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

17 May 2013

      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