diff --git a/.swift-version b/.swift-version
new file mode 100644
index 0000000..5186d07
--- /dev/null
+++ b/.swift-version
@@ -0,0 +1 @@
+4.0
diff --git a/Source/Argument.swift b/Source/Argument.swift
index 59187c0..3253f38 100644
--- a/Source/Argument.swift
+++ b/Source/Argument.swift
@@ -91,6 +91,6 @@ extension String {
     
     let actualRange = (characters.index(startIndex, offsetBy: range.lowerBound) ..< characters.index(endIndex, offsetBy: -distanceFromEndIndex))
     
-    return self[actualRange]
+    return String(self[actualRange])
   }
 }
diff --git a/Source/Ask.swift b/Source/Ask.swift
index 4336624..83d3c3d 100644
--- a/Source/Ask.swift
+++ b/Source/Ask.swift
@@ -30,7 +30,7 @@ public func ask(_ prompt: String, customizationBlock: ((AskSettings<String>) ->
  - returns: The string casted to the type requested
  - discussion: If the user enters a wrong type, ask will keep prompting until a correct value has been entered
  */
-public func ask<T: ArgConvertibleType>(_ prompt: String, type: T.Type, customizationBlock: ((AskSettings<T>) -> Void)? = nil) -> T {
+public func ask<T>(_ prompt: String, type: T.Type, customizationBlock: ((AskSettings<T>) -> Void)? = nil) -> T {
     
     PromptSettings.print(prompt)
     
diff --git a/Source/ChooseSettings.swift b/Source/ChooseSettings.swift
index b033e78..93b77ec 100644
--- a/Source/ChooseSettings.swift
+++ b/Source/ChooseSettings.swift
@@ -24,7 +24,7 @@ public enum ChoiceIndexType {
 public class ChooseSettings<T> {
     typealias Item = T
     
-    var choices: [(choice: String, callback: (Void) -> T)] = []
+    var choices: [(choice: String, callback: () -> T)] = []
     
     /// Prompt message to use
     public var promptQuestion = ""
@@ -41,7 +41,7 @@ public class ChooseSettings<T> {
      - parameter choice:   Item name
      - parameter callback: callback called when the item is selected, the value returned from this call back will be returned from choose
      */
-    public func addChoice(_ choice: String..., callback: @escaping (Void) -> T) {
+    public func addChoice(_ choice: String..., callback: @escaping () -> T) {
         choice.forEach {
             choices.append(($0, callback))
         }
@@ -85,14 +85,15 @@ public class ChooseSettings<T> {
     }
     
     func preparePromptItems() -> [String] {
-        return zip(indexChoices(), stringChoices()).map { index, string in
+        return zip(indexChoices(), stringChoices()).map { pair in
+            let (index, string) = pair
             return "\(index)\(indexSuffix)\(string)"
         }
     }
     
     func indexChoices() -> [String] {
-        return stringChoices().enumerated().map { itemIndex, string in
-
+        return stringChoices().enumerated().map { pair in
+            let (itemIndex, _) = pair
             if index == .numbers {
                 return "\(itemIndex + 1)"
             } else {
diff --git a/Source/Env.swift b/Source/Env.swift
index c09189c..da33456 100644
--- a/Source/Env.swift
+++ b/Source/Env.swift
@@ -89,7 +89,10 @@ public class Env {
    - parameter callback: callback to call on each key/value pair
    */
   public static func eachPair(_ callback: (_ key: String, _ value: String) -> ()) {
-    zip(self.keys, self.values).forEach(callback)
+    zip(self.keys, self.values).forEach { (pair) in
+        let (key, value) = pair
+        callback(key, value)
+    }
   }
   
 }
diff --git a/Source/RunResults.swift b/Source/RunResults.swift
index a3b0e87..d8c81c5 100644
--- a/Source/RunResults.swift
+++ b/Source/RunResults.swift
@@ -36,5 +36,5 @@ func splitCommandToArgs(_ command: String) -> [String] {
 
 func readPipe(_ pipe: TaskPipe) -> String {
     let data = pipe.read()
-    return NSString(data: data as Data, encoding: String.Encoding.utf8.rawValue) as? String ?? ""
+    return NSString(data: data as Data, encoding: String.Encoding.utf8.rawValue) as String? ?? ""
 }
diff --git a/Swiftline.xcodeproj/project.pbxproj b/Swiftline.xcodeproj/project.pbxproj
index 618abca..34ea6ea 100644
--- a/Swiftline.xcodeproj/project.pbxproj
+++ b/Swiftline.xcodeproj/project.pbxproj
@@ -180,7 +180,7 @@
 				LastUpgradeCheck = 9999;
 				TargetAttributes = {
 					"______Target_Swiftline" = {
-						LastSwiftMigration = 0800;
+						LastSwiftMigration = 0900;
 					};
 				};
 			};
@@ -252,7 +252,7 @@
 				PRODUCT_BUNDLE_IDENTIFIER = Swiftline;
 				PRODUCT_MODULE_NAME = "$(TARGET_NAME:c99extidentifier)";
 				PRODUCT_NAME = "$(TARGET_NAME:c99extidentifier)";
-				SWIFT_VERSION = 3.0;
+				SWIFT_VERSION = 4.0;
 			};
 			name = Release;
 		};
@@ -269,7 +269,7 @@
 				PRODUCT_MODULE_NAME = "$(TARGET_NAME:c99extidentifier)";
 				PRODUCT_NAME = "$(TARGET_NAME:c99extidentifier)";
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
-				SWIFT_VERSION = 3.0;
+				SWIFT_VERSION = 4.0;
 			};
 			name = Debug;
 		};