Re: [GHC] #11990: Custom Type Error not getting triggered in the nested Type function call

27 Jun 2017

      #11990: Custom Type Error not getting triggered in the nested Type function call
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        Reporter:  magesh.b          |                Owner:  diatchki
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.1-rc3
      Resolution:  fixed             |             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 tstr):

 Hi,

 for the example blow I'm wondering why GHC 8.0.2 fails on "TestError" with
 a custom type error, but at the same time happily accepts
 "NestedTypeError"? Is that the expected behavior of GHC?

 Many thanks for your help,

 Thomas

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

 module CustomErrorBug where

 import GHC.TypeLits

 -- Works as expected and makes GHC complain:
 type TestError = TypeError (Text "Top level custom errors work!")

 -- GHC 8.0.2 is absolutely happy with this:
 type family NestedError (x::Symbol) where
   NestedError x = TypeError (Text "NestedError: " :<>: ShowType x)

 type TestNestedError = NestedError "Why are nested custom errors not
 propagated?"
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11990#comment:10
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