Add a tail-recursive implementation of List.combine

Closes #108.

AUTHORS.adoc

@@ -19,3 +19,4 @@
 - Malcolm Matalka (`orbitz`)
 - David Sheets (@dsheets)
 - Glenn Slotte (glennsl)
+- @LemonBoy

src/core/CCList.ml

@@ -339,6 +339,26 @@ let partition_map f l =
   assert_equal [1;3] l2
 *)
 
+let combine l1 l2 =
+  let rec direct i l1 l2 = match l1, l2 with
+    | ([], []) -> []
+    | _ when i=0 -> safe l1 l2 []
+    | (x1::l1', x2::l2') -> (x1, x2) :: direct (i-1) l1' l2'
+    | (_, _) -> invalid_arg "CCList.combine"
+  and safe l1 l2 acc = match l1, l2 with
+    | ([], []) -> List.rev acc
+    | (x1::l1', x2::l2') -> safe l1' l2' @@ (x1, x2) :: acc
+    | (_, _) -> invalid_arg "CCList.combine"
+  in
+  direct direct_depth_default_ l1 l2
+
+(*$T
+  try ignore (combine [1] []); false with Invalid_argument _ -> true
+  try ignore (combine (1--1001) (1--1002)); false with Invalid_argument _ -> true
+  combine [1;2;3] [3;2;1] = List.combine [1;2;3] [3;2;1]
+  combine (1 -- 100_000) (1 -- 100_000) = List.combine (1 -- 100_000) (1 -- 100_000)
+*)
+
 let return x = [x]
 
 let (>>=) l f = flat_map f l

src/core/CCList.mli

@@ -72,6 +72,10 @@ val init : int -> (int -> 'a) -> 'a t
 (** Similar to {!Array.init}
     @since 0.6 *)
 
+val combine : ('a list) -> ('b list) -> ('a * 'b) list
+(** Similar to {!List.combine} but tail-recursive
+    @since NEXT_RELEASE *)
+
 val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int
 
 val equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool