test: add integration tests for args and $varname template variables

- args: [name] consumes CLI positional args as template vars
- Body with only {{ name }} effectively becomes the CLI arg
- $varname: Default provides defaults, --varname overrides
- Missing template variable detection

Author: johnlindquist

src/args.test.ts

@@ -0,0 +1,170 @@
+import { expect, test, describe } from "bun:test";
+import { parseFrontmatter } from "./parse";
+import { substituteTemplateVars, extractTemplateVars } from "./template";
+
+/**
+ * Tests for the args system:
+ * - args: [varname] in frontmatter defines positional arguments
+ * - CLI positional args fill template variables in the body
+ * - {{ varname }} in body gets replaced with the CLI arg value
+ */
+
+describe("args positional argument flow", () => {
+  test("args field defines template variables consumed from CLI", () => {
+    const content = `---
+args: [message]
+---
+Say: {{ message }}`;
+
+    const { frontmatter, body } = parseFrontmatter(content);
+
+    // Simulate CLI: ma file.md "Hello World"
+    const cliArgs = ["Hello World"];
+    const templateVars: Record<string, string> = {};
+
+    // Consume positional args (same logic as index.ts)
+    if (frontmatter.args && Array.isArray(frontmatter.args)) {
+      for (let i = 0; i < frontmatter.args.length; i++) {
+        if (i < cliArgs.length) {
+          templateVars[frontmatter.args[i]] = cliArgs[i];
+        }
+      }
+    }
+
+    expect(templateVars).toEqual({ message: "Hello World" });
+
+    // Apply substitution
+    const result = substituteTemplateVars(body, templateVars);
+    expect(result).toBe("Say: Hello World");
+  });
+
+  test("multiple args consume multiple positional CLI arguments", () => {
+    const content = `---
+args: [name, action]
+---
+{{ name }} will {{ action }}`;
+
+    const { frontmatter, body } = parseFrontmatter(content);
+
+    // Simulate CLI: ma file.md "Alice" "run"
+    const cliArgs = ["Alice", "run"];
+    const templateVars: Record<string, string> = {};
+
+    if (frontmatter.args && Array.isArray(frontmatter.args)) {
+      for (let i = 0; i < frontmatter.args.length; i++) {
+        if (i < cliArgs.length) {
+          templateVars[frontmatter.args[i]] = cliArgs[i];
+        }
+      }
+    }
+
+    expect(templateVars).toEqual({ name: "Alice", action: "run" });
+
+    const result = substituteTemplateVars(body, templateVars);
+    expect(result).toBe("Alice will run");
+  });
+
+  test("body consisting only of template var becomes the CLI arg", () => {
+    const content = `---
+args: [prompt]
+---
+{{ prompt }}`;
+
+    const { frontmatter, body } = parseFrontmatter(content);
+
+    // Simulate CLI: ma file.md "Write me a haiku about coding"
+    const cliArgs = ["Write me a haiku about coding"];
+    const templateVars: Record<string, string> = {};
+
+    if (frontmatter.args && Array.isArray(frontmatter.args)) {
+      for (let i = 0; i < frontmatter.args.length; i++) {
+        if (i < cliArgs.length) {
+          templateVars[frontmatter.args[i]] = cliArgs[i];
+        }
+      }
+    }
+
+    const result = substituteTemplateVars(body, templateVars);
+    expect(result).toBe("Write me a haiku about coding");
+  });
+
+  test("missing template variables are detected", () => {
+    const body = "Hello {{ name }}, welcome to {{ place }}";
+    const requiredVars = extractTemplateVars(body);
+
+    expect(requiredVars).toContain("name");
+    expect(requiredVars).toContain("place");
+
+    // If only one is provided, the other is "missing"
+    const templateVars = { name: "Alice" };
+    const missingVars = requiredVars.filter(v => !(v in templateVars));
+
+    expect(missingVars).toEqual(["place"]);
+  });
+});
+
+describe("$varname fields with defaults", () => {
+  test("$varname field with default value", () => {
+    const content = `---
+$feature_name: Authentication
+---
+Build {{ feature_name }}`;
+
+    const { frontmatter, body } = parseFrontmatter(content);
+
+    // Extract $varname fields
+    const namedVarFields = Object.keys(frontmatter)
+      .filter(key => key.startsWith("$") && !/^\$\d+$/.test(key));
+
+    const templateVars: Record<string, string> = {};
+
+    // Use frontmatter default (no CLI override)
+    for (const key of namedVarFields) {
+      const varName = key.slice(1);
+      const defaultValue = frontmatter[key];
+      if (defaultValue !== undefined && defaultValue !== null && defaultValue !== "") {
+        templateVars[varName] = String(defaultValue);
+      }
+    }
+
+    expect(templateVars).toEqual({ feature_name: "Authentication" });
+
+    const result = substituteTemplateVars(body, templateVars);
+    expect(result).toBe("Build Authentication");
+  });
+
+  test("CLI flag overrides $varname default", () => {
+    const content = `---
+$feature_name: Authentication
+---
+Build {{ feature_name }}`;
+
+    const { frontmatter, body } = parseFrontmatter(content);
+
+    // Simulate CLI: ma file.md --feature_name "Payments"
+    const cliArgs = ["--feature_name", "Payments"];
+
+    const namedVarFields = Object.keys(frontmatter)
+      .filter(key => key.startsWith("$") && !/^\$\d+$/.test(key));
+
+    const templateVars: Record<string, string> = {};
+
+    for (const key of namedVarFields) {
+      const varName = key.slice(1);
+      const defaultValue = frontmatter[key];
+
+      // Look for --varname in CLI args
+      const flagIndex = cliArgs.findIndex(arg => arg === `--${varName}`);
+      if (flagIndex !== -1 && flagIndex + 1 < cliArgs.length) {
+        templateVars[varName] = cliArgs[flagIndex + 1];
+      } else if (defaultValue !== undefined && defaultValue !== null && defaultValue !== "") {
+        templateVars[varName] = String(defaultValue);
+      }
+    }
+
+    expect(templateVars).toEqual({ feature_name: "Payments" });
+
+    const result = substituteTemplateVars(body, templateVars);
+    expect(result).toBe("Build Payments");
+  });
+});