feat: Variadic argument validation (#40)

* Add (optional) validator to VariadicArgument type

* changelog entry

* validator accepts array instead of single item

Author: djbe

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Enhancements
 
 - Renamed VaradicArgument to VariadicArgument.
+- VariadicArgument now supports an optional validator.
 - Adds support for variadic options, allowing the user to repeat options to
   provide additional values.
 

Sources/ArgumentDescription.swift

@@ -29,19 +29,28 @@ extension ArgumentConvertible {
 
 open class VariadicArgument<T : ArgumentConvertible> : ArgumentDescriptor {
   public typealias ValueType = [T]
+  public typealias Validator = (ValueType) throws -> ValueType
 
   open let name: String
   open let description: String?
+  open let validator: Validator?
 
   open var type: ArgumentType { return .argument }
 
-  public init(_ name: String, description: String? = nil) {
+  public init(_ name: String, description: String? = nil, validator: Validator? = nil) {
     self.name = name
     self.description = description
+    self.validator = validator
   }
 
   open func parse(_ parser: ArgumentParser) throws -> ValueType {
-    return try Array<T>(parser: parser)
+    let value = try Array<T>(parser: parser)
+
+    if let validator = validator {
+      return try validator(value)
+    }
+
+    return value
   }
 }