[GHC] #16249: no runtime error for -fdefer-type-errors with TypeError constraint

[GHC]

#16249: no runtime error for -fdefer-type-errors with TypeError constraint
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        Reporter:  guibou            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
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 simonpj):

 Hmm.   The trouble here is that, at runtime the `TypeError` dictionary is
 not actually used.  I think we get something like this
 {{{
 data Foo t = MkFoo (t -> t)

 foo :: Foo t -> t -> t
 foo (MkFoo f) = f

 $dfFooString :: TypeError (Text "String does not work") -> Foo String
 -- Arises from the instance
 $dfFooString d = MkFoo id


 derr:: TypeError (Text "String does not work")
 derr = error "String does not work"

 x :: String
 -- Haskell source code:
 --    x = foo "hello"
 x = foo ($dfFooString derr) "hello"
 }}}
 Evaluating `f` does not evaluate `derr`, hence the no-error run.

 I suppose someone could think about evaluating calls to `TypeError` more
 eagerly.  But I'd be inclined to think that an easier path is to say
 {{{
 instance (TypeError (Text "String does not work")) => Foo String where
   foo = error "String does not work"
 }}}
 I'm sure there is more than could be done to make `TypeError` better.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16249#comment:1>
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