#12442: Pure unifier usually doesn't need to unify kinds -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | dependent/should_compile/T12442 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2433 Wiki Page: | -------------------------------------+------------------------------------- Changes (by goldfire): * status: new => closed * resolution: => fixed Comment: Here's the Note: {{{ Note [tcMatchTy vs tcMatchTyKi] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This module offers two variants of matching: with kinds and without. The TyKi variant takes two types, of potentially different kinds, and matches them. Along the way, it necessarily also matches their kinds. The Ty variant instead assumes that the kinds are already eqType and so skips matching up the kinds. How do you choose between them? 1. If you know that the kinds of the two types are eqType, use the Ty variant. It is more efficient, as it does less work. 2. If the kinds of variables in the template type might mention type families, use the Ty variant (and do other work to make sure the kinds work out). These pure unification functions do a straightforward syntactic unification and do no complex reasoning about type families. Note that the types of the variables in instances can indeed mention type families, so instance lookup must use the Ty variant. (Nothing goes terribly wrong -- no panics -- if there might be type families in kinds in the TyKi variant. You just might get match failure even though a reducing a type family would lead to success.) 3. Otherwise, if you're sure that the variable kinds to not mention type families and you're not already sure that the kind of the template equals the kind of the target, then use the TyKi version. }}} This is referenced from the various matcher functions. Sorry for letting this slip! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12442#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler