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
[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.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.org http://www.haskell.org/mailman/listinfo/haskell-cafe