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

[Ivan Lazar Miljenovic]

On 15 February 2013 19:22, Raphael Gaschignard <dasuraga at gmail.com> wrote:
> Out of curiosity, what is the rationale for allowing programs to infer
> certain types, but not for the (inferred) type to be declared?

That's the type that's needed; the fact that you need an extension for
GHC to allow you to use it is irrelevant.

>
>
>
> 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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>
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com