RE: defaults

16 Jan 2007

      haskell-prime-bounces@haskell.org wrote:
...
Simon Marlow  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?
I don't know whether this is an acceptable loss of backwards compatibility, perhaps it is.  But I spotted the problem because it looks (to me) like the current Haskell 98 rule is designed specifically to handle cases like this.

Would you mind updating the wiki page?  I don't think it's too clear on the need for constraint simplification before applying your defaulting rules either.

Cheers,
        Simon