[Haskell] Re: Implicit type of numeric constants

[Robert Stroud]

On 21 Sep 2006, at 10:46, Robert Stroud wrote:

>> So k gets a monotype which is determined by its usage, you cannot  
>> do e.g.
>>
>>   let k = 2 ; f :: Int -> Int -> Int ; f x y = x * y in (f k k, 1/k)
>>
>> whereas   let k :: Num a => a; k = 2; ...   is possible.
>
> Thanks - that's a helpful example. But why is the following not  
> equivalent to the untyped "k = 2" case:
>
> let f :: Int -> Int -> Int ; f x y = x * y in (f 2 2, 1/2)
>
> Does the type of 2 effectively get decided twice, once as an Int,  
> and once as a Fractional, and is this the "repeated computation"  
> that the monomorphism restriction is intended to prevent?
>
> Otherwise, I would have expected that it wouldn't make any  
> difference whether I used a named 2 or an anonymous 2, but imposing  
> the monomorphism restriction on the named 2 seems to break  
> referential transparency.

Aha - light begins to dawn... :-)

Each 2 is a different 2, so the type of 2 can be different each time  
it's used, whereas there's only one k so it can only have one type.

Is that right?

Robert