[FEATURE EMBER_GLIMMER_FN_HELPER] Initial implementation of fn helper.

As proposed in https://emberjs.github.io/rfcs/0470-fn-helper.html.

packages/@ember/-internals/glimmer/lib/helpers/fn.ts

@@ -0,0 +1,18 @@
+import { Arguments, VM } from '@glimmer/runtime';
+import { InternalHelperReference } from '../utils/references';
+import { ICapturedArguments } from '@glimmer/runtime/dist/types/lib/vm/arguments';
+import { assert } from '@ember/debug';
+
+function fnHelper({ positional }: ICapturedArguments) {
+  assert(`You must pass a function as the \`fn\` helpers first argument, you passed ${positional.at(0).value()}`, typeof positional.at(0).value() === 'function');
+
+  return () => {
+    let [fn, ...args] = positional.value();
+
+    return fn!['apply'](null, args);
+  };
+}
+
+export default function(_vm: VM, args: Arguments) {
+  return new InternalHelperReference(fnHelper, args.capture());
+}

packages/@ember/-internals/glimmer/lib/resolver.ts

@@ -4,6 +4,7 @@ import { LookupOptions, Owner, setOwner } from '@ember/-internals/owner';
 import { lookupComponent, lookupPartial, OwnedTemplateMeta } from '@ember/-internals/views';
 import {
   EMBER_GLIMMER_ANGLE_BRACKET_BUILT_INS,
+  EMBER_GLIMMER_FN_HELPER,
   EMBER_MODULE_UNIFICATION,
 } from '@ember/canary-features';
 import { assert } from '@ember/debug';
@@ -32,6 +33,7 @@ import { default as action } from './helpers/action';
 import { default as array } from './helpers/array';
 import { default as concat } from './helpers/concat';
 import { default as eachIn } from './helpers/each-in';
+import { default as fn } from './helpers/fn';
 import { default as get } from './helpers/get';
 import { default as hash } from './helpers/hash';
 import { inlineIf, inlineUnless } from './helpers/if-unless';
@@ -61,7 +63,11 @@ function makeOptions(moduleName: string, namespace?: string): LookupOptions {
   };
 }
 
-const BUILTINS_HELPERS = {
+interface IBuiltInHelpers {
+  [name: string]: Helper | undefined;
+}
+
+const BUILTINS_HELPERS: IBuiltInHelpers = {
   if: inlineIf,
   action,
   array,
@@ -82,8 +88,13 @@ const BUILTINS_HELPERS = {
   '-mount': mountHelper,
   '-outlet': outletHelper,
   '-assert-implicit-component-helper-argument': componentAssertionHelper,
+  fn: undefined
 };
 
+if (EMBER_GLIMMER_FN_HELPER) {
+  BUILTINS_HELPERS.fn = fn;
+}
+
 const BUILTIN_MODIFIERS = {
   action: { manager: new ActionModifierManager(), state: null },
 };
@@ -96,9 +107,7 @@ export default class RuntimeResolver implements IRuntimeResolver<OwnedTemplateMe
   ];
   private objToHandle = new WeakMap<any, number>();
 
-  private builtInHelpers: {
-    [name: string]: Helper | undefined;
-  } = BUILTINS_HELPERS;
+  private builtInHelpers: IBuiltInHelpers = BUILTINS_HELPERS;
 
   private builtInModifiers: {
     [name: string]: ModifierDefinition;

packages/@ember/-internals/glimmer/tests/integration/helpers/fn-test.js

@@ -0,0 +1,55 @@
+import { EMBER_GLIMMER_FN_HELPER } from '@ember/canary-features';
+import { RenderingTestCase, moduleFor, runTask } from 'internal-test-helpers';
+
+import { set } from '@ember/-internals/metal';
+
+if (EMBER_GLIMMER_FN_HELPER) {
+  moduleFor(
+    'Helpers test: {{fn}}',
+    class extends RenderingTestCase {
+      beforeEach() {
+        this.registerHelper('invoke', function([fn]) {
+          return fn();
+        });
+      }
+
+      '@test invokes the provided function with arguments'() {
+        this.render(`{{invoke (fn this.myFunc this.arg1 this.arg2)}}`, {
+          myFunc(arg1, arg2) {
+            return `arg1: ${arg1}, arg2: ${arg2}`;
+          },
+
+          arg1: 'foo',
+          arg2: 'bar',
+        });
+
+        this.assertText('arg1: foo, arg2: bar');
+
+        this.assertStableRerender();
+
+        runTask(() => set(this.context, 'arg1', 'qux'));
+        this.assertText('arg1: qux, arg2: bar');
+
+        runTask(() => set(this.context, 'arg2', 'derp'));
+        this.assertText('arg1: qux, arg2: derp');
+
+        runTask(() => {
+          set(this.context, 'arg1', 'foo');
+          set(this.context, 'arg2', 'bar');
+        });
+
+        this.assertText('arg1: foo, arg2: bar');
+      }
+
+      '@test asserts if the first argument is not a function'() {
+        expectAssertion(() => {
+          this.render(`{{invoke (fn this.myFunc this.arg1 this.arg2)}}`, {
+            myFunc: null,
+            arg1: 'foo',
+            arg2: 'bar',
+          });
+        }, /You must pass a function as the `fn` helpers first argument, you passed null/);
+      }
+    }
+  );
+}

packages/@ember/canary-features/index.ts

@@ -22,6 +22,7 @@ export const DEFAULT_FEATURES = {
   EMBER_GLIMMER_ANGLE_BRACKET_NESTED_LOOKUP: true,
   EMBER_ROUTING_BUILD_ROUTEINFO_METADATA: true,
   EMBER_NATIVE_DECORATOR_SUPPORT: true,
+  EMBER_GLIMMER_FN_HELPER: null,
 };
 
 /**
@@ -88,3 +89,4 @@ export const EMBER_ROUTING_BUILD_ROUTEINFO_METADATA = featureValue(
   FEATURES.EMBER_ROUTING_BUILD_ROUTEINFO_METADATA
 );
 export const EMBER_NATIVE_DECORATOR_SUPPORT = featureValue(FEATURES.EMBER_NATIVE_DECORATOR_SUPPORT);
+export const EMBER_GLIMMER_FN_HELPER = featureValue(FEATURES.EMBER_GLIMMER_FN_HELPER);