What version of the GHC code are you looking at? The parser is currently stored in compiler/parser/Parser.y.pp (note the pp) and doesn’t have these lines. As far as I know, there is no way to refer to OpenKind from source.

You’re absolutely right about the type of `undefined`. `undefined` (and `error`) have magical types. GHC knows that GHC.Err defines an `undefined` symbol and gives it its type by fiat. There is no way (I believe) to reproduce this behavior.

If you have -fprint-explicit-foralls and -fprint-explicit-kinds enabled, quantified variables of kind * are not given kinds in the output. So, the lack of a kind annotation tells you that `a`’s kind is *. Any other kind (assuming these flags) would be printed.

I hope this helps!
Richard

On Apr 15, 2014, at 7:39 PM, Conal Elliott <conal@conal.net> wrote:

I see ‘#’ for unlifted and ‘?’ for open kinds in compiler/parser/Parser.y: 
akind   :: { IfaceKind }
        : '*'              { ifaceLiftedTypeKind }      
        | '#'              { ifaceUnliftedTypeKind }
        | '?'              { ifaceOpenTypeKind }
        | '(' kind ')'     { $2 }

kind    :: { IfaceKind }
        : akind            { $1 }
        | akind '->' kind  { ifaceArrow $1 $3 }

However, I don’t know how to get GHC to accept ‘#’ or ‘?’ in a kind annotation. Are these kinds really available to source programs.

I see that undefined has an open-kinded type:

*Main> :i undefined
undefined :: forall (a :: OpenKind). a      -- Defined in ‘GHC.Err’

Looking in the GHC.Err source, I just see the following:

undefined :: a
undefined =  error "Prelude.undefined"

However, if I try similarly,

q :: a
q = error "q"

I don’t see a similar type:

*X> :i q
q :: forall a. a        -- Defined at ../test/X.hs:12:1

I don't know what kind 'a' has here, nor how to find out.

-- Conal
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