#9334: Implement "instance chains" -------------------------------------+------------------------------------- Reporter: diatchki | Owner: diatchki Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: (Type checker) | Architecture: Unknown/Multiple Resolution: | Difficulty: Unknown Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: | None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by AntC): Replying to [comment:4 diatchki]:
Interestingly, even in my simple version, instance chains are a bit more expressive, because of the explicit ordering of instances. So we can write things like this: {{{ instance C Int a where ... else C a Int where ... }}} I am not sure how common cases like these are, but it is worth noting.
I suspect they're rare, but yes they are problematic. Can't you always resolve this today with an instance at the intersect? {{{ instance C Int Int where ... instance C Int a where ... instance C a Int where ... }}} (The `where`'s body for `C Int Int` would be the same as `C Int a` to match Ivor's example.) Probably for this to work all three instances must be in the same module. The main awkwardness is that GHC still sees the partially overlapping two instances and gets upset (wants `XIncoherentInstances`). If only it could realise there is no incoherence! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9334#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler