[GHC] #13872: Strange Typeable error message involving TypeInType

24 Jun 2017

      #13872: Strange Typeable error message involving TypeInType
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  Typeable                           |
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I originally discovered this when tinkering with #13871. This program:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Foo where

 import Data.Kind
 import Data.Typeable

 data Foo (a :: Type) (b :: Type) where
   MkFoo :: (a ~ Int, b ~ Char) => Foo a b

 data family Sing (a :: k)
 data SFoo (z :: Foo a b) where
   SMkFoo :: SFoo MkFoo

 f :: String
 f = show $ typeOf SMkFoo
 }}}

 Fails in GHC 8.0.1, 8.0.2, and 8.2 (after applying Phab:D3671) with a
 rather unsightly error message:

 {{{
 GHCi, version 8.3.20170624: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Foo              ( Foo.hs, interpreted )

 Foo.hs:19:12: error:
     • No instance for (Typeable <>) arising from a use of ‘typeOf’
     • In the second argument of ‘($)’, namely ‘typeOf SMkFoo’
       In the expression: show $ typeOf SMkFoo
       In an equation for ‘f’: f = show $ typeOf SMkFoo
    |
 19 | f = show $ typeOf SMkFoo
    |            ^^^^^^^^^^^^^
 }}}

 I'm not sure what this mysterious `<>` is, but I'm pretty sure it
 shouldn't be making an appearance here. (See also #13780, where `<>` also
 makes a surprise guest appearance.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13872
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