Use keyword for description, rules for everything else

When producing FSH for an Invariant, use the Description keyword for the
description element. For all other metadata elements, use assignment
rules instead. These rules will always come before any other rules on
the Invariant.

src/exportable/ExportableInvariant.ts

@@ -1,10 +1,36 @@
 import { fshtypes } from 'fsh-sushi';
 import { Exportable, ExportableAssignmentRule, ExportableInsertRule } from '.';
+import { fshifyString } from './common';
+import { EOL } from 'os';
 
 export class ExportableInvariant extends fshtypes.Invariant implements Exportable {
   rules: (ExportableAssignmentRule | ExportableInsertRule)[];
 
   constructor(name: string) {
     super(name);
   }
+
+  metadataToFSH(): string {
+    const resultLines: string[] = [];
+    resultLines.push(`Invariant: ${this.name}`);
+    if (this.description) {
+      // Description can be a multiline string.
+      // If it contains newline characters, treat it as a multiline string.
+      if (this.description.indexOf('\n') > -1) {
+        resultLines.push(`Description: """${this.description}"""`);
+      } else {
+        resultLines.push(`Description: "${fshifyString(this.description)}"`);
+      }
+    }
+    if (this.severity) {
+      resultLines.push(`* severity = ${this.severity}`);
+    }
+    if (this.expression) {
+      resultLines.push(`* expression = "${fshifyString(this.expression)}"`);
+    }
+    if (this.xpath) {
+      resultLines.push(`* xpath = "${fshifyString(this.xpath)}"`);
+    }
+    return resultLines.join(EOL);
+  }
 }

test/exportable/ExportableInvariant.test.ts

@@ -1,6 +1,6 @@
 import { EOL } from 'os';
 import { fshtypes } from 'fsh-sushi';
-import { ExportableInvariant } from '../../src/exportable';
+import { ExportableAssignmentRule, ExportableInvariant } from '../../src/exportable';
 
 describe('ExportableInvariant', () => {
   it('should export the simplest invariant', () => {
@@ -21,9 +21,9 @@ describe('ExportableInvariant', () => {
     const expectedResult = [
       'Invariant: inv-2',
       'Description: "This is an important condition."',
-      'Severity: #error',
-      'Expression: "requirement.exists()"',
-      'XPath: "f:requirement"'
+      '* severity = #error',
+      '* expression = "requirement.exists()"',
+      '* xpath = "f:requirement"'
     ].join(EOL);
     const result = input.toFSH();
     expect(result).toBe(expectedResult);
@@ -39,9 +39,38 @@ describe('ExportableInvariant', () => {
     const expectedResult = [
       'Invariant: inv-3',
       'Description: """Please do this.\nPlease always do this with a \\ character."""',
-      'Severity: #warning',
-      'Expression: "requirement.contains(\\"\\\\\\")"',
-      'XPath: "f:requirement"'
+      '* severity = #warning',
+      '* expression = "requirement.contains(\\"\\\\\\")"',
+      '* xpath = "f:requirement"'
+    ].join(EOL);
+    const result = input.toFSH();
+    expect(result).toBe(expectedResult);
+  });
+
+  it('should produce FSH for an invariant with additional rules', () => {
+    const input = new ExportableInvariant('inv-4');
+    input.description = 'This is an important condition.';
+    input.severity = new fshtypes.FshCode('error');
+    input.expression = 'requirement.exists()';
+    input.xpath = 'f:requirement';
+
+    const requirements = new ExportableAssignmentRule('requirements');
+    requirements.value = 'This is necessary because it is important.';
+    const extensionUrl = new ExportableAssignmentRule('human.extension[0].url');
+    extensionUrl.value = 'http://example.org/SomeExtension';
+    const extensionValue = new ExportableAssignmentRule('human.extension[0].valueString');
+    extensionValue.value = 'ExtensionValue';
+    input.rules.push(requirements, extensionUrl, extensionValue);
+
+    const expectedResult = [
+      'Invariant: inv-4',
+      'Description: "This is an important condition."',
+      '* severity = #error',
+      '* expression = "requirement.exists()"',
+      '* xpath = "f:requirement"',
+      '* requirements = "This is necessary because it is important."',
+      '* human.extension[0].url = "http://example.org/SomeExtension"',
+      '* human.extension[0].valueString = "ExtensionValue"'
     ].join(EOL);
     const result = input.toFSH();
     expect(result).toBe(expectedResult);