RE: defaults

haskell-prime-bounces@haskell.org wrote:

Simon Peyton-Jones writes:

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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.

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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