flesh/the_rewrite/src/eval.rs

/* relish: versatile lisp shell
 * Copyright (C) 2021 Aidan Hahn
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

use crate::func::FTable;
use crate::segment::{Seg, Ctr};
use crate::vars::VTable;

/* iterates over a syntax tree
 * returns a NEW LIST of values
 * representing the simplest possible form of the input
 */
pub fn eval<'a>(
    ast: &'a Seg<'a>,
    vars: &'a mut VTable<'a>,
    funcs: &'a mut FTable<'a>,
    sym_loose: bool,
) -> Result<Box<Ctr<'a>>, String> {
    let mut ret = Box::new(Ctr::Seg(Seg::new()));
    let mut iter: &mut Seg;
    if let Ctr::Seg(ref mut s) = *ret {
        iter = s;
    } else {
        return Err("Critfail: no fuckin clue whats up here".to_string())
    }

    let mut car = &ast.car;

    // doing an initial variable check here allows us
    // to find functions passed in as variables
    if let Ctr::Symbol(tok) = **car {
        if let Some(val) = vars.get(tok.clone()) {
            car = &(val.clone());
        }
    }

    // another check to detect if we may have a function call
    if let Ctr::Symbol(ref tok) = **car {
        match *ast.cdr {
            Ctr::Seg(ref ast) => {
                if let Some(func) = funcs.get(tok.clone()) {
                    return func.func_call(ast, vars, funcs);
                } else if !sym_loose {
                    return Err(format!("Couldnt find definition of {}.", tok));
                }
            }
            Ctr::None => {
                if let Some(func) = funcs.get(tok.clone()) {
                    return func.func_call(&Seg::new(), vars, funcs);
                } else if !sym_loose {
                    return Err(format!("Couldnt find definition of {}.", tok.clone()));
                }
            }
            _ => return Err(format!("Arguments to function not a list!")),
        }
    }

    let mut none = false;
    while !none {
        match **car {
            // if LIST: call eval inner on it with first_item=true
            Ctr::Seg(ref inner) => {
                match eval(inner, vars, funcs, sym_loose) {
                    Ok(res) => (*iter).car = res,
                    Err(e) => return Err(format!("Evaluation error: {}", e)),
                }
            }

            // if SYMBOL: unwrap naively
            Ctr::Symbol(ref tok) => {
                if let Some(val) = vars.get(tok.clone()) {
                    iter.car = val.clone();
                } else if sym_loose {
                    iter.car = car.clone()
                } else {
                    return Err(format!("Undefined variable: {}", tok.clone()));
                }
            }

            // if OTHER: clone and set
            _ => {
                iter.car = car.clone();
            }
        }

        match *ast.cdr {
            // if SYMBOL: unwrap naively, then end
            Ctr::Symbol(ref tok) => {
                if let Some(val) = vars.get(tok.clone()) {
                    iter.cdr = val.clone();
                } else if sym_loose {
                    iter.cdr = ast.cdr.clone()
                } else {
                    return Err(format!("Undefined variable: {}", tok.clone()));
                }

                none = true;
            }

            // if LIST:
            //    - iter.cdr = new_ast(None, None)
            //    - iter = iter.cdr
            //    - car = cdr.car
            //    - cdr = cdr.cdr
            //    - LOOP
            Ctr::Seg(next) => {
                iter.cdr = Box::new(Ctr::Seg(Seg::new()));
                ast = &next;
            }

            // if OTHER: clone and set, and then end
            _ => {
                iter.cdr = ast.cdr.clone();
                none = true;
            }
        }

        if let Ctr::None = **car {
            if let Ctr::None = *ast.cdr {
                none = true;
            }
        }
    }

    return Ok(ret);
}