feat: add init (#20)

src/BwdLabels.ml

@@ -14,6 +14,8 @@ let nth = BwdNoLabels.nth
 
 let nth_opt = BwdNoLabels.nth_opt
 
+let[@inline] init ~len ~f = BwdNoLabels.init len f
+
 let append = BwdNoLabels.append
 
 let prepend = BwdNoLabels.prepend

src/BwdLabels.mli

@@ -10,8 +10,8 @@
       {ul
       {- [cons] was replaced by {!val:snoc}.}
       {- {!val:append} was replaced by a new version that performs the "textual order yoga".}
-      {- The indices count from the right rather than the left:
-         {!val:nth}, {!val:nth_opt}, {!val:iteri}, {!val:mapi}, and {!val:filteri}.}
+      {- All indices count from the right rather than the left:
+         {!val:nth}, {!val:nth_opt}, {!val:init}, {!val:iteri}, {!val:mapi}, and {!val:filteri}.}
       {- All iteration functions work from the right rather than the left:
          {!val:iter}, {!val:map}, {!val:fold_left}, {!val:fold_right}, {!val:exists}, {!val:mem},
          {!val:find}, {!val:filter}, and other similar functions.}}}
@@ -41,6 +41,7 @@ val compare_length_with : 'a t -> len:int -> int
 val snoc : 'a t -> 'a -> 'a t
 val nth : 'a t -> int -> 'a
 val nth_opt : 'a t -> int -> 'a option
+val init : len:int -> f:(int -> 'a) -> 'a t
 val append : 'a t -> 'a list -> 'a t
 val prepend : 'a t -> 'a list -> 'a list
 

src/BwdNoLabels.ml

@@ -52,6 +52,17 @@ let nth_opt xs i =
       | Snoc (xs, _), i -> (go[@tailcall]) (xs, i - 1)
     in go (xs, i)
 
+let init len f =
+  if len < 0 then invalid_arg "Bwd.init"
+  else
+    let[@tail_mod_cons] rec go i =
+      if i >= len then Emp
+      else
+        let v = f i in
+        Snoc ((go[@tailcall]) (i+1), v)
+    in
+    go 0
+
 let append xs ys =
   let rec go =
     function

src/BwdNoLabels.mli

@@ -10,8 +10,8 @@
       {ul
       {- [cons] was replaced by {!val:snoc}.}
       {- {!val:append} was replaced by a new version that performs the "textual order yoga".}
-      {- The indices count from the right rather than the left:
-         {!val:nth}, {!val:nth_opt}, {!val:iteri}, {!val:mapi}, and {!val:filteri}.}
+      {- All indices count from the right rather than the left:
+         {!val:nth}, {!val:nth_opt}, {!val:init}, {!val:iteri}, {!val:mapi}, and {!val:filteri}.}
       {- All iteration functions work from the right rather than the left:
          {!val:iter}, {!val:map}, {!val:fold_left}, {!val:fold_right}, {!val:exists}, {!val:mem},
          {!val:find}, {!val:filter}, and other similar functions.}}}
@@ -41,6 +41,7 @@ val compare_length_with : 'a t -> int -> int
 val snoc : 'a t -> 'a -> 'a t
 val nth : 'a t -> int -> 'a
 val nth_opt : 'a t -> int -> 'a option
+val init : int -> (int -> 'a) -> 'a t
 val append : 'a t -> 'a list -> 'a t
 val prepend : 'a t -> 'a list -> 'a list
 

test/ListAsBwdLabels.ml

@@ -17,6 +17,10 @@ let nth_opt xs i =
   try L.nth_opt (L.rev xs) i with
   | Invalid_argument _ -> invalid_arg "Bwd.nth_opt"
 
+let init ~len ~f =
+  try L.init ~len ~f:(fun i -> f ((len-1) - i)) with
+  | Invalid_argument _ -> invalid_arg "Bwd.init"
+
 let append xs ys = xs @ ys
 
 let prepend xs ys = xs @ ys

test/TestBwdLabels.ml

@@ -53,6 +53,11 @@ let test_nth_opt =
        trap (fun () -> B.nth_opt (of_list xs) i)
        =
        trap (fun () -> L.nth_opt xs i))
+let test_init =
+  Q.Test.make ~count ~name:"init"
+    Q.Gen.(pair small_signed_int (Q.fun1 Q.Observable.int (opt int)))
+    ~print:Q.Print.(pair int Q.Fn.print)
+    (fun (len, Fun (_, f)) -> trap (fun () -> to_list (B.init ~len ~f)) = trap (fun () -> L.init ~len ~f))
 let test_append =
   Q.Test.make ~count ~name:"append" Q.Gen.(pair (small_list int) (small_list int))
     ~print:Q.Print.(pair (list int) (list int))
@@ -299,6 +304,7 @@ let () =
       test_snoc;
       test_nth;
       test_nth_opt;
+      test_init;
       test_append;
       test_prepend;
       test_equal;