GitLab

If I define an "opposite category" newtype Op cat a b = Op (cat b a) then representationally it forms an involution: applying Op twice gives the same representation as not applying it at all

$ ... -ignore-dot-ghci
GHCi, version 8.3.20170920: http://www.haskell.org/ghc/  :? for help
Prelude> import Data.Coerce
Prelude Data.Coerce> import Data.Type.Coercion 
Prelude Data.Coerce Data.Type.Coercion> newtype Op cat a b = Op (cat b a)
Prelude Data.Coerce Data.Type.Coercion> :t coerce :: Op (Op cat) a b -> cat a b
coerce :: Op (Op cat) a b -> cat a b :: Op (Op cat) a b -> cat a b
Prelude Data.Coerce Data.Type.Coercion> :t Coercion :: Coercion (Op (Op cat) a b) (cat a b)
Coercion :: Coercion (Op (Op cat) a b) (cat a b)
  :: Coercion (Op (Op cat) a b) (cat a b)

But these don't work:

Prelude Data.Coerce Data.Type.Coercion> :t Coercion :: Coercion (Op (Op cat) a) (cat a)

<interactive>:1:1: error:
    • Couldn't match representation of type ‘Op (Op cat1) a1’
                               with that of ‘cat1 a1’
        arising from a use of ‘Coercion’
    • In the expression: Coercion :: Coercion (Op (Op cat) a) (cat a)
Prelude Data.Coerce Data.Type.Coercion> :t Coercion :: Coercion (Op (Op cat) cat

<interactive>:1:38: error:
    parse error (possibly incorrect indentation or mismatched brackets)
Prelude Data.Coerce Data.Type.Coercion> :t Coercion :: Coercion (Op (Op cat)) cat

<interactive>:1:1: error:
    • Couldn't match representation of type ‘cat1’
                               with that of ‘Op (Op cat1)’
        arising from a use of ‘Coercion’
      ‘cat1’ is a rigid type variable bound by
        an expression type signature:
          forall (cat1 :: * -> * -> *). Coercion (Op (Op cat1)) cat1
        at <interactive>:1:13-38
    • In the expression: Coercion :: Coercion (Op (Op cat)) cat

I'm wondering if this is intentional

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture