gistfile1.rs

use std::fmt::Display;
use std::fmt::Formatter;
use std::fmt::Error;

//#[allow(dead_code)]
#[derive(Copy,Clone)]
enum Opb {
    PAIR,
    ADD,
    SUB,
    MUL,
    DIV,
    TYP,
    EQ  
}

impl Display for Opb {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        match *self {
           Opb::PAIR => write!(f, ","),
           Opb::ADD => write!(f, "+"),
           Opb::SUB => write!(f, "-"),
           Opb::MUL => write!(f, "*"),
           Opb::DIV => write!(f, "/"),
           Opb::TYP => write!(f, ":"),
           Opb::EQ  => write!(f, "=")
        }
    }   
}

#[derive(Copy,Clone)]
enum Opu {
    NOP,
    COMMUTE,
    RDIST
}

impl Display for Opu {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        match *self {
            Opu::COMMUTE => write!(f, "commute"),
            Opu::NOP => write!(f, "nop"),
            Opu::RDIST => write!(f, "rdist")
        }
    }   
}

#[derive(Clone)] 
enum Expr {
    Name(&'static str),
    Int(i32),
    BinOp(Box<Expr>, Opb, Box<Expr>),
    UOp(Box<Expr>,Opu)
}

impl Display for Expr {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        match *self {
            Expr::Name(ref s) => write!(f, "{}", s),
            Expr::Int(ref i)  => write!(f, "{}", i),
            Expr::BinOp(ref l,ref op,ref r) => write!(f, "({} {} {})", l, op, r),
            Expr::UOp(ref l,ref op) => write!(f, "({} {})", l, op)
        }
    }
}

////Relatively straightforward because nothing is copied
// (a op b) -> (b op a)
fn commute(expr: Expr) -> Expr {
    match expr {
        Expr::BinOp(l, op, r) => Expr::BinOp(r, op, l),
        _ => expr
    }
}

////This was rather tricky because of the boxes, and copying and cloning
// ( (ll opl rl) op r) -> ( (ll op r) opl (rl op r) )
fn rdistrib(expr: Expr) -> Expr {
    match expr.clone() {
        Expr::BinOp(l, op, r) =>
                match (*l).clone() {
                    Expr::BinOp(ll, opl, rl) =>
                        Expr::BinOp(
                            Box::new(
                                Expr::BinOp(
                                    Box::new((*ll).clone()),
                                    op,
                                    Box::new((*r).clone()),
                                )
                            ),
                            opl,
                            Box::new(
                                Expr::BinOp(
                                    Box::new((*rl).clone()),
                                    op,
                                    Box::new((*r).clone()),
                                )
                            ),
                        ),
                    _ => expr
                },
        _ => expr
    }
}

////Remove the tedium of constructing binops
fn do_opb(expr1: Expr, op: Opb, expr2: Expr) -> Expr {
    let x = Expr::BinOp(Box::new(expr1.clone()), op, Box::new(expr2.clone()));
    println!("{}", x);
    x
}

fn do_reduce(expr1: Expr) -> Expr {
    match expr1.clone() {
        Expr::UOp(x, uo) => {
            match uo {
                Opu::COMMUTE => commute(*x),
                Opu::RDIST => rdistrib(*x),
                Opu::NOP => *x
            }
        },
        _ => expr1
    }
}

fn do_opu(expr1: Expr, op: Opu) -> Expr {
    let mut x = Expr::UOp(Box::new(expr1.clone()), op);
    x = do_reduce(x);
    println!("{}", x);
    x
}

////Simple test of garbage collection free symbolic manipulation
fn main() {
    let mut x0 : Expr = Expr::Name("a");
    x0 = do_opb(x0, Opb::TYP, Expr::Name("t"));
    x0 = do_opb(x0, Opb::PAIR, Expr::Name("z"));
    x0 = do_opb(x0, Opb::ADD, Expr::Int(1));
    x0 = do_opu(x0, Opu::COMMUTE);
    x0 = do_opb(x0, Opb::MUL, Expr::Name("b"));
    x0 = do_opu(x0, Opu::RDIST);
    x0 = do_opb(x0, Opb::EQ, Expr::Name("c"));
    x0 = do_opb(x0, Opb::ADD, Expr::Int(42));
    x0 = do_opu(x0, Opu::RDIST);
    x0 = do_opb(x0, Opb::DIV, Expr::Int(2));
    x0 = do_opu(x0, Opu::RDIST);
    do_opu(x0, Opu::NOP);
}