#9911: Pattern synonyms with no signatures should yield warnings -------------------------------------+------------------------------------- Reporter: cactus | Owner: cactus Type: feature | Status: new request | Milestone: 7.10.1 Priority: normal | Version: 7.8.3 Component: Compiler | Keywords: PatternSynonyms (Type checker) | Architecture: Unknown/Multiple Resolution: | Difficulty: Unknown Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: Incorrect | warning at compile-time | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by cactus): The reason this doesnt Just Work(tm) is that the set of names that need signatures (`sig_names` in `TcRnDriver.tcTopSrcDecls`) is computed from the `HsBind Name`s on the input side (so for a pattern synonym `P`, the set would contain `P` itself), but then when checking against this set, the names from the typechecked bindings are used; so for a pattern synonym `P`, we try to look up `$mP` for the matcher and `$bP` for the builder. Another problem is that even if we would look up `P` (e.g. by changing matcher names to be the original pattern name without the added `$m` prefix), the warning would show the type of the matcher, instead of the pattern synonym type. So all in all, a real solution to this will require using the typechecked pattern synonym itself (a `PatSyn`) when doing the signature check, since that has the correct `Name` and the type. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9911#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler