diff --git a/src/commonMain/kotlin/com/infendro/cli/command/Argument.kt b/src/commonMain/kotlin/com/infendro/cli/command/Argument.kt
index 3175660..75c3b41 100644
--- a/src/commonMain/kotlin/com/infendro/cli/command/Argument.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/command/Argument.kt
@@ -1,21 +1,20 @@
 package com.infendro.cli.command
 
 import com.infendro.cli.parser.*
-import com.infendro.cli.parser.Parser
 
-sealed class Argument<T>(
+sealed class Argument<T : Any>(
     val parser: Parser<T>,
 ) {
     abstract val required: Boolean
 
-    class Required<T>(
+    class Required<T : Any>(
         parser: Parser<T>,
     ) : Argument<T>(parser) {
         override val required: Boolean
             get() = true
     }
 
-    class OrElse<T>(
+    class OrElse<T : Any>(
         val other: T,
         parser: Parser<T>,
     ) : Argument<T>(parser) {
@@ -23,14 +22,14 @@ sealed class Argument<T>(
             get() = false
     }
 
-    class OrNull<T>(
+    class OrNull<T : Any>(
         parser: Parser<T>,
     ) : Argument<T>(parser) {
         override val required: Boolean
             get() = false
     }
 
-    class Variable<T>(
+    class Variable<T : Any>(
         parser: Parser<T>,
     ) : Argument<T>(parser) {
         override val required: Boolean
diff --git a/src/commonMain/kotlin/com/infendro/cli/command/Command.kt b/src/commonMain/kotlin/com/infendro/cli/command/Command.kt
index 36794ad..877c916 100644
--- a/src/commonMain/kotlin/com/infendro/cli/command/Command.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/command/Command.kt
@@ -1,5 +1,7 @@
 package com.infendro.cli.command
 
+import com.infendro.cli.command.context.Context
+import com.infendro.cli.command.context.Parser
 import com.infendro.cli.exception.build.*
 import com.infendro.cli.exception.run.CliException
 import com.infendro.cli.util.Regex.COMMAND
@@ -22,7 +24,7 @@ class Command private constructor(
     }
 
     class Builder internal constructor(
-        private val index: Int,
+        private val level: Int,
         private val name: String?,
     ) {
         private val commands = mutableListOf<Command>()
@@ -30,22 +32,19 @@ class Command private constructor(
         private val options = mutableListOf<Option<*>>()
         private lateinit var _execute: Context.() -> Unit
 
-        private val nextIndex: Int
-            get() = index + 1
+        private val key: Key
+            get() = Key(level)
 
         fun command(name: String? = null, block: Builder.(Key) -> Unit) {
             // validate
             val last = commands.lastOrNull()
             when {
-                last?.fallback == true -> throw CommandOrderException()
-                name?.matches(COMMAND) == false -> throw IllegalCommandNameException(name)
+                last != null && last.fallback -> throw CommandOrderException()
+                name != null && !name.matches(COMMAND) -> throw IllegalCommandNameException(name)
                 commands.any { it.name == name } -> throw DuplicateCommandException(name)
             }
 
-            commands += Builder(nextIndex, name).also {
-                val key = Key(nextIndex)
-                it.block(key)
-            }.build()
+            commands += Builder(level + 1, name).also { it.block(it.key) }.build()
         }
 
         fun argument(argument: Argument<*>) {
@@ -53,7 +52,7 @@ class Command private constructor(
             val last = arguments.lastOrNull()
             when {
                 last is Argument.Variable -> throw ArgumentOrderException()
-                argument.required && last?.required == false -> throw ArgumentOrderException()
+                argument.required && last != null && !last.required -> throw ArgumentOrderException()
             }
 
             arguments += argument
@@ -92,7 +91,7 @@ class Command private constructor(
             // validate
             val last = commands.lastOrNull()
             when {
-                last?.fallback == true && arguments.isNotEmpty() -> throw InvalidFallbackException()
+                last != null && last.fallback && arguments.isNotEmpty() -> throw InvalidFallbackException()
             }
 
             if (!::_execute.isInitialized)
@@ -107,8 +106,5 @@ class Command private constructor(
     )
 }
 
-fun cli(block: Command.Builder.() -> Unit): Command {
-    val builder = Command.Builder(-1, null)
-    builder.block()
-    return builder.build()
-}
+fun cli(block: Command.Builder.() -> Unit) =
+    Command.Builder(-1, null).also(block).build()
diff --git a/src/commonMain/kotlin/com/infendro/cli/command/Context.kt b/src/commonMain/kotlin/com/infendro/cli/command/Context.kt
deleted file mode 100644
index 9801d79..0000000
--- a/src/commonMain/kotlin/com/infendro/cli/command/Context.kt
+++ /dev/null
@@ -1,56 +0,0 @@
-package com.infendro.cli.command
-
-import kotlin.reflect.KProperty
-
-class Context internal constructor(
-    val command: Command,
-    val commands: List<Cmd>,
-    val options: List<Opt>,
-    val arguments: List<Arg>,
-) {
-    class Cmd(
-        val name: String,
-    )
-
-    class Arg(
-        val value: Any,
-    )
-
-    class Opt(
-        val name: String,
-        val value: Any,
-    )
-
-    internal fun execute() = (command.execute)()
-
-    operator fun Command.Key.getValue(thisRef: Any?, property: KProperty<*>): String = commands[index].name
-
-    private val <T> Argument<T>.index: Int
-        get() = command.arguments.indexOf(this)
-
-    @Suppress("UNCHECKED_CAST")
-    private val <T> Argument<T>.value: T?
-        get() = arguments.getOrNull(index)?.value as? T
-
-    @Suppress("UNCHECKED_CAST")
-    private val <T> Argument.Variable<T>.values: List<T>
-        get() = arguments.drop(index).map { it.value as T }
-
-    operator fun <T> Argument.Required<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value!!
-    operator fun <T> Argument.OrElse<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value ?: other
-    operator fun <T> Argument.OrNull<T>.getValue(thisRef: Any?, property: KProperty<*>): T? = value
-    operator fun <T> Argument.Variable<T>.getValue(thisRef: Any?, property: KProperty<*>): List<T> = values
-
-    @Suppress("UNCHECKED_CAST")
-    private val <T> Option<T>.value: T?
-        get() = options.firstOrNull { it.name in names }?.value as? T
-
-    @Suppress("UNCHECKED_CAST")
-    private val <T> Option.Variable<T>.values: List<T>
-        get() = options.filter { it.name in names }.map { it.value as T }
-
-    operator fun <T> Option.Required<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value!!
-    operator fun <T> Option.OrElse<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value ?: other
-    operator fun <T> Option.OrNull<T>.getValue(thisRef: Any?, property: KProperty<*>): T? = value
-    operator fun <T> Option.Variable<T>.getValue(thisRef: Any?, property: KProperty<*>): List<T> = values
-}
diff --git a/src/commonMain/kotlin/com/infendro/cli/command/Option.kt b/src/commonMain/kotlin/com/infendro/cli/command/Option.kt
index 23f9af6..37e6eac 100644
--- a/src/commonMain/kotlin/com/infendro/cli/command/Option.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/command/Option.kt
@@ -1,13 +1,16 @@
 package com.infendro.cli.command
 
-import com.infendro.cli.exception.build.NoOptionNameException
+import com.infendro.cli.exception.build.MissingOptionNameException
 import com.infendro.cli.parser.*
-import com.infendro.cli.parser.Parser
 
-sealed class Option<T>(
-    val names: List<String>,
+sealed class Option<T : Any>(
     val parser: Parser<T>,
+    val fallback: T?,
+    val names: List<String>,
 ) {
+    val flag: Boolean
+        get() = fallback != null
+
     val name: String
         get() = names[0]
 
@@ -15,89 +18,114 @@ sealed class Option<T>(
 
     init {
         if (names.isEmpty())
-            throw NoOptionNameException()
+            throw MissingOptionNameException()
     }
 
-    class Required<T>(
-        names: List<String>,
+    class Required<T : Any>(
         parser: Parser<T>,
-    ) : Option<T>(names, parser) {
+        fallback: T?,
+        names: List<String>,
+    ) : Option<T>(parser, fallback, names) {
+        constructor(
+            parser: Parser<T>,
+            names: List<String>,
+        ) : this(parser, null, names)
+
         override val required: Boolean
             get() = true
     }
 
-    class OrElse<T>(
+    class OrElse<T : Any>(
+        parser: Parser<T>,
+        fallback: T?,
         names: List<String>,
         val other: T,
-        parser: Parser<T>,
-    ) : Option<T>(names, parser) {
+    ) : Option<T>(parser, fallback, names) {
+        constructor(
+            parser: Parser<T>,
+            names: List<String>,
+            other: T,
+        ) : this(parser, null, names, other)
+
         override val required: Boolean
             get() = false
     }
 
-    class OrNull<T>(
-        names: List<String>,
+    class OrNull<T : Any>(
         parser: Parser<T>,
-    ) : Option<T>(names, parser) {
+        fallback: T?,
+        names: List<String>,
+    ) : Option<T>(parser, fallback, names) {
+        constructor(
+            parser: Parser<T>,
+            names: List<String>,
+        ) : this(parser, null, names)
+
         override val required: Boolean
             get() = false
     }
 
-    class Variable<T>(
-        names: List<String>,
+    class Variable<T : Any>(
         parser: Parser<T>,
-    ) : Option<T>(names, parser) {
+        fallback: T?,
+        names: List<String>,
+    ) : Option<T>(parser, fallback, names) {
+        constructor(
+            parser: Parser<T>,
+            names: List<String>,
+        ) : this(parser, null, names)
+
         override val required: Boolean
             get() = false
     }
 
     companion object {
         fun string(vararg names: String) =
-            Required(names.asList(), StringParser)
+            Required(StringParser, names.asList())
 
         fun stringOrElse(vararg names: String, other: String) =
-            OrElse(names.asList(), other, StringParser)
+            OrElse(StringParser, names.asList(), other)
 
         fun stringOrNull(vararg names: String) =
-            OrNull(names.asList(), StringParser)
+            OrNull(StringParser, names.asList())
 
         fun strings(vararg names: String) =
-            Variable(names.asList(), StringParser)
+            Variable(StringParser, names.asList())
 
         fun int(vararg names: String) =
-            Required(names.asList(), IntParser)
+            Required(IntParser, names.asList())
 
         fun intOrElse(vararg names: String, other: Int) =
-            OrElse(names.asList(), other, IntParser)
+            OrElse(IntParser, names.asList(), other)
 
         fun intOrNull(vararg names: String) =
-            OrNull(names.asList(), IntParser)
+            OrNull(IntParser, names.asList())
 
         fun ints(vararg names: String) =
-            Variable(names.asList(), IntParser)
+            Variable(IntParser, names.asList())
 
         fun boolean(vararg names: String) =
-            Required(names.asList(), BooleanParser)
+            Required(BooleanParser, true, names.asList())
 
         fun booleanOrElse(vararg names: String, other: Boolean) =
-            OrElse(names.asList(), other, BooleanParser)
+            OrElse(BooleanParser, true, names.asList(), other)
 
         fun booleanOrNull(vararg names: String) =
-            OrNull(names.asList(), BooleanParser)
+            OrNull(BooleanParser, true, names.asList())
 
         fun booleans(vararg names: String) =
-            Variable(names.asList(), BooleanParser)
+            Variable(BooleanParser, true, names.asList())
 
         inline fun <reified T : Enum<T>> enum(vararg names: String) =
-            Required(names.asList(), enumParser<T>())
+            Required(enumParser<T>(), names.asList())
 
         inline fun <reified T : Enum<T>> enumOrElse(vararg names: String, other: T) =
-            OrElse(names.asList(), other, enumParser<T>())
+            OrElse(enumParser<T>(), names.asList(), other)
 
         inline fun <reified T : Enum<T>> enumOrNull(vararg names: String) =
-            OrNull(names.asList(), enumParser<T>())
+            OrNull(enumParser<T>(), names.asList())
 
         inline fun <reified T : Enum<T>> enums(vararg names: String) =
-            Variable(names.asList(), enumParser<T>())
+            Variable(enumParser<T>(), names.asList())
     }
 }
diff --git a/src/commonMain/kotlin/com/infendro/cli/command/context/Context.kt b/src/commonMain/kotlin/com/infendro/cli/command/context/Context.kt
new file mode 100644
index 0000000..037d2e4
--- /dev/null
+++ b/src/commonMain/kotlin/com/infendro/cli/command/context/Context.kt
@@ -0,0 +1,59 @@
+package com.infendro.cli.command.context
+
+import com.infendro.cli.command.Argument
+import com.infendro.cli.command.Command
+import com.infendro.cli.command.Option
+import kotlin.reflect.KProperty
+
+class Context internal constructor(
+    val command: Command,
+    val commands: List<Cmd>,
+    val options: List<Opt>,
+    val arguments: List<Arg>,
+) {
+    class Cmd(
+        val name: String,
+    )
+
+    class Arg(
+        val value: Any,
+    )
+
+    class Opt(
+        val name: String,
+        val value: Any,
+    )
+
+    internal fun execute() = (command.execute)()
+
+    operator fun Command.Key.getValue(thisRef: Any?, property: KProperty<*>): String = commands[index].name
+
+    private val <T : Any> Argument<T>.index: Int
+        get() = command.arguments.indexOf(this)
+
+    @Suppress("UNCHECKED_CAST")
+    private val <T : Any> Argument<T>.value: T?
+        get() = arguments.getOrNull(index)?.value as? T
+
+    @Suppress("UNCHECKED_CAST")
+    private val <T : Any> Argument.Variable<T>.values: List<T>
+        get() = arguments.drop(index).map { it.value as T }
+
+    operator fun <T : Any> Argument.Required<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value!!
+    operator fun <T : Any> Argument.OrElse<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value ?: other
+    operator fun <T : Any> Argument.OrNull<T>.getValue(thisRef: Any?, property: KProperty<*>): T? = value
+    operator fun <T : Any> Argument.Variable<T>.getValue(thisRef: Any?, property: KProperty<*>): List<T> = values
+
+    @Suppress("UNCHECKED_CAST")
+    private val <T : Any> Option<T>.value: T?
+        get() = options.firstOrNull { it.name in names }?.value as? T
+
+    @Suppress("UNCHECKED_CAST")
+    private val <T : Any> Option.Variable<T>.values: List<T>
+        get() = options.filter { it.name in names }.map { it.value as T }
+
+    operator fun <T : Any> Option.Required<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value!!
+    operator fun <T : Any> Option.OrElse<T>.getValue(thisRef: Any?, property: KProperty<*>): T = value ?: other
+    operator fun <T : Any> Option.OrNull<T>.getValue(thisRef: Any?, property: KProperty<*>): T? = value
+    operator fun <T : Any> Option.Variable<T>.getValue(thisRef: Any?, property: KProperty<*>): List<T> = values
+}
diff --git a/src/commonMain/kotlin/com/infendro/cli/command/Parser.kt b/src/commonMain/kotlin/com/infendro/cli/command/context/Parser.kt
similarity index 85%
rename from src/commonMain/kotlin/com/infendro/cli/command/Parser.kt
rename to src/commonMain/kotlin/com/infendro/cli/command/context/Parser.kt
index 9e95189..d6a8e7b 100644
--- a/src/commonMain/kotlin/com/infendro/cli/command/Parser.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/command/context/Parser.kt
@@ -1,6 +1,9 @@
-package com.infendro.cli.command
+package com.infendro.cli.command.context
 
-import com.infendro.cli.command.Context.*
+import com.infendro.cli.command.Argument
+import com.infendro.cli.command.Command
+import com.infendro.cli.command.Option
+import com.infendro.cli.command.context.Context.*
 import com.infendro.cli.exception.run.*
 
 internal object Parser {
@@ -41,6 +44,7 @@ internal object Parser {
             val current = args[i].drop(dashes)
 
             when {
+                // argument
                 endOfOptions || dashes == 0 -> {
                     val argument = command.arguments.getOrNull(argumentIndex)
                         ?: throw TooManyArgumentsException()
@@ -52,6 +56,7 @@ internal object Parser {
                     arg += Arg(value)
                 }
 
+                // option
                 dashes in 1..2 -> {
                     if (dashes == 2 && current.isEmpty()) {
                         endOfOptions = true
@@ -59,7 +64,7 @@ internal object Parser {
                         continue
                     }
 
-                    val (name, value) = when {
+                    val (name, text) = when {
                         current.contains('=') -> {
                             val (name, value) = current.split('=', limit = 2)
                             name to value
@@ -85,11 +90,16 @@ internal object Parser {
                         val option = command.options.firstOrNull { name in it.names }
                             ?: throw UnknownOptionException(name)
 
-                        val value = option.parser.parse(value) as Any
+                        val value = when {
+                            text != null -> option.parser.parse(text)
+                            option.flag -> option.fallback!!
+                            else -> throw MissingOptionValueException(name)
+                        }
                         opt += Opt(name, value)
                     }
                 }
 
+                // malformed option
                 else -> {
                     when {
                         args[i].contains('=') -> {
diff --git a/src/commonMain/kotlin/com/infendro/cli/exception/build/MissingOptionNameException.kt b/src/commonMain/kotlin/com/infendro/cli/exception/build/MissingOptionNameException.kt
new file mode 100644
index 0000000..bd969f8
--- /dev/null
+++ b/src/commonMain/kotlin/com/infendro/cli/exception/build/MissingOptionNameException.kt
@@ -0,0 +1,3 @@
+package com.infendro.cli.exception.build
+
+class MissingOptionNameException : Exception()
diff --git a/src/commonMain/kotlin/com/infendro/cli/exception/build/NoOptionNameException.kt b/src/commonMain/kotlin/com/infendro/cli/exception/build/NoOptionNameException.kt
deleted file mode 100644
index 475aca6..0000000
--- a/src/commonMain/kotlin/com/infendro/cli/exception/build/NoOptionNameException.kt
+++ /dev/null
@@ -1,3 +0,0 @@
-package com.infendro.cli.exception.build
-
-class NoOptionNameException : Exception()
diff --git a/src/commonMain/kotlin/com/infendro/cli/exception/run/MissingOptionValueException.kt b/src/commonMain/kotlin/com/infendro/cli/exception/run/MissingOptionValueException.kt
new file mode 100644
index 0000000..24b2cb7
--- /dev/null
+++ b/src/commonMain/kotlin/com/infendro/cli/exception/run/MissingOptionValueException.kt
@@ -0,0 +1,8 @@
+package com.infendro.cli.exception.run
+
+class MissingOptionValueException(
+    val option: String,
+) : CliException() {
+    override val message: String
+        get() = """no value provided for option "$option""""
+}
diff --git a/src/commonMain/kotlin/com/infendro/cli/parser/BooleanParser.kt b/src/commonMain/kotlin/com/infendro/cli/parser/BooleanParser.kt
index f60da95..ce894ab 100644
--- a/src/commonMain/kotlin/com/infendro/cli/parser/BooleanParser.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/parser/BooleanParser.kt
@@ -3,8 +3,8 @@ package com.infendro.cli.parser
 import com.infendro.cli.exception.run.ParseException
 
 object BooleanParser : Parser<Boolean> {
-    override fun parse(text: String?): Boolean = when (text) {
-        "true", "t", null -> true
+    override fun parse(text: String): Boolean = when (text) {
+        "true", "t" -> true
         "false", "f" -> false
         else -> throw ParseException(text, "Boolean")
     }
diff --git a/src/commonMain/kotlin/com/infendro/cli/parser/EnumParser.kt b/src/commonMain/kotlin/com/infendro/cli/parser/EnumParser.kt
index df04cbf..b4247a4 100644
--- a/src/commonMain/kotlin/com/infendro/cli/parser/EnumParser.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/parser/EnumParser.kt
@@ -7,7 +7,7 @@ class EnumParser<T : Enum<T>>(
     private val name: String,
     private val values: List<T>,
 ) : Parser<T> {
-    override fun parse(text: String?): T = try {
+    override fun parse(text: String): T = try {
         values.first { it.name == text }
     } catch (_: Exception) {
         throw ParseException(text, name)
diff --git a/src/commonMain/kotlin/com/infendro/cli/parser/IntParser.kt b/src/commonMain/kotlin/com/infendro/cli/parser/IntParser.kt
index 1ea0e77..750920a 100644
--- a/src/commonMain/kotlin/com/infendro/cli/parser/IntParser.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/parser/IntParser.kt
@@ -3,8 +3,8 @@ package com.infendro.cli.parser
 import com.infendro.cli.exception.run.ParseException
 
 object IntParser : Parser<Int> {
-    override fun parse(text: String?): Int = try {
-        text!!.toInt()
+    override fun parse(text: String): Int = try {
+        text.toInt()
     } catch (_: Exception) {
         throw ParseException(text, "Int")
     }
diff --git a/src/commonMain/kotlin/com/infendro/cli/parser/Parser.kt b/src/commonMain/kotlin/com/infendro/cli/parser/Parser.kt
index 44626f8..fc15d53 100644
--- a/src/commonMain/kotlin/com/infendro/cli/parser/Parser.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/parser/Parser.kt
@@ -1,5 +1,5 @@
 package com.infendro.cli.parser
 
 interface Parser<T> {
-    fun parse(text: String?): T
+    fun parse(text: String): T
 }
diff --git a/src/commonMain/kotlin/com/infendro/cli/parser/StringParser.kt b/src/commonMain/kotlin/com/infendro/cli/parser/StringParser.kt
index 3387c03..17248d5 100644
--- a/src/commonMain/kotlin/com/infendro/cli/parser/StringParser.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/parser/StringParser.kt
@@ -1,7 +1,5 @@
 package com.infendro.cli.parser
 
-import com.infendro.cli.exception.run.ParseException
-
 object StringParser : Parser<String> {
-    override fun parse(text: String?): String = text ?: throw ParseException(text, "String")
+    override fun parse(text: String): String = text
 }
diff --git a/src/commonMain/kotlin/com/infendro/cli/util/Regex.kt b/src/commonMain/kotlin/com/infendro/cli/util/Regex.kt
index aebe5f6..1eeb52c 100644
--- a/src/commonMain/kotlin/com/infendro/cli/util/Regex.kt
+++ b/src/commonMain/kotlin/com/infendro/cli/util/Regex.kt
@@ -1,8 +1,6 @@
 package com.infendro.cli.util
 
 internal object Regex {
-    private const val L = "[a-zA-Z]"
-    private const val AN = """[a-zA-Z0-9]"""
-    val COMMAND = Regex("""${L}${AN}*(?:-${AN}+)*""")
-    val OPTION = Regex("""${L}${AN}*(?:[\-.]${AN}+)*""")
+    val COMMAND = Regex("""[a-zA-Z][a-zA-Z0-9]*(?:-[a-zA-Z0-9]+)*""")
+    val OPTION = Regex("""[a-zA-Z][a-zA-Z0-9]*(?:[\-.][a-zA-Z0-9]+)*""")
 }