[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 tom-bop):

 Are we sure this has to do with `TypeError`s? I see the same problem
 without them:

 {{{#!haskell
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}

 import GHC.TypeLits
 import Data.Proxy

 type family Head xs where
   -- As written, this compiles, and it also compiles if you uncomment
 either
   --   of these lines:

   -- Head '[] = 'True ~ 'False
   -- Head '[] = TypeError ('Text "empty list")

   Head (x ': xs) = x

 main = print $ show (Proxy::Proxy (Head '[]))
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13775#comment:9>
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