implement type-safe fallback commands

src/commonMain/kotlin/com/infendro/cli/parser/Parser.kt

@@ -1,12 +1,24 @@
 package com.infendro.cli.parser
 
 import com.infendro.cli.exception.run.InvalidValue
+import com.infendro.cli.exception.run.RuntimeError
 import kotlin.reflect.KClass
 
-interface Parser<T : Any> {
-    val type: KClass<T>
+abstract class Parser<T : Any>(
+    val type: KClass<T>,
+) {
+    sealed class Result<T : Any> {
+        data class Success<T : Any>(
+            val value: T,
+        ) : Result<T>()
 
-    fun parse(text: String): T
+        data class Failure<T : Any>(
+            val error: RuntimeError,
+        ) : Result<T>()
+    }
 
-    fun invalid(text: String): Nothing = throw InvalidValue(text, type)
+    protected fun success(value: T) = Result.Success(value)
+    protected fun failure(text: String) = Result.Failure<T>(InvalidValue(text, type))
+
+    abstract fun parse(text: String): Result<T>
 }