[GHC] #13670: Improving Type Error Messages

[GHC]

#13670: Improving Type Error Messages
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        Reporter:  gridaphobe        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  TypeErrorMessages
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 > Finally, I've always liked how GHC helpfully explains the context in
 which the error occurs ("in the second argument ... in the expression ...
 etc"), but I think we've been outclassed by other compilers that just
 print the offending line with the error underlined. We could adopt this
 strategy.

 Thanks to @Rufflewind GHC 8.2 does precisely this. e.g.,
 {{{
 hi.hs:9:33: error:
     • Couldn't match type ‘a’ with ‘b’
       ‘a’ is a rigid type variable bound by
         the type signature for:
           fmap :: forall a b. (a -> b) -> List a -> List b
         at hi.hs:8:11-38
       ‘b’ is a rigid type variable bound by
         the type signature for:
           fmap :: forall a b. (a -> b) -> List a -> List b
         at hi.hs:8:11-38
       Expected type: List b
         Actual type: List a
     • In the second argument of ‘ListElement’, namely ‘xs’
       In the expression: ListElement (f x) xs
       In an equation for ‘fmap’: fmap f xs = ListElement (f x) xs
     • Relevant bindings include
         xs :: List a (bound at hi.hs:9:10)
         f :: a -> b (bound at hi.hs:9:8)
         fmap :: (a -> b) -> List a -> List b (bound at hi.hs:9:3)
   |
 9 |   fmap f xs = ListElement (f x) xs
   |                                 ^^
 }}}
 (with color even!)

 Arguably the "in the second argument of ..." text can now be dropped, but
 this won't be done for 8.2.

 > (Related: it seems redundant to provide this context if the user is
 inside their editor rather than at the command-line. What if we had a flag
 --editor-mode to prune such redundancies?)

 I think we are now getting into the problem of more structured (in the
 machine-readable sense) error messages. I suggest an approach to attacking
 this problem in ticket:8809#comment:3. Recently this project has been un-
 stuck by Alfredo Di Napoli, who has been doing some great work reconciling
 GHC's `Pretty` module with the upstream `pretty` library. This will allow
 us to use `pretty`'s annotated pretty-printer to embed Haskell values in
 error messages, giving consumers the ability to pick out exactly the
 details that they want to show.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13670#comment:2>
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