add ARRAY filter methods

Author: duart38

builder/ARRAY.ts

@@ -41,6 +41,25 @@ export class ARR<I> extends LogicalStack<ARR<I>>
     return this;
   }
 
+  /**
+   * Performs a filter on the inner value regardless if stack is true or false.
+   * @param predicate 
+   * @returns 
+   */
+  public filter(predicate: (val: I, index?: number, arr?:I[])=>boolean): this {
+    this.value = this.value.filter(predicate);
+    return this;
+  }
+    /**
+   * Performs a filter if the previous chain items result in a "true".
+   * @param predicate 
+   * @returns 
+   */
+     public thenFilter(predicate: (val: I, index?: number, arr?:I[])=>boolean): this {
+       if(this.lastStackMatched) this.filter(predicate);
+      return this;
+    }
+
   /**
      * Checks if the current array length is of size 'n'
      * @param n 

tests/ARRAY_test.ts

@@ -8,6 +8,30 @@ import {
 } from "https://deno.land/std@0.90.0/testing/asserts.ts";
 import { ARRAY } from "../mod.ts";
 
+
+Deno.test("Test array filter", () => {
+  let called = false;
+  ARRAY([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]).filter((x)=>x == 1).do((arr)=>{
+    assertEquals(arr[0], 1);
+    called = true;
+  })
+  if (!called) fail("clause was never called");
+});
+Deno.test("Test array thenFilter", () => {
+  let called = false;
+  ARRAY([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
+  .isOfSize(0)
+  .thenFilter((x)=>{fail("incorrect clause"); return false})
+  .or()
+  .isOfSize(10)
+  .thenFilter((x)=>x == 1).do((arr)=>{
+    assertEquals(arr[0], 1);
+    assertEquals(arr.length, 1);
+    called = true;
+  })
+  if (!called) fail("clause was never called");
+});
+
 Deno.test("Test array append()", () => {
   let called = false;
   ARRAY(["hello"]).append("world").do((cv) => {