Add defaultGshowsPrec using showsPrec

`GShow t` is isomorphic to `forall a. Show (t a)`, but we don't need the
quantified constraint to write a function to be used to define the
method.

The more specific type should make a nicer error if one didn't derive
the necessary polymorphic `Show` instance.

src/Data/GADT/Internal.hs

@@ -55,6 +55,11 @@ type GShow :: (k -> Type) -> Constraint
 class GShow t where
     gshowsPrec :: Int -> t a -> ShowS
 
+-- |If 'f' has a 'Show (f a)' instance, this function makes a suitable default
+-- implementation of 'gshowsPrec'.
+defaultGshowsPrec :: Show (t a) => Int -> t a -> ShowS
+defaultGshowsPrec = showsPrec
+
 gshows :: GShow t => t a -> ShowS
 gshows = gshowsPrec (-1)
 

src/Data/GADT/Show.hs

@@ -7,6 +7,7 @@
 module Data.GADT.Show (
     -- * Showing
     GShow  (..),
+    defaultGshowsPrec,
     gshows,
     gshow,
     -- * Reading