[GHC] #13775: Type family expansion is too lazy, allows accepting of ill-typed terms

[GHC]

#13775: Type family expansion is too lazy, allows accepting of ill-typed terms
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        Reporter:  fizruk            |                Owner:  diatchki
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |             Keywords:
      Resolution:                    |  CustomTypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Sorry -- I didn't mean to sound snarky!

 > A different design choice is to refuse to solve such constraints until
 GHC is sure that the type functions involved will produce a valid result

 I think you are suggesting that

 * Don't do any work on a constraint that mentions `TypeError`.   E.g. `C
 [TypeError ..]` would never be solved, regardless of the instances for
 `C`.

 That sounds attractive, but it's fragile.  Suppose we had `[W] C [alpha]`
 where `alpha` is a unification variable.  Can we use an instance
 declaration on that?  Well, no; it might turn out that, after hundreds
 more simplification steps on other constraints mentioning `alpha`, that
 `alpha := TypeError...`.

 Or it might be `[W] C (F alpha)`, where eventually `alpha := Int`, and
 then we reduce `F Int` to `TypeError...`.

 So I don't see how to make this approach robustly implementable.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13775#comment:8>
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