Add map method to List

src/List.ts

@@ -1,4 +1,4 @@
-import { Option } from './constants'
+import { Transform, Option } from './constants'
 import ArrayNode from './persistentVector/ArrayNode'
 import push from './listHelpers/push'
 import pop from './listHelpers/pop'
@@ -73,6 +73,24 @@ export default class List<T> {
     return pop<T>(this)
   }
 
+  map<G>(
+    transform: Transform<number, Option<T>, Option<G>>
+  ): List<G> {
+    const tail = this.tail.map<G>(transform, this.owner) as LeafNode<G>
+    const root = !this.root ?
+      undefined :
+      this.root.map<G>(transform, this.owner) as ArrayNode<G>
+
+    if (this.owner) {
+      const res = (this as List<any>)
+      res.root = root
+      res.tail = tail
+      return (res as List<G>)
+    }
+
+    return makeList<G>(tail, root)
+  }
+
   clear(): List<T> {
     return emptyList<T>()
   }

src/persistentVector/ArrayNode.ts

@@ -1,5 +1,5 @@
 import Node from './Node'
-import { Option, BUCKET_SIZE } from '../constants'
+import { Transform, Option, BUCKET_SIZE } from '../constants'
 import { replaceValue, copyArray, push, pop } from '../util/array'
 import { maskHash } from '../util/bitmap'
 import LeafNode from './LeafNode'
@@ -60,6 +60,31 @@ export default class ArrayNode<T> {
     )
   }
 
+  map<G>(
+    transform: Transform<number, Option<T>, Option<G>>,
+    owner?: Object
+  ): Node<G> {
+    const length = this.content.length
+    const content: Node<G>[] = new Array(length)
+    for (let i = 0; i < length; i++) {
+      const node = this.content[i]
+      content[i] = node.map<G>(transform, owner)
+    }
+
+    if (owner && owner === this.owner) {
+      const res = (this as ArrayNode<any>)
+      res.content = content
+      return (res as ArrayNode<G>)
+    }
+
+    return new ArrayNode(
+      this.level,
+      content,
+      this.size,
+      owner
+    )
+  }
+
   pushLeafNode(node: LeafNode<T>, owner?: Object): ArrayNode<T> {
     let content: Node<T>[]
     let size: number

src/persistentVector/LeafNode.ts

@@ -1,4 +1,5 @@
-import { Option } from '../constants'
+import Node from './Node'
+import { Transform, Option } from '../constants'
 import { replaceValue, spliceIn, spliceOut } from '../util/array'
 import { maskHash } from '../util/bitmap'
 
@@ -47,6 +48,26 @@ export default class LeafNode<T> {
     return new LeafNode<T>(content, owner)
   }
 
+  map<G>(
+    transform: Transform<number, Option<T>, Option<G>>,
+    owner?: Object
+  ): Node<G> {
+    const length = this.content.length
+    const content: Option<G>[] = new Array(length)
+    for (let i = 0; i < length; i++) {
+      const value = this.content[i]
+      content[i] = transform(value, i)
+    }
+
+    if (owner && owner === this.owner) {
+      const res = (this as LeafNode<any>)
+      res.content = content
+      return (res as LeafNode<G>)
+    }
+
+    return new LeafNode(content, owner)
+  }
+
   push(value: Option<T>, owner?: Object): LeafNode<T> {
     const content = spliceIn(this.content, this.size, value)