Add very rudimentary Lexer: it matches parens!

Sibil.xcodeproj/project.pbxproj

@@ -8,6 +8,7 @@
 
 /* Begin PBXBuildFile section */
 		C0496EB51E04533D0000E33E /* main.swift in Sources */ = {isa = PBXBuildFile; fileRef = C0496EB41E04533D0000E33E /* main.swift */; };
+		C0496EBC1E04534F0000E33E /* Lexer.swift in Sources */ = {isa = PBXBuildFile; fileRef = C0496EBB1E04534F0000E33E /* Lexer.swift */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXCopyFilesBuildPhase section */
@@ -25,6 +26,7 @@
 /* Begin PBXFileReference section */
 		C0496EB11E04533D0000E33E /* Sibil */ = {isa = PBXFileReference; explicitFileType = "compiled.mach-o.executable"; includeInIndex = 0; path = Sibil; sourceTree = BUILT_PRODUCTS_DIR; };
 		C0496EB41E04533D0000E33E /* main.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = main.swift; sourceTree = "<group>"; };
+		C0496EBB1E04534F0000E33E /* Lexer.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Lexer.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -58,6 +60,7 @@
 			isa = PBXGroup;
 			children = (
 				C0496EB41E04533D0000E33E /* main.swift */,
+				C0496EBB1E04534F0000E33E /* Lexer.swift */,
 			);
 			path = Sibil;
 			sourceTree = "<group>";
@@ -120,6 +123,7 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				C0496EBC1E04534F0000E33E /* Lexer.swift in Sources */,
 				C0496EB51E04533D0000E33E /* main.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;

Sibil/Lexer.swift

@@ -0,0 +1,52 @@
+//
+//  Lexer.swift
+//  Sibil
+//
+//  Created by Eryn Wells on 12/16/16.
+//  Copyright © 2016 Eryn Wells. All rights reserved.
+//
+
+import Foundation
+
+
+struct Token {
+    enum Kind {
+        case LeftParen
+        case RightParen
+    }
+    
+    let kind: Kind
+    let value: String
+}
+
+
+class Lexer: IteratorProtocol {
+    typealias Element = Token
+
+    let input: String
+
+    private var index: String.Index
+
+    init(input: String) {
+        self.input = input
+        self.index = input.startIndex
+    }
+
+    // MARK: IteratorProtocol
+
+    func next() -> Token? {
+        var token: Token?
+        while token == nil {
+            let c = input[index]
+            switch c {
+            case "(":
+                token = Token(kind: .LeftParen, value: String(c))
+            case ")":
+                token = Token(kind: .RightParen, value: String(c))
+            default:
+                break
+            }
+        }
+        return token
+    }
+}