move errors to dedicated file

src/error.rs

@@ -0,0 +1,20 @@
+use std::fmt;
+
+#[derive(Debug)]
+pub enum LexerError {
+    EmptyToken(usize),
+    UnexpectedCharacter(char, usize),
+}
+
+impl fmt::Display for LexerError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            LexerError::EmptyToken(line) => write!(f, "Empty token at line {}", line),
+            LexerError::UnexpectedCharacter(c, line) => {
+                write!(f, "Unexpected character '{}' at line {}", c, line)
+            }
+        }
+    }
+}
+
+impl std::error::Error for LexerError {}

src/lexer.rs

@@ -1,6 +1,6 @@
-use std::fmt;
 use std::fmt::Debug;
 
+use crate::error::LexerError;
 use crate::scanner::{Scanner, ScannerState};
 
 #[derive(Debug, Clone, PartialEq)]
@@ -70,25 +70,6 @@ pub trait Tokenizer: Scanner {
     fn add_token(&mut self, token_type: Self::TokenType);
 }
 
-#[derive(Debug)]
-pub enum LexerError {
-    EmptyToken(usize),
-    UnexpectedCharacter(char, usize),
-}
-
-impl fmt::Display for LexerError {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self {
-            LexerError::EmptyToken(line) => write!(f, "Empty token at line {}", line),
-            LexerError::UnexpectedCharacter(c, line) => {
-                write!(f, "Unexpected character '{}' at line {}", c, line)
-            }
-        }
-    }
-}
-
-impl std::error::Error for LexerError {}
-
 pub struct Lexer {
     source: String,
     tokens: Vec<Token>,

src/lib.rs

@@ -1,2 +1,3 @@
+mod error;
 mod lexer;
 mod scanner;