[GHC] #14771: TypeError reported too eagerly

[GHC]

#14771: TypeError reported too eagerly
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        Reporter:  hsyl20            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |             Keywords:
      Resolution:                    |  CustomTypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by hsyl20):

 > I guess a better solution, although more complex, would be too look at
 the available equations for T and see if evaluation is "stuck" due to the
 type error. If so, we report the custom error. Otherwise, we leave the
 expression as is.

 That would be good!

 ----

 Regarding the issue 2: say GHC can't solve `F n ~ Word` as in the given
 example.
 Currently it reports:
 {{{
     • Couldn't match type ‘If (n <=? 8) Int DUMMY’ with ‘Word’
 }}}

 Sometime in other places in my code (I don't know how to reproduce it in
 the smaller example) it reports the following, which is better because it
 mentions `F n`:
 {{{
     • Couldn't match type ‘If (n <=? 8) Int8 DUMMY’ with ‘Word’
       Expected type: Word
         Actual type: F n
 }}}

 I'd like `F n` to appear more often instead of having to remember that the
 complex expression `If (n <=? 8) ...` refers to `F n`. Maybe we could even
 suggest adding `F n ~ Word` as a constraint for `test2`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14771#comment:4>
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