defaults

[Malcolm Wallace]

Simon Marlow <simonmar at microsoft.com> wrote:

> 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