Add HELP message to command groups

src/command_test.ts

@@ -27,7 +27,7 @@ Deno.test("basic command errors", () => {
   });
 });
 
-Deno.test("help messages shows closest flag", () => {
+Deno.test("command help messages shows closest flag", () => {
   const cmd = new Command("A test command.")
     .required(string, "test", { flags: ["test"] });
 

src/distance.ts

@@ -10,7 +10,10 @@ function union<T>(a: Set<T>, b: Set<T>): number {
   return union;
 }
 
-export function closest(string: string, options: Set<string>): string | null {
+export function closest(
+  string: string,
+  options: Iterable<string>,
+): string | null {
   const stringSet = new Set(string);
   let bestItem = null;
   let bestScore = 0;

src/group.ts

@@ -1,6 +1,7 @@
 import type { Command } from "./command.ts";
 import { ArgumentError, HelpError } from "./error.ts";
 import { leftPad } from "./fmt.ts";
+import { closest } from "./distance.ts";
 
 /**
  * A group of `Command`s.
@@ -36,6 +37,19 @@ export class CommandGroup<T = Record<never, never>> {
     }`;
   }
 
+  private _fmtUnknownCommand(arg: string): string {
+    const unknownCommand = `Unknown command '${arg.replaceAll("'", "\\'")}'`;
+    const closestCommand = closest(
+      arg,
+      Object.getOwnPropertyNames(this._commands),
+    );
+    if (closestCommand) {
+      return `${unknownCommand}\n\nHELP:\n\tDid you mean ${closestCommand}?`;
+    } else {
+      return unknownCommand;
+    }
+  }
+
   /**
    * Add a sub command, which can either be a `Command`,
    * or another `CommandGroup`.
@@ -86,9 +100,7 @@ export class CommandGroup<T = Record<never, never>> {
       if (isHelp) {
         throw new HelpError(this.help(args.slice(0, skip)));
       } else {
-        throw new ArgumentError(
-          `Unknown command ${args[skip].replaceAll("'", "\\'")}`,
-        );
+        throw new ArgumentError(this._fmtUnknownCommand(args[skip]));
       }
     }
   }

src/group_test.ts

@@ -32,6 +32,22 @@ Deno.test("command group errors work", () => {
   assertThrows(() => group.parse(["first"]));
 });
 
+Deno.test("command group help messages shows closest subcommand", () => {
+  const group = new CommandGroup("A test command.")
+    .subcommand("test", cmd);
+
+  let message = "";
+  try {
+    group.parse(["tst"]);
+  } catch (error) {
+    message = error.message;
+  }
+  assertEquals(
+    message,
+    "Unknown command 'tst'\n\nHELP:\n\tDid you mean test?",
+  );
+});
+
 Deno.test("command group help output", () => {
   const innerGroup = new CommandGroup("Inner group.")
     .subcommand("first", cmd)