Add exemple for state parameters

doc/calculator/src/calculator7.lalrpop

@@ -0,0 +1,34 @@
+use std::str::FromStr;
+use ast::{Expr, Opcode};
+
+grammar(scale: i32);
+
+pub Expr: Box<Expr> = { // (1)
+    Expr ExprOp Factor => Box::new(Expr::Op(<>)), // (2)
+    Factor,
+};
+
+ExprOp: Opcode = { // (3)
+    "+" => Opcode::Add,
+    "-" => Opcode::Sub,
+};
+
+Factor: Box<Expr> = {
+    Factor FactorOp Term => Box::new(Expr::Op(<>)),
+    Term,
+};
+
+FactorOp: Opcode = {
+    "*" => Opcode::Mul,
+    "/" => Opcode::Div,
+};
+
+Term: Box<Expr> = {
+    Num => Box::new(Expr::Number(<>)),
+    "(" <Expr> ")"
+};
+
+Num: i32 = {
+    r"[0-9]+" => i32::from_str(<>).unwrap()*scale,
+};
+

doc/calculator/src/main.rs

@@ -125,6 +125,17 @@ fn calculator6() {
     assert_eq!(errors.len(), 4);
 }
 
+lalrpop_mod!(pub calculator7);
+
+#[test]
+fn calculator7() {
+    let scale = 2;
+    let expr = calculator7::ExprParser::new()
+        .parse(scale,"11 * 22 + 33")
+        .unwrap();
+    assert_eq!(&format!("{:?}", expr), "((22 * 44) + 66)");
+}
+
 #[cfg(not(test))]
 fn main() {
     println!("Hello, world!");