update

src/lexer/mod.rs

@@ -0,0 +1,23 @@
+struct Lexer {
+    input: String,
+    position: Option<i32>,
+    read_pos: Option<i32>,
+    ch: Option<i8>,
+}
+
+fn new(input_str: String) -> Lexer {
+    let l: Lexer = Lexer {
+        input: input_str,
+        position: None,
+        read_pos: None,
+        ch: None,
+    };
+    l
+}
+
+fn read_char(l: Lexer) {
+    let read_pos = usize::try_from(l.read_pos.unwrap());
+    if read_pos >= l.input.len() {
+        l.ch = 0;
+    }
+}

src/main.rs

@@ -0,0 +1,6 @@
+mod lexer;
+mod token;
+
+fn main() {
+    println!("Hello, world!");
+}

src/token/mod.rs

@@ -0,0 +1,19 @@
+struct Token {
+    type_: String,
+    literal: String,
+}
+
+const ILLEGAL: &str = "ILLEGAL";
+const EOF: &str = "EOF";
+const IDENT: &str = "IDENT";
+const INT: &str = "INT";
+const ASSIGN: &str = "=";
+const PLUS: &str = "+";
+const COMMA: &str = ",";
+const SEMICOLON: &str = ";";
+const LPAREN: &str = "(";
+const RPAREN: &str = ")";
+const LBRACE: &str = "{";
+const RBRACE: &str = "}";
+const FUNCTION: &str = "FUNCTION";
+const LET: &str = "LET";