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@cs.nott.ac.uk This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.