rhai/examples/repl.rs

use rhai::{Engine, EvalAltResult, Scope, AST};
use std::{
    io::{stdin, stdout, Write},
    iter,
};

fn print_error(input: &str, err: EvalAltResult) {
    fn padding(pad: &str, len: usize) -> String {
        iter::repeat(pad).take(len).collect::<String>()
    }

    let lines: Vec<_> = input.split("\n").collect();

    // Print error
    match err.position() {
        p if p.is_eof() => {
            // EOF
            let last = lines[lines.len() - 2];
            println!("{}", last);
            println!("{}^ {}", padding(" ", last.len() - 1), err);
        }
        p if p.is_none() => {
            // No position
            println!("{}", err);
        }
        p => {
            // Specific position
            let pos_text = format!(
                " (line {}, position {})",
                p.line().unwrap(),
                p.position().unwrap()
            );

            println!("{}", lines[p.line().unwrap() - 1]);
            println!(
                "{}^ {}",
                padding(" ", p.position().unwrap() - 1),
                err.to_string().replace(&pos_text, "")
            );
        }
    }
}

fn main() {
    let mut engine = Engine::new();
    let mut scope = Scope::new();

    let mut input = String::new();
    let mut ast: Option<AST> = None;

    loop {
        print!("rhai> ");
        stdout().flush().expect("couldn't flush stdout");

        input.clear();

        if let Err(err) = stdin().read_line(&mut input) {
            println!("input error: {}", err);
        }

        // Implement standard commands
        match input.as_str().trim() {
            "exit" | "quit" => break, // quit
            "ast" => {
                // print the last AST
                match &ast {
                    Some(ast) => println!("{:#?}", ast),
                    None => println!("()"),
                }
                continue;
            }
            _ => (),
        }

        if let Err(err) = engine
            .compile(&input)
            .map_err(EvalAltResult::ErrorParsing)
            .and_then(|r| {
                ast = Some(r);
                engine.consume_ast_with_scope(&mut scope, true, ast.as_ref().unwrap())
            })
        {
            println!("");
            print_error(&input, err);
            println!("");
        }
    }
}