# Building ASTs

Of course, most of the time, when you're parsing you don't want to compute a value, you want to build up some kind of data structure. Here's a quick example to show how that is done in LALRPOP. First, we need to define the data structure we will build. We're going to use a very simple enum:

#![allow(unused)]
fn main() {
pub enum Expr {
Number(i32),
Op(Box<Expr>, Opcode, Box<Expr>),
}

pub enum Opcode {
Mul,
Div,
Sub,
}
}

We put this code into an ast.rs module in our project, along with some Debug impls so that things pretty-print nicely. Now we will create the calculator4 example, which will build up this tree. To start, let's just look at the Expr nonterminal, which will show you most everything of how it is done (the most interesting lines have been flagged with comments):

use std::str::FromStr;
use ast::{Expr, Opcode}; // (0)

grammar;

pub Expr: Box<Expr> = { // (1)
Expr ExprOp Factor => Box::new(Expr::Op(<>)), // (2)
Factor,
};

ExprOp: Opcode = { // (3)
"-" => Opcode::Sub,
};


First off, we have to import these new names into our file by adding a use statement (0). Next, we want to produce Box<Expr> values, so we change the type of Expr (and Factor and Term) to Box<Expr> (1). The action code changes accordingly in (2); here we've used the <> expansion to supply three arguments to Expr::Op. Finally, just for concision, we introduced an ExprOp nonterminal (3) to cover the two opcodes, which now trigger the same action code (before they triggered different action code, so we could do an addition vs a subtraction).

The definition of Factor is transformed in a similar way:

Factor: Box<Expr> = {
Factor FactorOp Term => Box::new(Expr::Op(<>)),
Term,
};

FactorOp: Opcode = {
"*" => Opcode::Mul,
"/" => Opcode::Div,
};


And finally we adjust the definitions of Term and Num. Here, we convert from a raw i32 into a Box<Expr> when we transition from Num to Term (4):

Term: Box<Expr> = {
Num => Box::new(Expr::Number(<>)), // (4)
"(" <Expr> ")"
};

Num: i32 = {
r"[0-9]+" => i32::from_str(<>).unwrap()
};


And that's it! Now we can test it by adding some code to our main.rs file that parses an expression and formats it using the Debug impl:

#![allow(unused)]
fn main() {
lalrpop_mod!(pub calculator4);
pub mod ast;

#[test]
fn calculator4() {
let expr = calculator4::ExprParser::new()
.parse("22 * 44 + 66")
.unwrap();
assert_eq!(&format!("{:?}", expr), "((22 * 44) + 66)");
}
}