feat: make partition and partition_map tail-recursive (#25)

src/BwdLabels.mli

@@ -141,10 +141,8 @@ val find_all : f:('a -> bool) -> 'a t -> 'a t
 
 val filteri : f:(int -> 'a -> bool) -> 'a t -> 'a t
 
-(** Not tail-recursive. *)
 val partition : f:('a -> bool) -> 'a t -> 'a t * 'a t
 
-(** Not tail-recursive. *)
 val partition_map : f:('a -> ('b, 'c) Either.t) -> 'a t -> 'b t * 'c t
 
 (** {1 Lists of pairs} *)

src/BwdNoLabels.ml

@@ -324,32 +324,28 @@ let filteri f =
         (go[@tailcall]) (i + 1) xs
   in go 0
 
-let partition f =
-  let rec go =
-    function
-    | Emp -> Emp, Emp
+let partition f xs =
+  let rec go xs ys zs =
+    match xs with
+    | Emp -> append Emp ys, append Emp zs
     | Snoc (xs, x) ->
       if f x then
-        let ys, zs = go xs in
-        Snoc (ys, x), zs
+        (go[@tailcall]) xs (x :: ys) zs
       else
-        let ys, zs = go xs in
-        ys, Snoc (zs, x)
-  in go
+        (go[@tailcall]) xs ys (x :: zs)
+  in go xs [] []
 
-let partition_map f =
-  let rec go =
-    function
-    | Emp -> Emp, Emp
+let partition_map f xs =
+  let rec go xs ys zs =
+    match xs with
+    | Emp -> append Emp ys, append Emp zs
     | Snoc (xs, x) ->
       match f x with
       | Either.Left y ->
-        let ys, zs = go xs in
-        Snoc (ys, y), zs
+        (go[@tailcall]) xs (y :: ys) zs
       | Either.Right z ->
-        let ys, zs = go xs in
-        ys, Snoc (zs, z)
-  in go
+        (go[@tailcall]) xs ys (z :: zs)
+  in go xs [] []
 
 let rec split =
   function

src/BwdNoLabels.mli

@@ -141,10 +141,8 @@ val find_all : ('a -> bool) -> 'a t -> 'a t
 
 val filteri : (int -> 'a -> bool) -> 'a t -> 'a t
 
-(** Not tail-recursive. *)
 val partition : ('a -> bool) -> 'a t -> 'a t * 'a t
 
-(** Not tail-recursive. *)
 val partition_map : ('a -> ('b, 'c) Either.t) -> 'a t -> 'b t * 'c t
 
 (** {1 Lists of pairs} *)