Hi, On Mon, 2013-04-08 at 07:06 -0700, Conal Elliott wrote:
What you're suggesting is called "non-linear patterns", and it's a perfectly sensible, well-defined feature in a language with pattern-matching. As you point out, non-linearity allows for more direct & succinct programming. I've often wished for this feature when writing optimizations on data types, especially for syntactic types (languages).
AFAIK pattern-match overlap checking is well defined for linear patterns, but it is not fully implemented and buggy in ghc (I found ~10 open tickets, most of them are pretty old). Will not it be a nightmare to implement and maintain checker for overlapping/unused clauses for non-linear patterns? We already have a number of language extensions without good warnings (and even worse -- with incorrect warnings): view patterns, overloaded literals, GADTs, etc. Thanks, Yuras