#13775: Type family expansion is too lazy, allows accepting of ill-typed terms -------------------------------------+------------------------------------- Reporter: fizruk | Owner: diatchki Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Keywords: Resolution: | CustomTypeErrors Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * owner: (none) => diatchki Comment: This is really a question for Iavor, who implemented user-specified type errors. The [http://downloads.haskell.org/~ghc/master/users- guide/glasgow_exts.html#custom-compile-time-errors user manual section on custom errors] does not explain the behaviour of the feature (except with a single example), and need some serious love. What is happening is this: * GHC only reports custom type errors if * We have an ''unsolved'' constraint involving `TypeError` * We have a declared or inferred type involving `TypeError` * We get a constraint `Show (Proxy (TypeError "..."))`. * But the instance for `Proxy` is {{{ instance Show (Proxy s) where ... }}} so the `TypeError...` is discarded. * All constraints are solved, so no error is reported. Iavor: you may want to consider being more aggressive? Or at least documenting the expected behaviour better. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13775#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler