lib: add some list utilities

Adds `unfold` for constructing a list from a generating function, and
adds `range` for constructing a range of numbers.

Signed-off-by: Edward Pierzchalski <ed.pierzchalski@data61.csiro.au>

lib/ml-helpers/ListExtras.ML

@@ -28,6 +28,29 @@ sig
     found element.
   *)
   val find_index: ('a -> bool) -> 'a list -> int option;
+
+  (*
+    `unfold f init` repeatedly calls `f` to construct elements
+    of a list, until it returns NONE. For example:
+
+    `unfold
+      (fn idx => if idx < 10 then SOME ("hello", idx + 1) else NONE)
+      7
+      = ["hello", "hello", "hello"]`
+
+    `unfold (fn x => SOME (1, x)) anything` never returns.
+
+    `unfold (K NONE) anything = []`
+  *)
+  val unfold: ('acc -> ('item * 'acc) option) -> 'acc -> 'item list;
+
+  (* `range from to` produces the list of integers between
+     `from` (inclusive) and `to` (exclusive). For example:
+
+     `range 3 5 = [3, 4]`
+     `range ~1 2 = [~1, 0, 1]`
+  *)
+  val range: int -> int -> int list;
 end
 
 structure ListExtras: LIST_EXTRAS =
@@ -44,4 +67,12 @@ fun map_find_first (f: 'a -> 'b option) (xs: 'a list): 'b option =
 fun find_index test =
   Library.get_index (fn x => if test x then SOME () else NONE) #> Option.map fst
 
+fun unfold (f: 'acc -> ('item * 'acc) option) (acc: 'acc) =
+    case f acc of
+      NONE => []
+    | SOME (item, new_acc) => item :: unfold f new_acc;
+
+fun range from to =
+    unfold (fn i => if i < to then SOME (i, i + 1) else NONE) from;
+
 end