refactor(kernel): ensure optional args comes at the end

packages/ace/src/Kernel/index.ts

@@ -7,7 +7,7 @@
 * file that was distributed with this source code.
 */
 
-import { CommandConstructorContract, CommandFlag, GlobalFlagHandler } from '../Contracts'
+import { CommandConstructorContract, CommandFlag, GlobalFlagHandler, CommandArg } from '../Contracts'
 import { Parser } from '../Parser'
 
 /**
@@ -18,11 +18,25 @@ export class Kernel {
   public commands: { [name: string]: CommandConstructorContract } = {}
   public flags: { [name: string]: CommandFlag & { handler: GlobalFlagHandler } } = {}
 
+  private _validateArgs (command: CommandConstructorContract) {
+    let optionalArg: CommandArg
+    command.args.forEach((arg) => {
+      if (optionalArg && arg.required) {
+        throw new Error(`Required argument {${arg.name}} cannot come after optional argument {${optionalArg.name}}`)
+      }
+
+      if (!arg.required) {
+        optionalArg = arg
+      }
+    })
+  }
+
   /**
    * Register an array of commands
    */
   public register (commands: CommandConstructorContract[]): this {
     commands.forEach((command) => {
+      this._validateArgs(command)
       this.commands[command.commandName] = command
     })
 

packages/ace/test/kernel.spec.ts

@@ -11,7 +11,34 @@ import * as test from 'japa'
 import { Kernel } from '../src/Kernel'
 import { BaseCommand } from '../src/BaseCommand'
 
-test.group('Kernel', () => {
+test.group('Kernel | register', () => {
+  test('raise error when required argument comes after optional argument', (assert) => {
+    class Greet extends BaseCommand {
+      public static commandName = 'greet'
+      public static args = [
+        {
+          name: 'name',
+          required: false,
+        },
+        {
+          name: 'age',
+          required: true,
+        },
+      ]
+    }
+
+    const kernel = new Kernel()
+    const fn = () => kernel.register([Greet])
+    assert.throw(fn, 'Required argument {age} cannot come after optional argument {name}')
+  })
+
+  test('return null when unable to find command', (assert) => {
+    const kernel = new Kernel()
+    assert.isNull(kernel.find(['greet']))
+  })
+})
+
+test.group('Kernel | find', () => {
   test('find relevant command from the commands list', (assert) => {
     class Greet extends BaseCommand {
       public static commandName = 'greet'
@@ -27,7 +54,9 @@ test.group('Kernel', () => {
     const kernel = new Kernel()
     assert.isNull(kernel.find(['greet']))
   })
+})
 
+test.group('Kernel | handle', () => {
   test('raise exception when required argument is missing', async (assert) => {
     assert.plan(1)