Replace runMain with mainBody

runMain was error-prone, as it encouraged construction of args object outside
of try-catch block.

Author: xenomachina

README.md

@@ -186,14 +186,19 @@ throw a `SystemExitException` when parsing fails.
 Additional post-parsing validation can be performed on a delegate using
 `addValidator`.
 
-As a convenience, these exceptions can be handled by using the `runMain`
-extension function:
+As a convenience, these exceptions can be handled by using the `mainBody`
+function:
 
 ```kotlin
-fun main(args: Array<String>) =
-        MyArgs(ArgParser(args)).runMain(PROGRAM_NAME) {
+class ParsedArgs(parser: ArgParser) {
+    val name by positional("The user's name").default("world")
+}
+
+fun main(args: Array<String>) = mainBody("hello") {
+        ParsedArgs(ArgParser(args)).run {
             println("Hello, {name}!")
         }
+    }
 ```
 
 Note that parsing does not take place until at least one delegate is read, or

src/main/kotlin/SystemExitException.kt

@@ -57,19 +57,18 @@ open class SystemExitException(message: String, val returnCode: Int) : Exception
 }
 
 /**
- * Like [kotlin.run], but calls [SystemExitException.printAndExit] on any
- * `SystemExitException` that is caught.
+ * Calls [SystemExitException.printAndExit] on any `SystemExitException` that
+ * is caught.
  *
  * @param progName the name of the program, or null if not known
  * @param columns the number of columns to wrap any caught
  * `SystemExitException` to.  Specify null for reasonable defaults, or 0 to not
  * wrap at all.
- * @param f the main body. If a `SystemExitException` is caught its
- * `printAndExit` method will be invoked.
+ * @param body the code that may throw a `SystemExitException`
  */
-fun <T, R> T.runMain(progName: String? = null, columns: Int? = null, f: T.() -> R): R {
+fun <R> mainBody(progName: String? = null, columns: Int? = null, body: () -> R): R {
     try {
-        return f()
+        return body()
     } catch (e: SystemExitException) {
         e.printAndExit(progName, columns ?: System.getenv("COLUMNS")?.toInt() ?: 80)
     }