Add --help option for CLI

core/src/main/java/com/facebook/ktfmt/cli/Main.kt

@@ -80,6 +80,10 @@ class Main(
     val parsedArgs =
         when (processArgs) {
           is ParseResult.Ok -> processArgs.parsedValue
+          is ParseResult.ShowMessage -> {
+            err.println(processArgs.message)
+            return EXIT_CODE_SUCCESS
+          }
           is ParseResult.Error -> {
             err.println(processArgs.errorMessage)
             return EXIT_CODE_FAILURE

core/src/main/java/com/facebook/ktfmt/cli/ParsedArgs.kt

@@ -48,6 +48,50 @@ data class ParsedArgs(
       return parseOptions(arguments)
     }
 
+    val HELP_TEXT = """ktfmt - command line Kotlin source code pretty-printer
+      |
+      |Usage:
+      |  ktfmt [--style=dropbox | --style=google | --style=kotlinlang ] [-n | --dry-run] [--set-exit-if-changed] 
+      |        [--do-not-remove-unused-imports] <File1.kt> <File2.kt> ...
+      |  ktfmt @ARGFILE
+      |
+      |      ktfmt formats Kotlin source code files in-place, reporting for each file whether the
+      |      formatting succeeded or failed on standard error. If none of the style options are
+      |passed, Facebook's style is used.
+      |
+      |Alternatively, ktfmt can read Kotlin source code from standard in and write the 
+      |formatted result on standard output.
+      |
+      |Example:
+      |     $ ktfmt --dropbox-style Main.kt src/Parser.kt
+      |     Done formatting Main.kt
+      |     Error formatting src/Parser.kt: @@@ERROR@@@; skipping.
+      |    
+      |Commands options:
+      |  -n, --dry-run                     Don't write to files, only report files which 
+      |                                        would have changed
+      |  --style=dropbox                   Use 4-space block indenting
+      |  --style=google                    Google style
+      |  --style=kotlinlang                Kotlin language guidelines style
+      |  --set-exit-if-changed             Sets exit code to 1 if any input code was not 
+      |                                        correctly formatted
+      |  --do-not-remove-unused-imports    Leaves all imports in place, even if not used
+      |  
+      |ARGFILE:
+      |  If the only argument begins with '@', the remainder of the argument is treated
+      |  as the name of a file to read options and arguments from, one per line.
+      |  
+      |  e.g.
+      |      $ cat arg-file.txt
+      |      --google-style
+      |      -n
+      |      File1.kt
+      |      File2.kt
+      |      $ ktfmt @arg-file1.txt
+      |      Done formatting File1.kt
+      |      Done formatting File2.kt
+    """.trimMargin()
+
     /** parseOptions parses command-line arguments passed to ktfmt. */
     fun parseOptions(args: Array<out String>): ParseResult {
       val fileNames = mutableListOf<String>()
@@ -57,6 +101,9 @@ data class ParsedArgs(
       var removeUnusedImports = true
       var stdinName: String? = null
 
+      if (args.contains("--help"))
+        return ParseResult.ShowMessage(HELP_TEXT)
+
       for (arg in args) {
         when {
           arg == "--dropbox-style" -> formattingOptions = Formatter.DROPBOX_FORMAT
@@ -107,6 +154,6 @@ data class ParsedArgs(
 
 sealed interface ParseResult {
   data class Ok(val parsedValue: ParsedArgs) : ParseResult
-
+  data class ShowMessage(val message: String): ParseResult
   data class Error(val errorMessage: String) : ParseResult
 }

core/src/test/java/com/facebook/ktfmt/cli/MainTest.kt

@@ -493,4 +493,17 @@ class MainTest {
     assertThat(exitCode).isEqualTo(0)
     assertThat(file.readText(UTF_8)).isEqualTo("""fun f() = println("hello, world")""" + "\n")
   }
+
+  @Test
+  fun `--help gives return code of 0`() {
+    val exitCode = Main(
+      emptyInput,
+      PrintStream(out),
+      PrintStream(err),
+      arrayOf("--help"),
+    )
+      .run()
+
+    assertThat(exitCode).isEqualTo(0)
+  }
 }

core/src/test/java/com/facebook/ktfmt/cli/ParsedArgsTest.kt

@@ -133,6 +133,18 @@ class ParsedArgsTest {
     assertThat(parseResult).isInstanceOf(ParseResult.Error::class.java)
   }
 
+  @Test
+  fun `parseOptions recognises --help`() {
+    val parseResult = ParsedArgs.parseOptions(arrayOf("--help"))
+    assertThat(parseResult).isInstanceOf(ParseResult.ShowMessage::class.java)
+  }
+
+  @Test
+  fun `arg --help overrides all others`() {
+    val parseResult = ParsedArgs.parseOptions(arrayOf("--style=google", "@unknown", "--help", "file.kt"))
+    assertThat(parseResult).isInstanceOf(ParseResult.ShowMessage::class.java)
+  }
+
   @Test
   fun `processArgs use the @file option with non existing file`() {
     val e =