[GHC] #11599: Why is UndecidableInstances required for an obviously terminating type family?

[GHC]

#11599: Why is UndecidableInstances required for an obviously terminating type
family?
-------------------------------------+-------------------------------------
           Reporter:  heisenbug      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In below (working) snippet
 {{{#!hs
 {-# LANGUAGE DataKinds, KindSignatures, TypeFamilies, UndecidableInstances
 #-}

 type family Rev (acc :: [*]) (ts :: [*]) :: [*] where
   Rev acc '[] = acc
   Rev acc (t ': ts) = Rev (t ': acc) ts

 type Reverse ts = Rev '[] ts
 }}}
 the `Rev` does an exhaustive pattern match on its second argument and
 recurses on strictly smaller data.
 {{{#!hs
 *Main> :kind! Reverse [Char, Bool, Float]
 Reverse [Char, Bool, Float] :: [*]
 = '[Float, Bool, Char]
 }}}
 So when I remove `UndecidableInstances` it is not clear to me why this
 would be rejected :-(
 {{{
 error:
       The type family application  Rev (a : acc) as
         is no smaller than the instance head
       (Use UndecidableInstances to permit this)
       In the equations for closed type family  Rev
       In the type family declaration for  Rev
 Failed, modules loaded: none.
 }}}

 I tried this on GHC v8.1.today, but older GHCs behave the same.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11599>
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