GADT question

[Henrik Nilsson]

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
nhn at cs.nott.ac.uk


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