rudus/src/vm.rs

use crate::parser::*;
use crate::value::*;
use imbl::Vector;
use std::rc::Rc;

#[derive(Clone, Debug)]
pub struct LudusError {
    msg: String,
}

// oy
// lifetimes are a mess
// I need 'src kind of everywhere
// But (maybe) using 'src in eval
// for ctx
// means I can't borrow it mutably
// I guess the question is how to get
// the branches for Ast::Block and Ast::If
// to work with a mutable borrow of ctx
pub struct Ctx<'src> {
    pub locals: Vec<(&'src str, Value<'src>)>,
    // pub names: Vec<&'src str>,
    // pub values: Vec<Value<'src>>,
}

impl<'src> Ctx<'src> {
    pub fn resolve(&self, name: &'src str) -> Value {
        if let Some((_, val)) = self.locals.iter().rev().find(|(bound, _)| *bound == name) {
            val.clone()
        } else {
            unreachable!()
        }
    }

    pub fn store(&mut self, name: &'src str, value: Value<'src>) {
        self.locals.push((name, value));
    }
}

pub fn match_eq<T, U>(x: T, y: T, z: U) -> Option<U>
where
    T: PartialEq,
{
    if x == y {
        Some(z)
    } else {
        None
    }
}

pub fn matchh<'src, 'a>(
    patt: &Pattern<'src>,
    val: &Value<'src>,
    ctx: &'a mut Vec<(&'src str, Value<'src>)>,
) -> Option<&'a mut Vec<(&'src str, Value<'src>)>> {
    match (patt, val) {
        (Pattern::Nil, Value::Nil) => Some(ctx),
        (Pattern::Placeholder, _) => Some(ctx),
        (Pattern::Number(x), Value::Number(y)) => match_eq(x, y, ctx),
        (Pattern::Boolean(x), Value::Boolean(y)) => match_eq(x, y, ctx),
        (Pattern::Keyword(x), Value::Keyword(y)) => match_eq(x, y, ctx),
        (Pattern::String(x), Value::String(y)) => match_eq(x, y, ctx),
        (Pattern::Word(w), val) => {
            ctx.push((w, val.clone()));
            Some(ctx)
        }
        // todo: add splats to these match clauses
        (Pattern::Tuple(x), Value::Tuple(y)) => {
            if x.len() != y.len() {
                return None;
            };
            let to = ctx.len();
            for i in 0..x.len() {
                if let None = matchh(&x[i].0, &y[i], ctx) {
                    while ctx.len() > to {
                        ctx.pop();
                    }
                    return None;
                }
            }
            Some(ctx)
        }
        (Pattern::List(x), Value::List(y)) => {
            if x.len() != y.len() {
                return None;
            };
            let to = ctx.len();
            for i in 0..x.len() {
                if let None = matchh(&x[i].0, y.get(i).unwrap(), ctx) {
                    while ctx.len() > to {
                        ctx.pop();
                    }
                    return None;
                }
            }
            Some(ctx)
        }
        (Pattern::Pair(_, _), _) => todo!("dictionary patterns still to do"),
        (Pattern::Dict(_), _) => todo!("dictionary patterns still to do"),
        _ => None,
    }
}

pub fn eval<'src, 'a>(
    ast: &Ast<'src>,
    ctx: &'a mut Vec<(&'src str, Value<'src>)>,
) -> Result<Value<'src>, LudusError> {
    match ast {
        Ast::Nil => Ok(Value::Nil),
        Ast::Boolean(b) => Ok(Value::Boolean(*b)),
        Ast::Number(n) => Ok(Value::Number(*n)),
        Ast::Keyword(k) => Ok(Value::Keyword(k)),
        Ast::String(s) => Ok(Value::String(s)),
        Ast::Block(exprs) => {
            let to = ctx.len();
            let mut result = Value::Nil;
            for (expr, _) in exprs {
                result = eval(expr, ctx)?;
            }
            while ctx.len() > to {
                ctx.pop();
            }
            Ok(result)
        }
        Ast::If(cond, if_true, if_false) => {
            let truthy = eval(&cond.0, ctx)?.bool();
            if truthy {
                eval(&if_true.0, ctx)
            } else {
                eval(&if_false.0, ctx)
            }
        }
        Ast::List(members) => {
            let mut vect = Vector::new();
            for member in members {
                vect.push_back(eval(&member.0, ctx)?)
            }
            Ok(Value::List(Rc::new(vect)))
        }
        Ast::Tuple(members) => {
            let mut vect = Vec::new();
            for member in members {
                vect.push(eval(&member.0, ctx)?);
            }
            Ok(Value::Tuple(Rc::new(vect)))
        }
        Ast::Word(w) => {
            let val = if let Some((_, value)) = ctx.iter().rev().find(|(name, _)| w == name) {
                value.clone()
            } else {
                unreachable!()
            };
            Ok(val)
        }
        Ast::Let(patt, expr) => {
            let val = eval(&expr.0, ctx)?;
            match matchh(&patt.0, &val, ctx) {
                Some(_) => Ok(val),
                None => Err(LudusError {
                    msg: "No match".to_string(),
                }),
            }
        }
        Ast::Placeholder => todo!(),
        Ast::Error => todo!(),
        Ast::Dict(_) => todo!(),
        Ast::Arguments(_) => todo!(),
        Ast::Pair(_, _) => todo!(),
        Ast::Box(_, _) => todo!(),
        Ast::Synthetic(_, _, _) => todo!(),
        Ast::When(clauses) => {
            for clause in clauses.iter() {
                let WhenClause { cond, body } = &clause.0;
                if eval(&cond.0, ctx)?.bool() {
                    return eval(&body.0, ctx);
                };
            }
            return Err(LudusError {
                msg: "no match".to_string(),
            });
        }
        Ast::Match(_, _) => todo!(),
        Ast::MatchClause(_, _) => todo!(),
        Ast::Fn(_, _) => todo!(),
        Ast::FnDeclaration(_) => todo!(),
        Ast::FnClause(_, _) => todo!(),
        Ast::Panic(msg) => {
            let msg = eval(&msg.0, ctx)?;
            Err(LudusError {
                msg: msg.to_string(),
            })
        }
        Ast::Repeat(times, body) => {
            let mut times_num = 0;
            match eval(&times.0, ctx) {
                Ok(Value::Number(n)) => {
                    times_num = n as usize;
                }
                _ => {
                    return Err(LudusError {
                        msg: "repeat may only take numbers".to_string(),
                    })
                }
            }
            for _ in 0..times_num {
                let _ = eval(&body.0, ctx)?;
            }
            Ok(Value::Nil)
        }
        Ast::Do(_) => todo!(),
        Ast::Loop(_, _) => todo!(),
        Ast::Recur(_) => todo!(),
    }
}