On Fri, May 04, 2007 at 07:20:03PM -0700, Conal Elliott wrote:
[switching to haskell-cafe]
thanks for the explanation, John. doesn't the list cases mentioned in the definition of (+) below assume whole program, in order to know all Num instances?
Yes it does in the current implementation, but the type class implementation is relatively separate from the rest of jhc, so it could be modified in various ways. my current thinking for separate compilation is to allow separate compilation, but have a special pass at the end right before linking that goes through and assigns a unique discriminator to each type, so it would sort of be a built in extensible data type. it can actually be done without a special linker by just placing each descriptor in a special section and using their offset from the beginning of the section as its unique id. FWIW, I would _love_ some standard way to create closed classes in haskell'. it would greatly facilitate this sort of optimization without resorting to much trickery. My current thinking is something like allowing 'class closed Foo' in export lists, which exports the class name except won't let you create new instances for it (via that import). I know the standard trick using a 'Fail' superclass, but having the compiler recognize that special case feels very unclean. John -- John Meacham - ⑆repetae.net⑆john⑈