Rename emptyQ as nilQ

src/Control/Arrow/Free.hs

@@ -63,7 +63,7 @@ arrArr bc = Arr bc Id
 mapArr :: f b c
        -> Arr f a b
        -> Arr f a c
-mapArr bc ac = Cons bc emptyQ . ac
+mapArr bc ac = Cons bc nilQ . ac
 
 foldArr :: forall f arr a b.
            Arrow arr
@@ -103,7 +103,7 @@ type instance AlgebraType0 Arr f = ()
 type instance AlgebraType  Arr c = Arrow c
 
 instance FreeAlgebra2 Arr where
-  liftFree2 = \fab -> Cons fab emptyQ
+  liftFree2 = \fab -> Cons fab nilQ
   {-# INLINE liftFree2 #-}
 
   foldNatFree2 = foldArr

src/Control/Category/Free.hs

@@ -153,7 +153,7 @@ instance Show (Cat f a b) where
 arrCat :: forall (f :: k -> k -> *) a b.
           f a b
        -> Cat f a b
-arrCat ab = Cat emptyQ ab
+arrCat ab = Cat nilQ ab
 {-# INLINE arrCat #-}
 
 foldCat :: forall f c a b.
@@ -180,7 +180,7 @@ foldCat nat (Cat q0 tr0) =
 -- @
 -- op :: Cat f x y -> Cat (Op f) y x
 -- op Id = Id
--- op (Cat q tr) = Cat emptyQ (Op tr) . foldNatQ unDual q
+-- op (Cat q tr) = Cat nilQ (Op tr) . foldNatQ unDual q
 -- @
 --
 -- where
@@ -202,7 +202,7 @@ foldCat nat (Cat q0 tr0) =
 -- > op Id = Id
 -- >       ~ Id
 -- > op (Cat q tr)
--- >   = c₀@(Cat emptyQ (Op tr)) . foldNatQ unDual q
+-- >   = c₀@(Cat nilQ (Op tr)) . foldNatQ unDual q
 -- >        Note that `.` here denotes composition in `Cat (Op f)`.
 -- >        Let us assume that `q = c₁ : c₂ :  ⋯  : cₙ : NilQ`,
 -- >        where each `cᵢ :: Cat (Op (Op tr)) aᵢ aᵢ₊₁`
@@ -211,7 +211,7 @@ foldCat nat (Cat q0 tr0) =
 -- >        `unDual cᵢ :: Cat tr aᵢ aᵢ₊₁` has the same representation as cᵢ,
 -- >        at this step we also need to rewrite `.` composition in
 -- >        `Cat (Op f)` using composition in `Cat f`, this reverses the order
--- >   ~ cₙ . ⋯ . c₁ . Cat emptyQ tr
+-- >   ~ cₙ . ⋯ . c₁ . Cat nilQ tr
 -- >        By definition of composition in `Cat f`
 -- >   = Cat q tr
 --
@@ -232,7 +232,7 @@ unOp :: forall (f :: k -> k -> *) x y.
      -> Cat f y x
 unOp = unsafeCoerce
 -- unOp Id = Id
--- unOp (Cat q (Op tr)) = Cat emptyQ tr . foldNatQ unDual q
+-- unOp (Cat q (Op tr)) = Cat nilQ tr . foldNatQ unDual q
 {-# INLINE unOp #-}
 
 instance Arrow f => Arrow (Cat f) where

src/Control/Category/Free/Internal.hs

@@ -32,7 +32,7 @@ module Control.Category.Free.Internal
   , ListTr (..)
   , lengthListTr
   , Queue (NilQ, ConsQ)
-  , emptyQ
+  , nilQ
   , consQ
   , ViewL (..)
   , unconsQ
@@ -267,8 +267,8 @@ instance ArrowChoice f => ArrowChoice (Queue f) where
   NilQ +++ (ConsQ fxb cax) = (arr id +++ fxb) `ConsQ` (NilQ +++ cax)
   NilQ +++ NilQ            = NilQ
 
-emptyQ :: Queue (f :: k -> k -> *) a a
-emptyQ = Queue NilTr NilTr NilTr
+nilQ :: Queue (f :: k -> k -> *) a a
+nilQ = Queue NilTr NilTr NilTr
 
 consQ :: forall (f :: k -> k -> *) a b c.
          f b c
@@ -303,7 +303,7 @@ pattern ConsQ a as <- (unconsQ -> a :< as) where
 
 pattern NilQ :: () => a ~ b => Queue f a b
 pattern NilQ <- (unconsQ -> EmptyL) where
-    NilQ = emptyQ
+    NilQ = nilQ
 
 #if __GLASGOW_HASKELL__ > 802
 {-# complete NilQ, ConsQ #-}