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

18 May 2013

      On 18 May 2013 11:16, TP  wrote:
<snip>
...
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
The foralls should be in front of all the data constructor parameters, like
with the
Dead constructor, since you want to quantify a and b over the whole type
signature:

data Person :: Gender -> * where
    Dead :: forall (a :: Gender). Person a
    Alive :: forall (a :: Gender) (b :: Gender).
             { name :: String
             , weight :: Float
             , father :: Person a } -> Person b
...
deriving instance Show (forall (a :: Gender). Person a)
Again, you should quantify over Show as well:

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

Note that all of this would work even without explicit quantification since
you
have already specified that Person accepts an argument of kind Gender. In
other
words, this works as expected:

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

deriving instance Show (Person a)

You also probably want father :: Person Male, since a father cannot be
Female
(presumably!).

Regards,

-- 
Denis Kasak