feat: error reporting

example/hello_world.vy

@@ -1 +1,3 @@
 (print '(Hello, World!))
+(print (format "Hello" (format "World")))
+)

src/main.rs

@@ -36,6 +36,8 @@ fn main() {
                         if args_index < args.len() {
                             let file_path: &str = &args[args_index];
                             let file_content: String = read_to_string(file_path).unwrap();
+                            // Used for error reporting
+                            let file_content_joined: String = file_content.split("\n").collect::<Vec<&str>>().join(" ");
 
                             let parsed = parser::parse(&file_content);
                             let mut ast = Vec::new();
@@ -43,7 +45,7 @@ fn main() {
                                 match node {
                                     Ok(node) => { ast.push(node); },
                                     Err(error) => {
-                                        eprintln!("ERROR: {}", error);
+                                        eprintln!("{}", error.at(&file_content_joined));
                                         exit(1);
                                     }
                                 }

src/parser.rs

@@ -167,16 +167,15 @@ pub struct ParseError {
     pub pos: (usize, usize),
 }
 
-impl fmt::Display for ParseError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{} at {}", self.kind, self.pos.0)
-    }
-}
-
 impl ParseError {
     fn new(kind: ParseErrorKind, pos: (usize, usize)) -> Self {
         ParseError { kind, pos }
     }
+
+    pub fn at(&self, src: &str) -> String {
+        let snip = &src[(self.pos.0.saturating_sub(5))..(if self.pos.0 + 5 > src.len() { src.len() } else { self.pos.0 + 5 })];
+        format!("\n{}..{}\n{}\nError: {} at {}", " ".repeat(3), snip, format!("{}^", " ".repeat(10)), self.kind, self.pos.0)
+    }
 }
 
 fn read<'a>(
@@ -192,7 +191,10 @@ fn read<'a>(
         match token {
             "(" => {
                 parenths += 1;
+                
+                if parenths == 1 {
                     block_start = start;
+                }
 
                 stack.push(Tree::List {
                     vec: Vec::new(),