Added REPL

Started REPL addition.  The parser does not handle multiple eof's yet,
but lines of code are otherwise processing and the REPL can be exited
with "quit".

Author: BuggusMageevers

swift/Sources/Lexer/Lexer.swift

@@ -23,11 +23,14 @@ extension String {
 }
 
 struct Lexer {
-    let input: String
+    var input: String = ""
     private var currentIndex: String.Index
     private var currentChar: Character { input[currentIndex] }
     private var keywords: [String : Token] = Token.defaultKeywords
 
+    init() {
+        self.currentIndex = input.startIndex
+    }
     init(input: String) {
         self.input = input
         self.currentIndex = input.startIndex

swift/Sources/Lexer/REPL.swift

@@ -0,0 +1,21 @@
+import ArgumentParser
+
+@main
+struct REPL: ParsableCommand {
+    @Argument private var prompt = ">> "
+
+    mutating func run() {
+        print("Welcome the Monkey language REPL")
+        print("Enter 'quit' to exit")
+        print(prompt, terminator: "")
+        while let line = readLine(), line != "quit" {
+            var lexer = Lexer(input: line)
+            var tokens = [Token]()
+            for _ in line {
+               tokens += [lexer.nextToken()]
+            }
+            print(tokens)
+            print(prompt, terminator: "")
+        }
+    }
+}