[Haskell-cafe] question about GADT and deriving automatically a Show instance

[TP]

Adam Gundry wrote:

[...]
> To use standalone deriving, you need to write
> 
>> deriving instance Show (Person gender)
> 
> and everything will work fine. By writing
> 
>> instance Show (Person gender)
> 
> you are instead giving an instance with no methods, and the default
> methods in the Show class contain a loop (so that one can define either
> show or showsPrec).

Thanks a lot. I did not remember that "Standalone Deriving" has a meaning as 
a GHC extension. My idea was correct, but the employed syntax was incorrect. 
Just for reference (future Google search by others), the corresponding link 
in the GHC documentation:

http://www.haskell.org/ghc/docs/7.6.2/html/users_guide/deriving.html

> P.S. You might like to check out the new DataKinds extension, which
> would allow Male and Female to be data constructors rather than type
> constructors.

Thanks a lot for pointing out this subject (it compells me to work on it - I 
am not an advanced user of GHC).
I have just tried to understand all this stuff about type and data 
constructor promotion:

http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/promotion.html

If I understand well, your idea is to avoid letting someone write:

let a = Alive "Joe" 60 Dead :: Person Int

which is nonsense (a Person cannot have a gender of type "Integer"), but 
legal code.
I have tried to use the technique described at the beginning of the article 
"Given Haskell a Promotion", but I'm stuck. See the code below. My problem 
is that now Gender is a kind, no more a type, such that I cannot use it in 
the type definition of the GADT; but I am compelled to write something after 
"::", and I cannot write for instance "Dead :: Person Male" because I want a 
dead person to be either a man or woman, of course. In fact, what I need is 
Gender both as a type and as a kind, if I am correct? What do I miss?

So the following version does not work:
----------------------------------------
{-# LANGUAGE GADTs #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}

data Gender = Male | Female

data Person :: Gender -> * where
    Dead :: Person Gender  -- WHAT DO I PUT HERE
    Alive :: { name :: String
              , weight :: Float
              , father :: Person Gender } -> Person Gender

deriving instance Show (Person Gender)

main = do

let a = Alive "Joe" 60 Dead :: Person Male
let b = Alive "Jim" 70 a :: Person Male

print a
print b
----------------------------------------

How to modify it?

Thanks a lot,

TP