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

18 May 2013

      Chris Wong wrote:
...
...
data Person :: Gender -> * where
    Dead :: Person Gender  -- WHAT DO I PUT HERE
    Alive :: { name :: String
              , weight :: Float
              , father :: Person Gender } -> Person Gender
Here's the problem. In the line:
Dead :: Person Gender
you are referring to the Gender *type*, not the Gender kind.
To refer to the kind instead, change this to:
Dead :: forall (a :: Gender). Person a
This means "for all types A which have the kind Gender, I can give you
a Person with that type." The Alive declaration and deriving clause
can be fixed in a similar way.
Also, to enable the "forall" syntax, you need to add
{-# LANGUAGE ExplicitForAll #-}
at the top of the file.
Thanks a lot for your help. I did not realize the possible usage of "a::b" 
to indicate "any type a of kind b". So I have adapted my code, and the 
following version is working correctly:

----------------------------
{-# LANGUAGE GADTs #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}

data Gender = Male | Female

data Person :: Gender -> * where
    Dead :: Person (a :: Gender)
    Alive :: { name :: String
              , weight :: Float
              , father :: Person (a::Gender) } -> Person (b :: Gender)

deriving instance Show (Person (a::Gender))

main = do

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

However, I have not managed to make the version with forall work. Below, the 
first occurrence of forall is ok, but the three following yield error.

----------------------------
{-# LANGUAGE GADTs #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ExplicitForAll #-}

data Gender = Male | Female

data Person :: Gender -> * where
    Dead :: forall (a :: Gender). Person a
    Alive :: { name :: String
              , weight :: Float
              , father :: forall (a :: Gender). Person a } -> forall (b :: 
Gender). Person b

deriving instance Show (forall (a :: Gender). Person a)

main = do

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

Thanks,

TP