Fix: Handle package.json exports field for sub-path imports (#47)

experiments/debug-resolution.mjs

@@ -0,0 +1,41 @@
+#!/usr/bin/env node
+
+/**
+ * Debug script to trace the resolution logic
+ */
+
+import { parseModuleSpecifier } from '../use.mjs';
+import { readFile } from 'node:fs/promises';
+import path from 'node:path';
+
+const moduleSpecifier = 'yargs/helpers';
+const { packageName, version, modulePath } = parseModuleSpecifier(moduleSpecifier);
+
+console.log('Parsed module specifier:', { packageName, version, modulePath });
+
+// This is what happens in the code:
+const packagePath = '/home/hive/.nvm/versions/node/v20.19.5/lib/node_modules/yargs-v-latest';
+const packageModulePath = modulePath ? path.join(packagePath, modulePath) : packagePath;
+
+console.log('Package path:', packagePath);
+console.log('Package module path:', packageModulePath);
+
+// The problem: we're reading package.json from packageModulePath, but we should read from packagePath
+const wrongPackageJsonPath = path.join(packageModulePath, 'package.json');
+const correctPackageJsonPath = path.join(packagePath, 'package.json');
+
+console.log('\nWrong package.json path:', wrongPackageJsonPath);
+console.log('Correct package.json path:', correctPackageJsonPath);
+
+try {
+  const packageJson = await readFile(correctPackageJsonPath, 'utf8');
+  const parsed = JSON.parse(packageJson);
+  console.log('\nExports field:', JSON.stringify(parsed.exports, null, 2));
+
+  // Check for subPath
+  const dottedSubPath = `.${modulePath}`;
+  console.log('\nLooking for:', dottedSubPath);
+  console.log('Found:', parsed.exports[dottedSubPath]);
+} catch (error) {
+  console.error('Error:', error.message);
+}

experiments/root-cause-analysis.md

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+# Root Cause Analysis for Issue #47
+
+## Problem
+Cannot import sub-paths like 'yargs/helpers' - use-m fails to resolve the path.
+
+## Error Message
+```
+Error: Failed to resolve the path to 'yargs/helpers' from '/home/hive/.nvm/versions/node/v20.19.5/lib/node_modules/yargs-v-latest/helpers'.
+```
+
+## Root Cause
+
+### What Should Happen
+When importing `yargs/helpers`:
+1. Parse the module specifier into: packageName='yargs', version='latest', modulePath='/helpers'
+2. Install package as `yargs-v-latest`
+3. Read `yargs-v-latest/package.json`
+4. Look up `./helpers` in the `exports` field
+5. Resolve to the mapped file: `./helpers/helpers.mjs`
+6. Import from `/node_modules/yargs-v-latest/helpers/helpers.mjs`
+
+### What Actually Happens
+1. ✓ Parse correctly: packageName='yargs', version='latest', modulePath='/helpers'
+2. ✓ Install package as `yargs-v-latest`
+3. ✓ Construct path: `packageModulePath = /node_modules/yargs-v-latest/helpers`
+4. ✗ **BUG**: `tryResolveModule` tries to resolve the directory `/helpers` directly
+5. ✗ When that fails, it reads package.json but only checks for the root "." export
+6. ✗ It never checks if there's a sub-path export for "./helpers"
+7. ✗ Returns null, causing the error
+
+### Code Location
+File: `use.mjs`, lines 476-522 (npm resolver) and lines 606-650 (bun resolver)
+
+The `tryResolveModule` function:
+- Only handles the root export (`exp['.']`)
+- Does NOT handle sub-path exports like `exp['./helpers']`
+
+### The Fix Needed
+In the `tryResolveModule` function, when we have a modulePath (like '/helpers'):
+1. Check if the exports field has a matching sub-path entry
+2. If found, resolve to the mapped file
+3. Otherwise, fall back to current behavior
+
+### Example from yargs/package.json
+```json
+{
+  "exports": {
+    "./package.json": "./package.json",
+    "./helpers": "./helpers/helpers.mjs",  // ← This needs to be checked!
+    "./browser": {
+      "types": "./browser.d.ts",
+      "import": "./browser.mjs"
+    },
+    ".": "./index.mjs",
+    "./yargs": "./index.mjs"
+  }
+}
+```
+
+When importing `yargs/helpers`:
+- Current code: Only checks `exports["."]` → doesn't find "./helpers"
+- Fixed code: Should check `exports["./helpers"]` → finds "./helpers/helpers.mjs"

experiments/test-yargs-helpers.mjs

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+#!/usr/bin/env node
+
+/**
+ * Test script to reproduce issue #47: Cannot import sub-paths like 'yargs/helpers'
+ *
+ * This script tests importing yargs/helpers to verify the exports field handling.
+ */
+
+console.log('Testing yargs with use-m...\n');
+
+// Import use-m from the local source
+import { use } from '../use.mjs';
+
+try {
+  console.log('1. Testing yargs main import...');
+  const yargs = await use('yargs');
+  console.log('✓ yargs imported successfully');
+  console.log('  Type:', typeof yargs);
+} catch (error) {
+  console.error('✗ Failed to import yargs:', error.message);
+  console.error('  Stack:', error.stack);
+  process.exit(1);
+}
+
+try {
+  console.log('\n2. Testing yargs/helpers import...');
+  const helpers = await use('yargs/helpers');
+  console.log('✓ yargs/helpers imported successfully');
+  console.log('  Type:', typeof helpers);
+  console.log('  Has hideBin:', 'hideBin' in helpers);
+
+  // Test hideBin function
+  if (helpers.hideBin) {
+    const testArgs = ['node', 'script.js', 'arg1', 'arg2'];
+    const result = helpers.hideBin(testArgs);
+    console.log('  hideBin test:', JSON.stringify(result));
+  }
+} catch (error) {
+  console.error('✗ Failed to import yargs/helpers:', error.message);
+  console.error('  Stack:', error.stack);
+  process.exit(1);
+}
+
+console.log('\n✅ All imports successful!');

tests/exports-field.test.mjs

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+import { use } from '../use.mjs';
+import { describe, test, expect } from '../test-adapter.mjs';
+
+// Mock jest object for Deno compatibility
+const jest = typeof Deno !== 'undefined' ? { setTimeout: () => {} } : (await import('@jest/globals')).jest;
+
+const moduleName = `[${import.meta.url.split('.').pop()} module]`;
+
+jest.setTimeout(60000);
+
+describe(`${moduleName} exports field handling tests`, () => {
+  // Test for issue #47: Cannot import sub-paths like 'yargs/helpers'
+  test(`${moduleName} should import yargs/helpers using exports field`, async () => {
+    const helpers = await use('yargs/helpers');
+    expect(helpers).toBeDefined();
+    expect(typeof helpers).toBe('object');
+    expect(typeof helpers.hideBin).toBe('function');
+  });
+
+  test(`${moduleName} should import yargs main module`, async () => {
+    const yargs = await use('yargs');
+    expect(yargs).toBeDefined();
+    expect(typeof yargs).toBe('object');
+  });
+
+  test(`${moduleName} should import yargs@17.7.2/helpers`, async () => {
+    const helpers = await use('yargs@17.7.2/helpers');
+    expect(helpers).toBeDefined();
+    expect(typeof helpers).toBe('object');
+    expect(typeof helpers.hideBin).toBe('function');
+  });
+
+  test(`${moduleName} should import yargs@18.0.0/helpers`, async () => {
+    const helpers = await use('yargs@18.0.0/helpers');
+    expect(helpers).toBeDefined();
+    expect(typeof helpers).toBe('object');
+    expect(typeof helpers.hideBin).toBe('function');
+  });
+
+  // Test the hideBin function actually works
+  test(`${moduleName} yargs/helpers hideBin should work correctly`, async () => {
+    const { hideBin } = await use('yargs/helpers');
+    const testArgs = ['node', 'script.js', 'arg1', 'arg2'];
+    const result = hideBin(testArgs);
+    expect(result).toEqual(['arg1', 'arg2']);
+  });
+
+  // Test that main package import still works
+  test(`${moduleName} should import @octokit/core using exports field`, async () => {
+    const { Octokit } = await use('@octokit/core@6.1.5');
+    expect(Octokit).toBeDefined();
+    expect(typeof Octokit).toBe('function');
+  });
+});