diff --git a/sketching/sketch.dm b/sketching/sketch.dm
new file mode 100644
index 0000000..efd5ba4
--- /dev/null
+++ b/sketching/sketch.dm
@@ -0,0 +1,174 @@
+ns nonavosa_rail
+
+pub mod train {
+
+    pub int Train {
+        name: String
+        primary_color: Color
+        destination: String
+
+        fn blow_horn(): IO
+        fn set_volume_by(factor: Number)
+    }
+
+    pub enum Color {
+        White, Blue, Red
+    }
+
+    pub fn int Printer {
+        (message: String): IO
+    }
+
+    pub fn create(props: Props<Train> & { printer: Printer }): Train = SimpleTrain { ...props } with { volume = 80.0 }
+
+    abs impl : Train {
+
+        #get #set
+        fld volume: Double
+
+    }
+
+    impl SimpleTrain : AbstractTrain {
+
+        printer: Printer
+
+        override fn set_volume(volume) {
+            if (volume < 0 || volume > 100) {
+                throw IllegalArgumentException {
+                    message: `Volume $volume is outside of bounds 0-100.`,
+                    argument: volume
+                }
+            }
+
+            this.volume = volume
+        }
+
+        impl fn blow_horn() = printer(`Train named $name is blowing it's horn at volume $volume!`)
+             -> { s => s.append('Hello from SimpleTrain!') }
+
+        impl fn set_volume_by(factor) = set_volume(volume * factor)
+
+    }
+
+}
+
+fn main(args) {
+    let train = train::create {
+        name: 'Test Train',
+        primary_color: train::Color::Blue,
+        destination: 'Nonavosa Central',
+        printer: { msg => println(msg) }
+    }
+    train.set_volume_by(1.1)
+    train.blow_horn()
+
+    let anonymous_train = train::Train {
+        name: 'Anonymous Train',
+        primary_color: train::Color::Red,
+        destination: 'Nonavosa North',
+
+        impl fn blow_horn() = println('Choo Choo!')
+        impl fn set_volume_by = throw MethodNotSupported
+    }
+
+    try {
+        anonymous_train.set_volume_by(3.0)
+    } catch (e: MethodNotSupported) {
+        println e.message
+    }
+
+    anonymous_train.blow_horn() -> { println('After blow horn, assuming nothing went wrong before') }
+}
+
+// some other file
+
+fn log_printer(message) impl nonavosa_rail::train::Printer = msg =>
+    io::open_file('log.txt', io::Mode::Append) ->
+        { f => f.append(msg) } ->
+        io::close_file
+
+fn main() {
+    nonavosa_rail::main(log_printer)
+}
+
+// io.dm
+
+ns io
+
+decl extern fn println(s: String)
+
+#target[lua]
+decl extern fn _open_file(filename: String, mode: String): LuaTable
+
+#target[lua]
+pub fn open_file(filename: String, mode: Mode): IO<File, LuaError> {
+    try {
+        let lua_file_handle = lua {% fs.open(${filename}, ${mode.as_lua_string()}) %}
+        Success(LuaFile { lua_file_handle })
+    } catch (e: LuaError) {
+        Failure(e)
+    }
+}
+
+#target[lua]
+pub fn close_file(file): impl IO<File, LuaError> = file.close()
+
+pub int File : Stream
+
+#target[lua]
+impl LuaFile : File {
+
+    #get
+    fld lua_file_handle
+
+    impl fn append(data) {
+        try {
+            lua {% ${lua_file_handle}.write(${data}) %}
+            Success(self)
+        } catch (e: LuaError) {
+            Failure(e)
+        }
+    }
+
+    impl fn close() {
+        try {
+            lua {% ${lua_file_handle}.close() %}
+            Success
+        } catch (e: LuaError) {
+            Failure(e)
+        }
+    }
+
+}
+
+pub enum Mode {
+    Append, Write, Read
+}
+
+pub enum IO<S: Stream = Stream, E?> : Chainable<IO<S, E>> {
+    Success(stream?: S) {
+        impl fn call() {
+            try {
+                do_call(stream)
+            } catch (e: E) {
+                Failure(e)
+            }
+        }
+    },
+    Failure(_) {
+        impl fn call() = self
+    };
+
+    fn <N: Stream> do_call(stream: S): IO<N, E>
+}
+
+pub int Stream {
+    fn append(data: String): IO<Self>
+    fn close(): IO
+}
+
+// chain.dm
+
+pub fn int Chainable<N> {
+    (): N
+}
diff --git a/src/lexer/mod.rs b/src/lexer/mod.rs
index 8f07ee3..00b2218 100644
--- a/src/lexer/mod.rs
+++ b/src/lexer/mod.rs
@@ -31,7 +31,7 @@ pub enum Token {
     Abstract
 }
 
-pub fn tokenize(input: &String) -> Vec<Token> {
+pub fn tokenize(input: &String) -> Result<Vec<Token>, &'static str> {
     let mut tokens: Vec<Token> = Vec::new();
     let mut peekable = input.chars().peekable();
     while let Some(c) = peekable.next() {
@@ -80,7 +80,7 @@ pub fn tokenize(input: &String) -> Vec<Token> {
                 match count {
                     1 => tokens.push(Token::Dot),
                     3 => tokens.push(Token::Ellipsis),
-                    _ => panic!("Too many dots.")
+                    _ => return Err("Unexpected number of tokens after '.'"),
                 }
             }
             _ => {
@@ -90,7 +90,7 @@ pub fn tokenize(input: &String) -> Vec<Token> {
             }
         }
     }
-    tokens
+    Ok(tokens)
 }
 
 fn match_identifier_or_keyword(start_char: char, peekable: &mut Peekable<Chars>) -> Option<Token> {
@@ -143,7 +143,7 @@ mod tests {
 
     #[test]
     fn simple_ns() {
-        let result = tokenize(&String::from("ns simple"));
+        let result = tokenize(&String::from("ns simple")).unwrap();
         assert_eq!(Token::Namespace, result[0]);
         assert_eq!(Token::Identifier(String::from("simple")), result[1]);
     }
@@ -153,14 +153,14 @@ mod tests {
         let mut src_file = File::open(Path::new("test-data/lexer/simple_ns.dm")).unwrap();
         let mut src = String::new();
         let _ = src_file.read_to_string(&mut src);
-        let result = tokenize(&src);
+        let result = tokenize(&src).unwrap();
         assert_eq!(Token::Namespace, result[0]);
         assert_eq!(Token::Identifier(String::from("simple")), result[1]);
     }
 
     #[test]
     fn pub_mod_simple() {
-        let result = tokenize(&String::from("pub mod simple"));
+        let result = tokenize(&String::from("pub mod simple")).unwrap();
         assert_eq!(Token::Public, result[0]);
         assert_eq!(Token::Module, result[1]);
         assert_eq!(Token::Identifier(String::from("simple")), result[2]);
@@ -168,21 +168,22 @@ mod tests {
 
     #[test]
     fn curly_open_and_close() {
-        let result = tokenize(&String::from("{ }"));
+        let result = tokenize(&String::from("{ }")).unwrap();
         assert_eq!(Token::CurlyOpen, result[0]);
         assert_eq!(Token::CurlyClose, result[1]);
     }
 
     #[test]
     fn simple_int() {
-        let result = tokenize(&String::from("int simple"));
+        let result = tokenize(&String::from("int simple")).unwrap();
         assert_eq!(Token::Interface, result[0]);
         assert_eq!(Token::Identifier(String::from("simple")), result[1]);
     }
 
     #[test]
     fn ns_pub_mod_simple() {
-        let result = tokenize(&String::from("ns simple_ns\npub mod simple { }"));
+        let result = tokenize(&String::from("ns simple_ns\npub mod simple { }"))
+            .unwrap();
         assert_eq!(Token::Namespace, result[0]);
         assert_eq!(Token::Identifier(String::from("simple_ns")), result[1]);
         assert_eq!(Token::Public, result[2]);
@@ -194,14 +195,14 @@ mod tests {
 
     #[test]
     fn curly_open_and_close_no_space() {
-        let result = tokenize(&String::from("{}"));
+        let result = tokenize(&String::from("{}")).unwrap();
         assert_eq!(Token::CurlyOpen, result[0]);
         assert_eq!(Token::CurlyClose, result[1]);
     }
 
     #[test]
     fn interface_function() {
-        let result = tokenize(&String::from("fn test(): Test"));
+        let result = tokenize(&String::from("fn test(): Test")).unwrap();
         assert_eq!(Token::Function, result[0]);
         assert_eq!(Token::Identifier(String::from("test")), result[1]);
         assert_eq!(Token::ParenOpen, result[2]);
@@ -212,7 +213,7 @@ mod tests {
 
     #[test]
     fn interface_prop() {
-        let result = tokenize(&String::from("test: Test"));
+        let result = tokenize(&String::from("test: Test")).unwrap();
         assert_eq!(Token::Identifier(String::from("test")), result[0]);
         assert_eq!(Token::Colon, result[1]);
         assert_eq!(Token::Identifier(String::from("Test")), result[2]);
@@ -220,7 +221,7 @@ mod tests {
 
     #[test]
     fn enum_decl() {
-        let result = tokenize(&String::from("enum Test {}"));
+        let result = tokenize(&String::from("enum Test {}")).unwrap();
         assert_eq!(Token::Enum, result[0]);
         assert_eq!(Token::Identifier(String::from("Test")), result[1]);
         assert_eq!(Token::CurlyOpen, result[2]);