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
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