Add unary operator parsing

Now we can also parse unary +, i.e.
  +17,
and also expressions that contain it, i.e.
  +17 + 23.

We also now have custom Debug implementation for Expression, which
prints them in more useful, s-expression-like syntax, i.e.
  +17 + 23 => (+ +17 23).

We also change implementation of `TokenStream`s `next` and `parse_next`
methods to allow to easily implement `peek` method for looking at the
(possible) next token without advancing the stream.

src/main.rs

@@ -91,10 +91,12 @@ fn main() {
 	let path = path.unwrap();
 	eprintln!("compiling `{}`", path);
 
-	#[allow(non_upper_case_globals)] const source: &'static str = "23 + 21;";
+	#[allow(non_upper_case_globals)] const source: &'static str = "+17 + 23 + +21;";
 	let mut tokens = TokenStream::from(source);
-	let expr = parse_expression(&mut tokens);
+	let expr = parse_expression(&mut tokens, 0);
 	eprintln!("{:?}", expr);
+	// let expr = Some(Expression::Binary(Token::Plus, box Expression::Literal(Token::IntegerLiteral(OffsetStr::from("42"))), box Expression::Unary(Token::Plus, box Expression::Literal(Token::IntegerLiteral(OffsetStr::from("22"))))));
+	// eprintln!("{:?}", expr);
 
 	match output_pretty {
 		Some("dot" | "graphdotviz") => expr.then(|e| {
@@ -107,7 +109,7 @@ fn main() {
 		// a single bit.
 		Some(_) => unreachable!(),
 		None => {},
-	}
+	};
 }
 
 // Represents a type characterised by a parameter T (either the type itself, or
@@ -135,12 +137,22 @@ impl<T> Then<T> for std::option::Option<T> {
 	}
 }
 
-#[derive(Debug)]
 enum Expression {
 	Literal(Token),
+	Unary(Token, Box<Expression>),
 	Binary(Token, Box<Expression>, Box<Expression>),
 }
 
+impl std::fmt::Debug for Expression {
+	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+		return match self {
+			Expression::Literal(token) => write!(f, "{}", token),
+			Expression::Unary(token, expr) => write!(f, "{}{:?}", token, expr),
+			Expression::Binary(token, left, right) => write!(f, "({} {:?} {:?})", token, left, right),
+		};
+	}
+}
+
 struct Counter {
 	state: usize,
 }
@@ -168,6 +180,14 @@ impl Expression {
 				format!("Node{} [label = \"{}\"]", id, i)
 			},
 			Expression::Literal(_) => unreachable!(),
+			Expression::Unary(op, expr) => {
+				let expr_id = unsafe { GLOBAL_COUNTER.next() };
+
+				format!("Node{} -> Node{}\nNode{} [label = \"{}\"]\n{}",
+					id, expr_id,
+					id, op,
+					expr.create_graphviz_graph(expr_id))
+			},
 			Expression::Binary(op, left, right) => {
 				let left_id = unsafe { GLOBAL_COUNTER.next() };
 				let right_id = unsafe { GLOBAL_COUNTER.next() };
@@ -181,22 +201,57 @@ impl Expression {
 	}
 }
 
-fn parse_expression<'a>(tokens: &'a mut TokenStream<'a>) -> Option<Expression> {
+fn unary_precedence(token: Token) -> Option<usize> {
+	return match token {
+		Token::Plus => Some(2),
+		_ => None,
+	};
+}
+
+fn binary_precedence(token: Token) -> Option<usize> {
+	return match token {
+		Token::Plus => Some(1),
+		_ => None,
+	};
+}
+
+fn parse_expression<'a, 'b: 'a>(tokens: &'a mut TokenStream<'b>, highest_precedence: usize) -> Option<Expression> {
 	let lhs = match tokens.next()? {
 		token @ Token::IntegerLiteral(_) => Expression::Literal(token),
-		_ => return None,
-	};
-
-	return match tokens.next()? {
-		operator @ Token::Plus => {
-			let rhs = parse_expression(tokens)?;
-
-			Some(Expression::Binary(operator, box lhs, box rhs))
+		token => {
+			if let Some(precedence) = unary_precedence(token) {
+				let expr = parse_expression(tokens, precedence)?;
+				Expression::Unary(token, box expr)
+			} else {
+				return None;
+			}
 		},
-		// If it's not a valid operator, then caller can get rest of the input in the token stream
-		// it has provided to us.
-		_ => Some(lhs),
 	};
+
+	loop {
+		let operator = match tokens.peek()? {
+			operator @ Token::Plus => operator,
+			_ => return None,
+		};
+
+		let precedence =  binary_precedence(operator)?;
+
+		if precedence <= highest_precedence {
+			return Some(lhs);
+		}
+
+		return match operator {
+			Token::Plus => {
+				tokens.next();
+				let rhs = parse_expression(tokens, precedence)?;
+
+				Some(Expression::Binary(operator, box lhs, box rhs))
+			},
+			// If it's not a valid operator, then caller can get rest of the input in the token stream
+			// it has provided to us.
+			_ => Some(lhs),
+		};
+	}
 }
 
 struct TokenStream<'a> {
@@ -259,11 +314,17 @@ impl<'a> TokenStream<'a> {
 			c => todo!("character unsupported: `{}`", natural_char_representation(c)),
 		};
 
-		self.cursor += token.len();
 		return Some(token);
 	}
 
 	pub fn next(&mut self) -> Option<Token> {
+		self.skip_whitespace();
+		let token = self.parse_next()?;
+		self.cursor += token.len();
+		return Some(token);
+	}
+
+	pub fn peek(&mut self) -> Option<Token> {
 		self.skip_whitespace();
 		return self.parse_next();
 	}