#14380: Compile error for PatternSynonyms together with OverloadedLists -------------------------------------+------------------------------------- Reporter: lehins | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.3 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 simonpj): OK, I know what is happening here. * The message is utterly bogus. * It arise because, with ''implicitly-bidirectional'' pattern synonyms, we have to translate a pattern into an expression. See `TcPatSyn` {{{ tcPatToExpr :: Name -> [Located Name] -> LPat GhcRn -> Either MsgDoc (LHsExpr GhcRn) }}} Note that this happens to a renamed, but not typechecked, pattern. * With rebindable syntax, a renamed list pattern `[p1, p2]`, implemented with `ListPat`, looks like {{{ ListPat [p1, p2] (Just toList) }}} where the `toList` is the name of the in-scope `toList` function. Matching on the pattern will call `toList` to convert the actual arg to a list, and match that list against `[p1, p2]` * But when converting a `ListPat` ''patttern'' to a to an `ExplicitList` ''expression'' we need `fromList` not `toList`!! Stupidly, `tcPatToExpr` just plops the `toList` in the `ExplicitList`. Wrong wrong. What do to? The typechecker (which is where `tcPatToExpr` is currently called) isn't really the right place to look up `fromList`, although it might be possible. E.g. the renamer would then not see this use of `fromList`, and hence perhaps complain about an unused import. I think the Right Thing is to move `tcPatToExpr` to the renamer, and which can look up that `fromList` just as it does the `toList`. To do that we'd need to alter `HsPatSynDir` thus {{{ data HsPatSynDir id = Unidirectional | ImplicitBidirectional (PostRn (LHsExpr id)) | ExplicitBidirectional (MatchGroup id (LHsExpr id)) }}} This adds a `LHsExpr` field to `ImplicitBidirectional`, the result of doing that conversion. I suppose that `tcPatToExpr` would then become monadic so that it can look up `fromList`. And then it can report errors rather than returning them in an `Either`. Would anyone like to try this? Meanwhile, I think I'll just reject programs that use implicitly- bidirectional syntax with overloaded lists, pointing to this ticket so that users can yell if it happens. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14380#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler