flesh/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 std::rc::Rc;
use std::cell::RefCell;
use crate::segment::{Ast, Ctr, new_ast};
use crate::func::{FTable, func_call};
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(
    ast: Ast,
    vars: Rc<RefCell<VTable>>,
    funcs: Rc<RefCell<FTable>>,
    sym_loose: bool
) -> Result<Ctr, String> {
    let mut car = ast.borrow().clone().car;
    let mut cdr = ast.borrow().clone().cdr;
    let ret = new_ast(Ctr::None, Ctr::None);
    let mut iter = ret.clone();

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

    // another check to detect if we may have a function call
    if let Ctr::Symbol(ref tok) = car {
        match cdr {
            Ctr::Seg(ast) => {
                if let Some(func) = funcs.borrow().get(tok) {
                    return func_call(func.clone(), ast.clone(), vars.clone(), funcs.clone())
                } else {
                    return Err(format!("Couldnt find function: {}.", tok))
                }
            },
            Ctr::None => {
                if let Some(func) = funcs.borrow().get(tok) {
                    return func_call(func.clone(), new_ast(Ctr::None, Ctr::None), vars.clone(), funcs.clone())
                } else {
                    return Err(format!("Couldnt find function: {}.", tok))
                }
            },
            _ => 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.clone(), vars.clone(), funcs.clone(), sym_loose) {
                    Ok(res) => (*iter).borrow_mut().car = res,
                    Err(e) => return Err(format!("Evaluation error: {}", e))
                }
            },

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

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

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

                none = true;
            },

            // if LIST:
            //    - iter.cdr = new_ast(None, None)
            //    - iter = iter.cdr
            //    - car = cdr.car
            //    - cdr = cdr.cdr
            //    - LOOP
            Ctr::Seg(next) => {
                let n = new_ast(Ctr::None, Ctr::None);
                iter.borrow_mut().cdr = Ctr::Seg(n.clone());
                iter = n;
                car = next.borrow().clone().car;
                cdr = next.borrow().clone().cdr;
            }

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

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

    return Ok(Ctr::Seg(ret))
}