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

[Raphael Gaschignard]

Out of curiosity, what is the rationale for allowing programs to infer
certain types, but not for the (inferred) type to be declared?



On Fri, Feb 15, 2013 at 4:54 PM, David McBride <toad3k at gmail.com> wrote:

> 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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>
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