#11581: TypeError requires UndecidableInstances unnecessarily -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): In the (necessarily conservative) story about the "size" of the RHS, we could certainly ignore all definitely-non-recursive type functions. `TypeError` is a case in point. By "recursive" I mean: given {{{ F t1 ... tn = ...(G s1 .. sm)... }}} can `G s1 .. sm` ever rewrite to call of `F` that is no smaller than the LHS? For open families `G` we can't know for sure, because `G` might get new RHSs. For closed `G` we could make a stab, perhaps. For `TypeError` we know for sure since it is built in. For now we could make it a special case: easy to do and useful. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11581#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler