#8776: Displaying pattern synonym for a GADT -------------------------------------+------------------------------------ Reporter: monoidal | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): Gergo writes (to ghc-devs):The problem is that the two type variables occurring in the provided context both have "t" as their user-visible name; even though the second one is projected as "t1" in the tau type on the right-hand side. The code to generate this output is in `HsBinds.ppr_sig`, using `pprPatSynSig` which just adds the "pattern" keyword and the separating "=>" symbols etc: {{{ ppr_sig (PatSynSig name arg_tys ty prov req) = pprPatSynSig (unLoc name) False args (ppr ty) (pprCtx prov) (pprCtx req) where args = fmap ppr arg_tys pprCtx lctx = case unLoc lctx of [] -> Nothing ctx -> Just (pprHsContextNoArrow ctx) }}} My guess is that the problem is `pprHsContextNoArrow` projects individual constraints one-by-one and so doesn't notice the name clash on 't' between the two constraints in the example; whereas 'ppr ty' walks the whole right-hand tau type and thus has the opportunity to maintain a set of type variable names used. My question is, where is that logic, and how can I use that in this instance? My hope is to be shown a design where I can run 'ppr ty', 'pprCtx prov' and 'pprCtx req' in the same "naming environment" (I hope such a thing exists) so that they use a consistent naming scheme. This looks like a problem that must have popped up at a lot of places in GHC already and must have an idiomatic solution. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8776#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler