Types of generated data constructors for dictionaries

[Simon Peyton-Jones]

> class Eq a => Eq2 a where eq :: a -> a -> Bool


Based on the runtime representation above, I expect the type of Main.:DEq2 to be

Main.:DEq2 :: GHC.Base.:TEq a -> (a -> a -> Bool) -> Main.:TEq2 a


but GHC tells me, via (fmap dataConRepType . tcLookupDatacon), the following

Main.:DEq2 :: (a -> a -> Bool) -> Main.:TEq2 a

Why does the type of :DEq2 differ from what :print sees at runtime ?



Are you sure it's different?  Here's what I get (below). Note the type of :DEq2.

Can you tell me how to reproduce what you are seeing?

Simon


module Foo where
class Eq a => Eq2 a where
op :: a -> a -> Bool


ghc -c -ddump-types Foo.hs -dppr-debug
TYPE SIGNATURES
    main:Foo.$p1Eq2{v r5P} [gid] :: forall a{tv a5D} [tv].
                                    (main:Foo.Eq2{tc r5z} a{tv a5D} [tv]) =>
                                    <pred>base:GHC.Base.Eq{tc 23} a{tv a5D} [tv]
    main:Foo.:DEq2{v r5R} [gid] :: forall a{tv a5D} [tv].
                                   (base:GHC.Base.Eq{tc 23} a{tv a5D} [tv]) =>
                                   (a{tv a5D} [tv] -> a{tv a5D} [tv] -> base:GHC.Base.Bool{(w) tc 3c})
                                   -> (main:Foo.:TEq2{tc r5Q}) a{tv a5D} [tv]
    main:Foo.op{v r5B} [gid] :: forall a{tv a5D} [tv].
                                (main:Foo.Eq2{tc r5z} a{tv a5D} [tv]) =>
                                a{tv a5D} [tv] -> a{tv a5D} [tv] -> base:GHC.Base.Bool{(w) tc 3c}
TYPE CONSTRUCTORS
    data (:TEq2{tc}) a
        RecFlag NonRecursive
        Generics: no
        = :DEq2{d} :: forall a.
                      base:GHC.Base.Eq{tc 23} a =>
                      (a -> a -> base:GHC.Base.Bool{(w) tc 3c}) -> :TEq2{tc} a
              Stricts: _
        FamilyInstance: none
Tycons with generics:
Dependent modules: []
Dependent packages: [base]
bash-3.2$
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