Re: [GHC] #11974: `default` declaration doesn't allow higher-kinded types

9 May 2016

      #11974: `default` declaration doesn't allow higher-kinded types
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2136
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Let's say you have

 {{{#!hs
 type Type₁ = Type
 type Num₁  = Num

 data Exp₁ (ty :: Type₁) where
   N₁ :: Num₁ n => Exp₁ n

 deriving instance Show (Exp₁ ty)
 }}}

 {{{
 ghci> :set -fwarn-type-defaults
 ghci> N₁
 ... warning: [-Wtype-defaults]
     • Defaulting the following constraint to type ‘Integer’
         Num n0 arising from a use of ‘it’
 ...
 N₁
 }}}

 and you avoid writing `N₁ :: Exp₁ Integer` because of defaulting.

 If however one indexes it with a promoted data type

 {{{#!hs
 data Type₂ = Integer₂ | Int32₂ | Bool₂

 class    Num₂ (a :: Type₂)
 instance Num₂ 'Integer₂
 instance Num₂ 'Int32₂

 data Exp₂ (ty :: Type₂) where
   N₂ :: Num₂ n => Exp₂ n

 deriving instance Show (Exp₂ ty)
 }}}

 but showing it errors

 {{{
 ghci> N₂

 <interactive>:3:1: error:
     • Ambiguous type variable ‘n0’ arising from a use of ‘it’
       prevents the constraint ‘(Num₂ n0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘n0’ should be.
       These potential instances exist:
         instance Num₂ 'Int32₂ -- Defined at /tmp/tyGL.hs:11:10
         instance Num₂ 'Integer₂ -- Defined at /tmp/tyGL.hs:10:10
     • In the first argument of ‘print’, namely ‘it’
       In a stmt of an interactive GHCi command: print it
 ghci>
 }}}

 I needed this and remembered this ticket. I want to write something like

 {{{#!hs
 default ('Integer₂)

 -- Using https://prime.haskell.org/wiki/Defaulting#Proposal1-nametheclass
 default Num₂ ('Integer₂)
 }}}

 Comment got to long, may split it

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11974#comment:4
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