Hi Garrett,
> I get an error "No instance for (Fractional a) arising from the use of
> '/'" This seems odd to me, since Div is constrained to have
> fractional arguments. Is there something obvious I'm missing?
Unless GADTs are handled specially, and I don't think they are in this
case, this problem is not specific to GADTs but related to how
type constraints on constructors are handled in general. Basically, a
constraint on a constructor has no effect beyond beyond constraining
what the constructor can be applied to (see the Haskell 98 report
4.2.1). In particular, when you take a constructed value apart through
pattern matching, the constraints do not come into play: hence the "no
instance" message.
It has been suggested a number of times that constraints on
constructors should have a more profound meaning, and
maybe GADTs make this even more desirable, as your example suggest.
Best regards,
/Henrik
--
Henrik Nilsson
School of Computer Science and Information Technology
The University of Nottingham
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