#9334: Implement "instance chains" -------------------------------------+------------------------------------- Reporter: diatchki | Owner: diatchki Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.9 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Replying to [comment:1 goldfire]:
2. This doesn't necessarily mean that we'll never need overlapping instances -- instance chains seem to only work when the overlap would be contained only in one module. Some programs require inter-module overlap (say, for a global "default" instance).
Our first question was how frequently the open-endedness of overlapping instances was necessary in practice. To answer this question, we determined whether the instances in each set were located in the same module, in different modules within the same package, or in different
Here's some data from [http://homepages.inf.ed.ac.uk/jmorri14/d/final.pdf Type Classes and Instance Chains: A Relational Approach] packages (Figure 3.1). Out of the 123 sets, 19 included overlapping instances from different modules, and 6 (of those 19) included overlapping instances from different packages. THe[sic] majority (104, or 85%) of the sets only included instances from a single module. This suggests that, while applications exist for instances overlapping across modules, most overlapping instances are defined locally.
— page 37
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9334#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler