#8809: Prettier error messages? -------------------------------------+------------------------------------- Reporter: joelteon | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by bgamari): goldfire, indeed the an ADT-per-compiler-phase is exactly what I was thinking (and I have the beginnings of a branch looking at `TcErrors` in particular. In my case though, I was thinking of at least starting by merely annotating a few semantically-important elements of the message (e.g. `Name`s, `Type`s, `TyVar`s, etc.). This would enable, for instance, allow links to the definition span of a symbol, printing an expanded representation of a type, etc. That being said, there is no reason why one couldn't go further with this same approach and encode the entire error as a value. This certainly offers further advantages, although also implies a bit more work (which is why I'm starting with the atoms listed above). As far as the indexing issue goes, I was thinking we would give `Doc` a `Monad` instance. This would allow a number of quite convenient patterns. For instance, have `msgs :: Doc TcErrDoc` containing some errors you'd like to print: If you have `pprTcErrDoc :: TcErrDoc -> Doc Void`, you could trivially flatten the document with `msgs >>= pprTcErrDoc`. Further if you want to combine a `Doc TcErrDoc` with a `Doc ParserErrDoc`, you'd simply lift them both into an ADT `data GhcErrDoc = TcErrDoc TcErrDoc | ParserErrDoc ParserErrDoc` with `Applicative`. Alternatively, if you'd rather keep the universe of error types open, you could opt to lift them into a universally quantified `newtype`, roughly like you suggest. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8809#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler