[GHC] #14668: Ordering of declarations can cause typechecking to fail

[GHC]

#14668: Ordering of declarations can cause typechecking to fail
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        Reporter:  heptahedron       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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 danilo2):

 Hi! Yet another example which seems related! If you move the data
 declaration to be the first line, it compiles fine:

 {{{

 {-# LANGUAGE TypeInType #-}

 module Type.Data.Map where

 import Prelude
 import Data.Kind


 type family KeyKind (obj :: Type) :: Type
 type family ValKind (obj :: Type) :: Type

 type family Get (key :: KeyKind a) (obj :: a) :: ValKind a

 data Map (k :: Type) (v :: Type) = Map [(k,v)]

 type instance Get k ('Map ('(k,v) ': _)) = v

 type instance KeyKind (Map k v) = k
 type instance ValKind (Map k v) = v

 }}}

 Otherwise it gives error:

 {{{

 Main.hs:16:19: error:
     • Occurs check: cannot construct the infinite kind:
         k0 ~ KeyKind (Map k0 v0)
       The type variables ‘v0’, ‘k0’ are ambiguous
     • In the first argument of ‘Get’, namely ‘k’
       In the type instance declaration for ‘Get’
    |
 16 | type instance Get k ('Map ('(k,v) ': _)) = v
    |                   ^

 Main.hs:16:44: error:
     • Occurs check: cannot construct the infinite kind:
         v0 ~ ValKind (Map k0 v0)
       The type variables ‘k0’, ‘v0’ are ambiguous
     • In the type ‘v’
       In the type instance declaration for ‘Get’
    |
 16 | type instance Get k ('Map ('(k,v) ': _)) = v

 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14668#comment:4>
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