allow optional return types on functions

examples/hello.v

@@ -52,3 +52,5 @@ fn main(a int) int {
 
 	v()()()
 }
+
+fn voidfunc() {}

src/parser.zig

@@ -375,6 +375,16 @@ pub const Parser = struct {
         return root;
     }
 
+    /// Copy a token with a different lexeme.
+    fn mkToken(self: *@This(), ttype: TokenType, lexeme: []const u8, line: usize) !Token {
+        const owned_lexeme = try std.mem.dupe(self.allocator, u8, lexeme);
+        return Token{
+            .ttype = ttype,
+            .lexeme = owned_lexeme,
+            .line = line,
+        };
+    }
+
     fn parseFnDecl(self: *@This()) !*Node {
         var param_list = ast.ParamList.init(self.allocator);
         errdefer param_list.deinit();
@@ -387,7 +397,7 @@ pub const Parser = struct {
         while (self.peek().ttype != .RightParen) {
             const param_name = try self.consumeSingle(.Identifier);
 
-            // TODO dedicated function to consume a type
+            // TODO dedicated function to consume a type?
             const param_type = try self.consumeSingle(.Identifier);
 
             try param_list.append(ast.ParamDecl{
@@ -398,8 +408,15 @@ pub const Parser = struct {
 
         _ = try self.consumeSingle(.RightParen);
 
-        // TODO dedicated function to consume a type
+        // the return type is default void if a type
-        const return_type = try self.consumeSingle(.Identifier);
+        // is not provided
+        var return_type: Token = undefined;
+        if (self.check(.Identifier)) {
+            return_type = try self.consumeSingle(.Identifier);
+        } else {
+            return_type = try self.mkToken(.Identifier, "void", name.line);
+        }
+
         var block_node = try self.parseBlock();
         return try self.mkFnDecl(name, param_list, return_type, block_node.Block);
     }