add list.insert

src/prelude/list.ts

@@ -73,6 +73,13 @@ export const list = {
     const realItems = items.map(item => item.value)
     return toValue(realList.splice(realStart, realDeleteCount, ...realItems))
   },
+  insert: (list: Value, index: Value, item: Value) => {
+    if (list.type !== 'array') return toNull()
+    const realList = list.value as any[]
+    const realIndex = index.value as number
+    realList.splice(realIndex, 0, item)
+    return toValue(realList.length)
+  },
 
   // sequence operations
   reverse: (list: any[]) => list.slice().reverse(),
@@ -143,3 +150,4 @@ export const list = {
   ; (list.pop as any).raw = true
   ; (list.shift as any).raw = true
   ; (list.unshift as any).raw = true
+  ; (list.insert as any).raw = true

src/prelude/tests/prelude.test.ts

@@ -349,6 +349,22 @@ describe('collections', () => {
     await expect(`arr = [1 2 3 4 5]; list.splice arr 3 2; arr`).toEvaluateTo([1, 2, 3])
   })
 
+  test('list.insert adds element at index and mutates array', async () => {
+    await expect(`arr = [1 2 4 5]; list.insert arr 2 3; arr`).toEvaluateTo([1, 2, 3, 4, 5])
+  })
+
+  test('list.insert returns array length', async () => {
+    await expect(`list.insert [1 2 4] 2 3`).toEvaluateTo(4)
+  })
+
+  test('list.insert at start', async () => {
+    await expect(`arr = [2 3]; list.insert arr 0 1; arr`).toEvaluateTo([1, 2, 3])
+  })
+
+  test('list.insert at end', async () => {
+    await expect(`arr = [1 2]; list.insert arr 2 99; arr`).toEvaluateTo([1, 2, 99])
+  })
+
   test('list.sort with no callback sorts ascending', async () => {
     await expect(`list.sort [3 1 4 1 5] null`).toEvaluateTo([1, 1, 3, 4, 5])
   })