GHC asks me to add FlexibleContexts language extension.
>
> I would appreciate explanation on this issue.
>
> Thanks,
> Sheng
>
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