[Haskell-cafe] Problem with type signature in local function

[Arthur Baars]

You could also do it with the current implementation of scoped type 
variables. So you don't need to compile the CVS HEAD version. Note that 
you need the pattern (n1::a) and/or (n2::a) to introduce the scoped 
type variable, which was not necessary in Ralf's solution.

{-# OPTIONS -fglasgow-exts #-}
add :: Num a => a -> a -> a
add (n1::a) n2 = addToN1 n2
    where addToN1 :: a -> a
          addToN1 number = n1 + number

Cheers,

Arthur
On 24-dec-04, at 13:04, Ralf Laemmel wrote:

> Timely question. :-)
>
> GHC supports lexically scoped type variables.
> And this support has just been _extended_ to be readily useful for you
> (see discussion on the ghc list just a few days back).
> With GHC CVS HEAD the following program works fine.
> (Note that I removed the inner Num constraint.)
>
> {-# OPTIONS -fglasgow-exts #-}
> add :: Num a => a -> a -> a
> add n1 n2 = addToN1 n2
>   where addToN1 :: a -> a
>         addToN1 number = n1 + number
>
> ... even though I would not claim that this is a really strong example
> of lexically scoped variables, but you probably just simplied a real 
> code example.
> And most of my recent code I use lexically scoped type variables
> extensively. They are great.
>
> (In principle, one does not need them at the cost of encoding. Using 
> asTypeOf and friends ...)
>
> Merry Xmas,
> Ralf
>
>
> Christian Hofer wrote:
>
>> Hi,
>>
>> in order to understand the reason for strange compiler error 
>> messages, I often find it convenient to add type signatures to every 
>> function. But sometimes this seems to be impossible when using type 
>> variables.
>>
>> I try to give a very easy example:
>>
>> add :: Num a => a -> a -> a
>> add n1 n2 = addToN1 n2
>>    where addToN1 :: Num a => a -> a
>>          addToN1 number = n1 + number
>>
>> ghc says "Inferred type is less polymorphic than expected", and 
>> justly so, because the type variable "a" in addToN1 must of course be 
>> the same as in add. Is there a way to specify this?
>>
>> Regards,
>> Chris
>>
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>
>
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