a/src/repl.rs

//! Interactive read-eval-print loop.
//!
//! Each line is compiled and run in exactly the same way scripts are run, 
//! keeping the Lua state between lines.

use mlua::prelude::{LuaFunction, LuaMultiValue, LuaTable};
use mlua::Lua;
use rustyline::error::ReadlineError;
use rustyline::DefaultEditor;

const PROMPT: &str = ">>> ";
const CONTINUATION_PROMPT: &str = "..> ";

/// Outcome of trying to compile a piece of REPL input.
enum Compiled {
    /// A ready-to-run chunk.
    Chunk(LuaFunction),
    /// Input is syntactically incomplete; more lines are needed.
    Incomplete,
    /// Input is invalid and cannot become valid by appending more lines.
    Error(mlua::Error),
}

/// Compile a complete piece of REPL input.
///
/// Mirrors stock Lua's `lua.c`: first try the input wrapped in `return ( ... )`
/// so that bare expressions print their value, and fall back to running it as a
/// statement. Only the statement form's error decides whether the input is
/// merely incomplete (so a half-typed expression keeps reading more lines).
fn compile(lua: &Lua, source: &str) -> Compiled {
    if let Ok(func) = lua
        .load(format!("return {source}"))
        .set_name("=stdin")
        .into_function()
    {
        return Compiled::Chunk(func);
    }

    match lua.load(source).set_name("=stdin").into_function() {
        Ok(func) => Compiled::Chunk(func),
        Err(mlua::Error::SyntaxError {
            incomplete_input: true,
            ..
        }) => Compiled::Incomplete,
        Err(err) => Compiled::Error(err),
    }
}

/// Pretty-print the results of an evaluated chunk, one per line, using the
/// project's own `utils.dump`. Nothing is printed when the chunk returns no
/// values (e.g. a plain statement).
fn print_results(lua: &Lua, values: LuaMultiValue) -> mlua::Result<()> {
    if values.is_empty() {
        return Ok(());
    }
    let dump: LuaFunction = lua
        .globals()
        .get::<LuaTable>("utils")?
        .get::<LuaFunction>("dump")?;
    for value in values {
        let rendered: String = dump.call(value)?;
        println!("{rendered}");
    }
    Ok(())
}

/// Run the interactive REPL against an already-sandboxed Lua state.
pub(crate) async fn run(lua: &Lua) -> Result<(), Box<dyn std::error::Error>> {
    let mut editor = DefaultEditor::new()?;
    let history_path = std::env::var_os("HOME").map(|home| {
        let mut p = std::path::PathBuf::from(home);
        p.push(".a_history");
        p
    });
    if let Some(path) = &history_path {
        // A missing history file on first run is not an error.
        let _ = editor.load_history(path);
    }

    println!(
        "a {} — interactive Lua. Ctrl-D to exit.",
        env!("CARGO_PKG_VERSION")
    );

    // Accumulated lines of an as-yet-incomplete statement.
    let mut buffer = String::new();
    // Guards against an unbroken stream of read errors spinning forever (e.g. a
    // reader that keeps failing without ever reaching EOF).
    let mut consecutive_errors = 0u32;

    loop {
        let prompt = if buffer.is_empty() {
            PROMPT
        } else {
            CONTINUATION_PROMPT
        };

        match editor.readline(prompt) {
            Ok(line) => {
                consecutive_errors = 0;
                if buffer.is_empty() {
                    buffer = line;
                } else {
                    buffer.push('\n');
                    buffer.push_str(&line);
                }

                match compile(lua, &buffer) {
                    Compiled::Incomplete => continue, // keep reading continuation lines
                    Compiled::Chunk(func) => {
                        let _ = editor.add_history_entry(buffer.as_str());
                        match func.call_async::<LuaMultiValue>(()).await {
                            Ok(values) => {
                                if let Err(err) = print_results(lua, values) {
                                    eprintln!("{err}");
                                }
                            }
                            Err(err) => eprintln!("{err}"),
                        }
                        buffer.clear();
                    }
                    Compiled::Error(err) => {
                        let _ = editor.add_history_entry(buffer.as_str());
                        eprintln!("{err}");
                        buffer.clear();
                    }
                }
            }
            // Ctrl-C: abandon the current input and start fresh.
            Err(ReadlineError::Interrupted) => {
                buffer.clear();
                consecutive_errors = 0;
            }
            // Ctrl-D (on an empty line) or end of piped input: quit.
            Err(ReadlineError::Eof) => break,
            // Any other read error (e.g. a non-UTF-8 input line) is reported but
            // does not end the session — a stray pasted byte shouldn't kill the
            // REPL. Bail only if errors keep coming with no successful read in
            // between, so a permanently broken stream can't spin forever.
            Err(err) => {
                eprintln!("{err}");
                buffer.clear();
                consecutive_errors += 1;
                if consecutive_errors >= 100 {
                    break;
                }
            }
        }
    }

    if let Some(path) = &history_path {
        let _ = editor.save_history(path);
    }

    Ok(())
}