bug in report? empty datatypes

Hal Daume III hdaume@ISI.EDU
Fri, 30 Nov 2001 09:53:12 -0800 (PST)

we can now say:

> data T

for an empty datatype

what about:

> data T deriving (Show)  -- or Eq, Ord, ...

personally I think this should be legal.  the actual semantics are
irrelevant, as far as I can tell, since you won't ever have something of
type T other than _|_ and so you could simply define the derived instance
of equals to be something like:

> instance Eq T where { (==) = undefined }


> instance Eq T where { _ (==) _ = True }

seems also to be resonable.

this isn't a big deal, but i'd like it fixed.  ghc panics when you try to
derive something.  hugs gives you a syntax error and i can't get nhc to
accept empty datatypes at all...

 - hal

Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume