Show instances for ListTr and Queue (with QuantifiedConstraints)

src/Control/Category/Free/Internal.hs

@@ -7,6 +7,10 @@
 {-# LANGUAGE TypeFamilies          #-}
 {-# LANGUAGE ViewPatterns          #-}
 
+#if __GLASGOW_HASKELL__ >= 806
+{-# LANGUAGE QuantifiedConstraints #-}
+#endif
+
 {-# OPTIONS_HADDOCK show-extensions #-}
 
 #if __GLASGOW_HASKELL__ <= 802
@@ -53,6 +57,7 @@ import           Control.Algebra.Free2 ( AlgebraType0
 -- from @b@ to @a@ in the original category.
 --
 newtype Op (f :: k -> k -> *) (a :: k) (b :: k) = Op { runOp :: f b a }
+  deriving Show
 
 -- | 'Op' is an endo-functor of the category of categories.
 --
@@ -91,6 +96,16 @@ data ListTr :: (k -> k -> *) -> k -> k -> * where
   NilTr  :: ListTr f a a
   ConsTr :: f b c -> ListTr f a b -> ListTr f a c
 
+lengthListTr :: ListTr f a b -> Int
+lengthListTr NilTr = 0
+lengthListTr (ConsTr _ xs) = 1 + lengthListTr xs
+
+#if __GLASGOW_HASKELL__ >= 806
+instance (forall (x :: k) (y :: k). Show (f x y)) => Show (ListTr f a b) where
+    show NilTr         = "NilTr"
+    show (ConsTr x xs) = "ConsTr " ++ show x ++ " " ++ show xs
+#endif
+
 instance Category (ListTr f) where
   id = NilTr
   NilTr    . ys = ys
@@ -154,6 +169,12 @@ data Queue (f :: k -> k -> *) (a :: k) (b :: k) where
           -> !(ListTr f b x)
           -> Queue f a c
 
+#if __GLASGOW_HASKELL__ >= 806
+instance (forall (x :: k) (y :: k). Show (f x y))
+      => Show (Queue f a b) where
+    show (Queue f r s) = "Queue (" ++ show f ++ ") (" ++ show r ++ ") " ++ show (lengthListTr s)
+#endif
+
 emptyQ :: Queue (f :: k -> k -> *) a a
 emptyQ = Queue NilTr NilTr NilTr