Array.fromFormula with docs and tests

src/Core__Array.mjs

@@ -138,6 +138,22 @@ function findMap(arr, f) {
   };
 }
 
+function fromFormula(state, generator) {
+  var result = [];
+  var state$1 = state;
+  var $$break = false;
+  while(!$$break) {
+    var match = Curry._1(generator, state$1);
+    if (match !== undefined) {
+      result.push(match[0]);
+      state$1 = match[1];
+    } else {
+      $$break = true;
+    }
+  };
+  return result;
+}
+
 export {
   make ,
   fromInitializer ,
@@ -155,5 +171,6 @@ export {
   shuffle ,
   shuffleInPlace ,
   findMap ,
+  fromFormula ,
 }
 /* No side effect */

src/Core__Array.res

@@ -214,3 +214,19 @@ let findMap = (arr, f) => {
 }
 
 @send external at: (array<'a>, int) => option<'a> = "at"
+
+let fromFormula = (state, generator) => {
+  let result = []
+  let state = ref(state)
+  let break = ref(false)
+  while !break.contents {
+    switch generator(state.contents) {
+    | None => break := true
+    | Some((item, nextState)) => {
+        result->push(item)
+        state := nextState
+      }
+    }
+  }
+  result
+}

src/Core__Array.resi

@@ -955,3 +955,19 @@ let findMap: (array<'a>, 'a => option<'b>) => option<'b>
 */
 @send
 external at: (array<'a>, int) => option<'a> = "at"
+
+/**
+`fromFormula(state, f)` creates an array that contains the elements generated by a computation. The function `f` is first passed `state` and then is repeatedly called to build the array. When `f` returns a tuple of `Some(item, state)`, `item` is pushed onto the result array, and `state` is fed into the next computation. When `f` returns `None`, the computation is complete and the new array is returned.
+
+## Examples
+
+```rescript
+let f = i => i<100 ? Some(i, i*2) : None
+Array.fromFormula(1, f) // [1, 2, 4, 8, 16, 32, 64]
+Array.fromFormula(100, f) // []
+
+let g = i => i >= 0.0 ? Some(i, i -. 0.5) : None 
+Array.fromFormula(3.0, g) // [3.0, 2.5, 2.0, 1.5, 1.0, 0.5, 0.0]
+```
+*/
+let fromFormula: ('state, 'state => option<('a, 'state)>) => array<'a>

test/ArrayTests.mjs

@@ -401,7 +401,71 @@ Test.run([
 
           })), eq, undefined);
 
+function generator(i) {
+  if (i < 100) {
+    return [
+            i,
+            (i << 1)
+          ];
+  }
+
+}
+
+Test.run([
+      [
+        "ArrayTests.res",
+        104,
+        20,
+        33
+      ],
+      "fromFormula"
+    ], Core__Array.fromFormula(1, generator), eq, [
+      1,
+      2,
+      4,
+      8,
+      16,
+      32,
+      64
+    ]);
+
+Test.run([
+      [
+        "ArrayTests.res",
+        105,
+        20,
+        33
+      ],
+      "fromFormula"
+    ], Core__Array.fromFormula(12, generator), eq, [
+      12,
+      24,
+      48,
+      96
+    ]);
+
+Test.run([
+      [
+        "ArrayTests.res",
+        106,
+        20,
+        33
+      ],
+      "fromFormula"
+    ], Core__Array.fromFormula(99, generator), eq, [99]);
+
+Test.run([
+      [
+        "ArrayTests.res",
+        107,
+        20,
+        33
+      ],
+      "fromFormula"
+    ], Core__Array.fromFormula(100, generator), eq, []);
+
 export {
   eq ,
+  generator ,
 }
 /*  Not a pure module */

test/ArrayTests.res

@@ -98,3 +98,10 @@ Test.run(
   eq,
   None,
 )
+
+let generator = i => i < 100 ? Some(i, i * 2) : None
+
+Test.run(__POS_OF__("fromFormula"), Array.fromFormula(1, generator), eq, [1, 2, 4, 8, 16, 32, 64])
+Test.run(__POS_OF__("fromFormula"), Array.fromFormula(12, generator), eq, [12, 24, 48, 96])
+Test.run(__POS_OF__("fromFormula"), Array.fromFormula(99, generator), eq, [99])
+Test.run(__POS_OF__("fromFormula"), Array.fromFormula(100, generator), eq, [])

test/TestSuite.mjs

@@ -26,6 +26,8 @@ var Concurrently = PromiseTest.Concurrently;
 
 var panicTest = ErrorTests.panicTest;
 
+var generator = ArrayTests.generator;
+
 var eq = IntTests.eq;
 
 var $$catch = IntTests.$$catch;
@@ -41,6 +43,7 @@ export {
   Catching ,
   Concurrently ,
   panicTest ,
+  generator ,
   eq ,
   $$catch ,
 }