From 26177f7e25370d27e6e2f04857269129a74d1f44 Mon Sep 17 00:00:00 2001
From: Robert Sesek <rsesek@bluestatic.org>
Date: Wed, 19 Feb 2020 21:29:09 -0500
Subject: [PATCH] Make SupportsMultipleCopies an interface.

---
 src/Form.ts           | 12 ++++++++----
 src/TaxReturn.test.ts |  6 +++---
 src/TaxReturn.ts      |  4 ++--
 3 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/Form.ts b/src/Form.ts
index 507a611..07e9ccc 100644
--- a/src/Form.ts
+++ b/src/Form.ts
@@ -14,10 +14,6 @@ export default abstract class Form<I = unknown> {
 
   protected abstract getLines(): Line<any>[];
 
-  get allowMultipleCopies(): boolean {
-    return false;
-  }
-
   private addLine(line: Line<any>) {
     if (line.form !== undefined) {
       throw new InconsistencyError('Line is already in a Form');
@@ -47,3 +43,11 @@ export default abstract class Form<I = unknown> {
     return this._input[name];
   }
 };
+
+export interface SupportsMultipleCopies extends Form {
+  aggregate(forms: Form[]): this;
+};
+
+export function supportsMultipleCopies(f: object): f is SupportsMultipleCopies {
+  return f instanceof Form && 'aggregate' in f;
+};
diff --git a/src/TaxReturn.test.ts b/src/TaxReturn.test.ts
index 74c82f4..852ae65 100644
--- a/src/TaxReturn.test.ts
+++ b/src/TaxReturn.test.ts
@@ -1,6 +1,6 @@
 import TaxReturn from './TaxReturn';
 import { Person } from './Person';
-import Form from './Form';
+import Form, { SupportsMultipleCopies } from './Form';
 import { NotFoundError, InconsistencyError } from './Errors';
 
 test('constructor', () => {
@@ -67,10 +67,10 @@ test('single-copy forms', () => {
 });
 
 test('multiple-copy forms', () => {
-  class TestForm extends Form {
+  class TestForm extends Form implements SupportsMultipleCopies {
     get name(): string { return 'Test Form'; }
 
-    get allowMultipleCopies(): boolean { return true; }
+    aggregate(forms: Form[]): this { return null; }
 
     protected getLines() { return []; }
   };
diff --git a/src/TaxReturn.ts b/src/TaxReturn.ts
index 18a0bbb..7bab4d8 100644
--- a/src/TaxReturn.ts
+++ b/src/TaxReturn.ts
@@ -1,4 +1,4 @@
-import Form from './Form';
+import Form, { SupportsMultipleCopies, supportsMultipleCopies } from './Form';
 import { Person, Relation } from './Person';
 import { NotFoundError, InconsistencyError, UnsupportedFeatureError } from './Errors';
 
@@ -38,7 +38,7 @@ export default class TaxReturn {
   }
 
   addForm(form: Form) {
-    if (!form.allowMultipleCopies) {
+    if (!supportsMultipleCopies(form)) {
       const other = this.getForms(form.name);
       if (other.length > 0) {
         throw new InconsistencyError(`Cannot have more than one type of form ${form.name}`);
-- 
2.22.5