Surprised to type in (1+n) as a type index!

3 Feb 2009

      I was writing a document in lhs form and typed in something like this.
...
data Exp n where
  Atom :: Exp ()
  List :: [Exp n] -> Exp (1+n)
I wasn't expecting this to actually load up on GHCi,
but when I load this up with the magical -fglasgow-exts option,
it just worked !!!

*Main> :t Atom
Atom :: Exp ()
*Main> :t List [Atom, Atom]
List [Atom, Atom] :: forall (+ :: * -> * -> *).
                     Exp (+ GHC.Generics.Unit ())

It seems GHC is treating + as just a type variable for binary type
constructor of kind * -> * -> *.

*Main> :t List [Atom, Atom]
List [Atom, Atom] :: forall (+ :: * -> * -> *).
                     Exp (+ GHC.Generics.Unit ())
*Main> :t List [List [Atom, Atom], List []]
List [List [Atom, Atom], List []] :: forall (+ :: * -> * -> *)
                                            (+1 :: * -> * -> *).
                                     Exp (+1 GHC.Generics.Unit (+
GHC.Generics.Unit ()))
*Main> :t List [List [List [Atom, Atom], List []]]
List [List [List [Atom, Atom], List []]] :: forall (+ :: *
                                                         -> *
                                                         -> *)
                                                   (+1 :: * -> * -> *)
                                                   (+2 :: * -> * -> *).
                                            Exp
                                              (+2 GHC.Generics.Unit (+1
GHC.Generics.Unit (+ GHC.Generics.Unit ())))

But, what is this GHC.Generics.Unit thing and where is this documented?

Thanks,

Ahn, Ki Yung