defaults

[Simon Marlow]

haskell-prime-bounces at haskell.org wrote:
> Simon Peyton-Jones <simonpj at microsoft.com> writes:
>
>>> My suggestion is that there should be a single default type
>>> per class.
>>
>> Ah, I missed that.
>
> I suppose, because the proposed new rule is so simple and short, it is
> rather easy to miss its implications, especially if you are used to
> thinking in terms of the existing more complicated rule.
>
>> Suppose I have
>>         default Eq Integer
>>         default Fractional Float
>> and I have (Eq a, Fractional a).  Does 'a' resolve to
> Integer or to Float?  Perhaps a few examples on the proposal
> page would be useful for readers?
>
> The proposed rule is that defaulting applies *after* simplification of
> the context.  So, although you initially infer
>     (Eq a, Fractional a) => a
> but after simplification, this becomes
>     (Fractional a) => a
> so there is a single class to be defaulted, and Float is chosen.

Is your proposal supposed to be backwards compatible with Haskell 98 for programs that don't have default declarations?  If so, then I offer a counter example:

  toRational pi

will default pi to Double in Haskell 98, but will be an error under your proposal, because the two constraints (Real and Floating) disagree on the default.

dflt.hs:3:7:
    Warning: Defaulting the following constraint(s) to type `Double'
             `Real a' arising from use of `toRational' at dflt.hs:3:7-19
             `Floating a' arising from use of `pi' at dflt.hs:3:18-19
             In the expression: toRational pi
             In the definition of `test': test = toRational pi

You can generate more examples using Enum/Fractional combinations, but it's dubious whether Float & Double should be instances of Enum anyway.

Cheers,
        Simon