Re: [Haskell-cafe] Domain specific error messages

I think this is very similar to what Idris supports for reflecting on errors: http://www.itu.dk/people/drc/drafts/error-reflection-submission.pdf Aha! Yes, thank you for jogging my memory. I had a good conversation with David Christiansen about this at ICFP, about this very topic, and whether we could steal Idris’s clever ideas and put them in GHC. · He subsequently wrote a blog posthttp://www.davidchristiansen.dk/2014/09/06/pretty-printing-idris/ · Incidentally, in case it’s not clear this thread on Haskell Caféhttps://www.haskell.org/pipermail/haskell-cafe/2014-November/117054.html goes back to Nov 14. The first message in the series had more useful links David’s idea is would have much broader scope than just TcErrors. Much of the infrastructure is in place already, since error message are SDocs (an abstract type, private to GHC), not strings. Design work needed. Start a wiki page. Articulate a vision. Discuss alternatives. Simon From: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Alejandro Serrano Mena Sent: 06 February 2015 08:24 To: Adam Gundry Cc: Haskell Cafe Subject: Re: [Haskell-cafe] Domain specific error messages I think this is very similar to what Idris supports for reflecting on errors: http://www.itu.dk/people/drc/drafts/error-reflection-submission.pdf 2015-02-06 8:53 GMT+01:00 Adam Gundry mailto:adam@well-typed.com>: [Re-sending to haskell-cafe since I used the wrong From address...] This is something that has been on my mind for a while, particularly following discussions at the last ICFP and prompted by the work on typechecker plugins in GHC [1]. I think I see a way to proceed, though I don't know whether I will have time to implement it for a while. I'd be interested to hear about other approaches. Suppose we define an ADT representation of all the typechecker error messages, e.g. something roughly like data TcError = CustomError String | CouldNotUnify Type Type | NoClassInstance ... In GHC all the relevant messages are generated in the typechecker/TcErrors module, so this shouldn't be too hard to arrange. Now we can extend plugins with the ability to supply a function tcPluginAdjustErrors :: [TcError] -> TcPluginM [TcError] and run the plugged-in function between generating and reporting the errors. Thus a library defining a DSL could also provide a plugin to supply more informative error messages. This is a fairly quick-and-dirty approach, because the plugin would be quite tightly coupled to GHC, even though it could be written and distributed separately. But it might be one way of making progress. (It also occurs to me that such an ADT might allow GHC to define a machine-readable serialisable format for its error messages, to allow post-processing by external programs.) Adam [1] https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker On 05/02/15 09:54, Corentin Dupont wrote:

Hi all, I have been very interested by this discussion when Alberto started it. As there been any progress? The problem is very acute in the Nomyx game I'm developing (www.nomyx.comhttp://www.nomyx.com http://www.nomyx.com). The game is based on a DSL, it's working well, but at the moment only expert Haskell developers can play... I think cryptic error messages is part of the problem. How to improve that? For example, a common error message is the following:

Won't Compile <interactive>:5:28: Couldn't match type ‘'NoEffect’ with ‘'Effect’ Expected type: Exp Effect () Actual type: Exp NoEffect () In the expression: e_1 In the expression: (let e_1 = do { ... } in e_1) :: Exp Effect ()

It's not so helpful and exposing Nomyx internals. A better error message would be to hint that the player forgot a "liftEffect" instruction.

The only quick-and-dirty solution I see is to pattern-match for it and display an additional hint line...

-- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.orgmailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe