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

17 May 2013

      Hi everybody,

I have a question about deriving automatically a Show instance when using 
GADT.
It works in this situation:

----------------------------
{-# LANGUAGE GADTs #-}

data Male
data Female

data Person gender where
    Dead :: Person gender
    Alive :: { name :: String
              , weight :: Float
              , father :: Person gender } -> Person gender
     deriving Show

main = do

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

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

Indeed:

$ runghc test_show.hs 
Alive {name = "Joe", weight = 60.0, father = Dead}
Alive {name = "Jim", weight = 70.0, father = Alive {name = "Joe", weight = 
60.0, father = Dead}}

But when I replace "father :: Person gender" by "father :: Person gender2" 
in the code (this is one of the advantages of GADT: with a classical 
algebraic data type declaration, gender2 would have to be a type variable 
for the data type), I obtain:

Can't make a derived instance of `Show (Person gender)':
      Constructor `Alive' must have a Haskell-98 type
      Possible fix: use a standalone deriving declaration instead
    In the data declaration for `Person'

So I modify my code by removing "deriving Show", and adding the line:
----------------------------
instance Show (Person gender)
----------------------------

But now, I obtain:

$ runghc test_show.hs 
GHC stack-space overflow: current limit is 536870912 bytes.
Use the `-K<size>' option to increase it.

Why (I imagine this is because there is an infinite loop in the construction 
of the show function)? Is there any workaround?

Thanks,

TP