[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 diatchki):

 As for issue 1), I am a bit weary of doing the change, as the current
 behavior has a fairly simple explanation;  the alternative I mentioned
 would be more of a heuristic, as I don't think we have a specific order of
 evaluation, and it could make some custom errors not show up, when you'd
 like them to show.

 Generally, even though `TypeError` is implemented just as a type function,
 I really think of it more of a special right-hand side in type function
 definitions.  If you use it in this way, then you don't encounter the
 problems you found, because the type error is only ever generated when
 there is no other alternative.  So this is how I'd structure your example
 in this style:
 {{{
 type family Assert (prop :: Bool) (val :: k) (msg :: ErrorMessage) where
    Assert 'True  val msg = val
    Assert 'False val msg = TypeError msg
 }}}
 In fact, we should probably add this type function to `Data.Type.Bool`.

 With this combinator, you could define `F` simply as:
 {{{
 type F n = Assert (n <=? 8) Int (Text "Input to F should be no greater
 than 8")
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14771#comment:6>
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