iterateWhile

Author: jmagaram

CHANGELOG.md

@@ -1,3 +1,7 @@
+## Version 4.3.0
+
+- `Seq.iterateWhile` generates a sequence until the interation function returns `None`.
+
 ## Version 4.2.0
 
 - `Seq.at` returns the item at a specified zero-based index.

README.md

@@ -62,6 +62,7 @@ let fromArray: (~start: int=?, ~end: int=?, array<'a>) => t<'a>
 let fromList: list<'a> => t<'a>
 let init: (int, int => 'a) => t<'a>
 let iterate: ('a, 'a => 'a) => t<'a>
+let iterateWhile: ('a, 'a => option<'a>) => t<'a>
 let once: 'a => t<'a>
 let onceWith: (unit => 'a) => t<'a>
 let permutations: (t<'a>, int) => t<(int, t<'a>)>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@jmagaram/rescript-seq",
-  "version": "4.2.0",
+  "version": "4.3.0",
   "module": "",
   "description": "Lazy sequences for ReScript.",
   "keywords": [

src/Seq.res

@@ -943,3 +943,12 @@ let at = (xx, inx) => {
     }
   go(xx, 0)
 }
+
+let iterateWhile = (seed, f) => () => {
+  let rec go = seed => () =>
+    switch f(seed) {
+    | None => End
+    | Some(seed) => Next(seed, go(seed))
+    }
+  Next(seed, go(seed))
+}

src/Seq.resi

@@ -70,11 +70,17 @@ let forever: 'a => t<'a>
 let foreverWith: (unit => 'a) => t<'a>
 
 /**
-`iterate(seed, f)` creates an infinite sequence starting with `seed` and
-repeatedly calling function `f` on it.
+`iterate(seed, f)` creates an infinite sequence that starts with `seed`.
+Subsequent items are generated by calling `f` on the previous item.
 */
 let iterate: ('a, 'a => 'a) => t<'a>
 
+/**
+`iterate(seed, f)` creates a sequence that starts with `seed`. Subsequent items
+are generated by calling `f` on the previous item until `None`.
+*/
+let iterateWhile: ('a, 'a => option<'a>) => t<'a>
+
 /**
 `cycle` is the sequence that consists of an infinite number of repetitions of
 the input sequence. If the original sequence is empty, the resulting sequence

tests/Seq__SeqTests.res

@@ -624,12 +624,36 @@ let iterateTests = makeSeqEqualsTests(
 )->Array.concat([
   willNotThrow(~title="iterate", ~expectation="lazy", () =>
     S.iterate(0, i => {
-      Exn.raiseError("Boom!")->raise->ignore
+      Exn.raiseError("Boom!")->ignore
       i
     })
   ),
 ])
 
+let iterateWhileTests = {
+  let increment = (max, seed) => {
+    let next = seed + 1
+    next <= max ? Some(next) : None
+  }
+  makeSeqEqualsTests(
+    ~title="iterateWhile",
+    [
+      (S.iterateWhile(4, increment(10, _)), [4, 5, 6, 7, 8, 9, 10], ""),
+      (S.iterateWhile(9, increment(10, _)), [9, 10], ""),
+      (S.iterateWhile(10, increment(10, _)), [10], ""),
+      (S.iterateWhile(999, increment(10, _)), [999], "Always includes seed"),
+      (S.iterateWhile(1, increment(1_000_000, _))->Seq.drop(999_998), [999_999, 1_000_000], ""),
+    ],
+  )->Array.concat([
+    willNotThrow(~title="iterateWhile", ~expectation="lazy", () =>
+      S.iterateWhile(0, i => {
+        Exn.raiseError("Boom!")->ignore
+        Some(i)
+      })
+    ),
+  ])
+}
+
 let fromArrayTests = {
   let basicTests = makeSeqEqualsTests(
     ~title="fromArray",
@@ -2230,6 +2254,7 @@ let tests =
     isEmptyTests,
     isSortedByTests,
     iterateTests,
+    iterateWhileTests,
     joinTests,
     lastTests,
     lengthTests,