Simon Marlow
Is your proposal supposed to be backwards compatible with Haskell 98 for programs that don't have default declarations?
Yes, it is supposed to be backwards compatible.
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.
Well spotted. So there are some expressions that are defaulted in H'98 but would not pass type-checking with my proposal. (1) Are examples like this common? I am guessing not. You mention Enum/Fractional combinations, but arguably Float and Double do not belong in Enum anyway. (2) The new rule is conservative - it does not silently change the semantics, but it does reject more programs. Such programs are easily fixed by adding a type signature. If these two points are valid, then I think the slight loss of backward compatibility is acceptable? Regards, Malcolm