For those who find this question by searches for Rust, Lazy, and LazyList, I here post the final generic working code for Lazy and LazyList with both non and thread-safe versions working with current stable Rust version 1.56.1.
The code contains some methods not actually used by the included test code, and especially the unwrap() methods are useless here as we can't consume a type embedded in another type (unless we replace an interior mutable value); a few more tests would need to be devised for the singleton() method, the unwrap() methods, and the tail()method.
Because we can't generally unwrap, the embedded type must be Clone; this costs some performance in the copy operations involved, so when types are large (copy-wise), one may want to wrap them in an Rc for faster reference-counted cloning.
The code is as follows:
pub struct Thunk<'a, R>(Box<dyn FnOnce() -> R + 'a>);
impl<'a, R: 'a> Thunk<'a, R> {
#[inline(always)]
fn new<F: 'a + FnOnce() -> R>(func: F) -> Thunk<'a, R> {
Thunk(Box::new(func))
}
#[inline(always)]
fn invoke(self) -> R { self.0() }
}
// Lazy is lazily evaluated contained value using the above Thunk implementation...
mod lazy {
use crate::Thunk;
use std::cell::UnsafeCell;
use std::mem::replace;
use std::ops::Deref;
// Lazy is lazily evaluated contained value using the above Thunk implementation...
pub struct Lazy<'a, T: 'a>(UnsafeCell<LazyState<'a, T>>);
enum LazyState<'a, T: 'a> {
Unevaluated(Thunk<'a, T>),
EvaluationInProgress,
Evaluated(T),
}
use self::LazyState::*;
impl<'a, T: 'a> Lazy<'a, T> {
#[inline]
pub fn new<F: 'a + FnOnce() -> T>(func: F) -> Lazy<'a, T> {
Lazy(UnsafeCell::new(Unevaluated(Thunk::new(func))))
}
#[inline]
pub fn evaluated(val: T) -> Lazy<'a, T> {
Lazy(UnsafeCell::new(Evaluated(val)))
}
#[inline(always)]
fn force<'b>(&'b self) {
unsafe {
match *self.0.get() {
Evaluated(_) => return, // nothing required; already Evaluated
EvaluationInProgress => panic!("Lazy::force called recursively!!!"),
_ => {
let ue = replace(&mut *self.0.get(), EvaluationInProgress);
if let Unevaluated(thnk) = ue {
*self.0.get() = Evaluated(thnk.invoke());
} // no other possiblity!
}
}
}
}
#[inline]
pub fn unwrap<'b>(self) -> T where T: 'b { // consumes the object to produce the value
self.force(); // evaluatate if not evealutated
match self.0.into_inner() {
Evaluated(v) => v,
_ => unreachable!() // previous code guarantees never not Evaluated
}
}
}
impl<'a, T: 'a> Deref for Lazy<'a, T> {
type Target = T;
#[inline]
fn deref<'b>(&'b self) -> &'b T {
self.force(); // evaluatate if not evalutated
match unsafe { &*self.0.get() } {
&Evaluated(ref v) => return v,
_ => unreachable!(),
}
}
}
}
mod lazy_sync {
use crate::Thunk;
use std::cell::UnsafeCell;
use std::mem::replace;
use std::sync::Mutex;
use std::sync::atomic::AtomicBool;
use std::sync::atomic::Ordering::Relaxed;
use std::ops::Deref;
pub struct Lazy<'a, T: 'a + Send + Sync>(
UnsafeCell<LazyState<'a, T>>, AtomicBool, Mutex<()>);
enum LazyState<'a, T: 'a + Send + Sync> {
Unevaluated(Thunk<'a, T>),
EvaluationInProgress,
Evaluated(T),
}
use self::LazyState::*;
unsafe impl<'a, T: 'a + Send + Sync> Send for Lazy<'a, T> {}
unsafe impl<'a, T: 'a + Send + Sync> Sync for Lazy<'a, T> {}
impl<'a, T: 'a + Send + Sync> Lazy<'a, T> {
#[inline]
pub fn new<F: 'a + FnOnce() -> T>(func: F) -> Lazy<'a, T> {
Lazy(UnsafeCell::new(Unevaluated(Thunk::new(func))),
AtomicBool::new(false), Mutex::new(()))
}
#[inline]
pub fn evaluated(val: T) -> Lazy<'a, T> {
Lazy(UnsafeCell::new(Evaluated(val)),
AtomicBool::new(true), Mutex::new(()))
}
#[inline(always)]
fn force<'b>(&'b self) {
unsafe {
if !self.1.load(Relaxed) {
let _ = self.2.lock();
// if we don't get the false below, means
// another thread already handled the thunk,
// including setting to true, still processing when checked
if !self.1.load(Relaxed) {
match *self.0.get() {
Evaluated(_) => return, // nothing required; already Evaluated
EvaluationInProgress => unreachable!(), // because lock race recursive evals...
_ => {
if let Unevaluated(thnk) = replace(&mut *self.0.get(), EvaluationInProgress) {
*self.0.get() = Evaluated(thnk.invoke());
} // no other possiblity!
}
}
self.1.store(true, Relaxed);
}
}
}
}
#[inline]
pub fn unwrap<'b>(self) -> T where T: 'b { // consumes the object to produce the value
self.force(); // evaluatate if not evealutated
match self.0.into_inner() {
Evaluated(v) => v,
_ => unreachable!() // previous code guarantees never not Evaluated
}
}
}
impl<'a, T: 'a + Send + Sync> Deref for Lazy<'a, T> {
type Target = T;
#[inline]
fn deref<'b>(&'b self) -> &'b T {
self.force(); // evaluatate if not evalutated
match unsafe { &*self.0.get() } {
&Evaluated(ref v) => return v,
_ => unreachable!(),
}
}
}
}
// LazyList is an immutable lazily-evaluated persistent (memoized) singly-linked list
// similar to lists in Haskell, although here only tails are lazy...
// depends on the contained type being Clone so that the LazyList can be
// extracted from the reference-counted Rc heap objects in which embedded.
mod lazylist {
use crate::lazy::Lazy;
use std::rc::Rc;
use std::mem::{replace, swap};
#[derive(Clone)]
pub enum LazyList<'a, T: 'a + Clone> {
Empty,
Cons(T, RcLazyListNode<'a, T>),
}
pub use self::LazyList::Empty;
use self::LazyList::Cons;
type RcLazyListNode<'a, T> = Rc<Lazy<'a, LazyList<'a, T>>>;
// impl<'a, T:'a> !Sync for LazyList<'a, T> {}
impl<'a, T: 'a + Clone> LazyList<'a, T> {
#[inline]
pub fn singleton(v: T) -> LazyList<'a, T> {
Cons(v, Rc::new(Lazy::evaluated(Empty)))
}
#[inline]
pub fn cons<F>(v: T, cntf: F) -> LazyList<'a, T>
where F: 'a + FnOnce() -> LazyList<'a, T>
{
Cons(v, Rc::new(Lazy::new(cntf)))
}
#[inline]
pub fn head<'b>(&'b self) -> &'b T {
if let Cons(ref hd, _) = *self {
return hd;
}
panic!("LazyList::head called on an Empty LazyList!!!")
}
#[inline]
pub fn tail<'b>(&'b self) -> &'b Lazy<'a, LazyList<'a, T>> {
if let Cons(_, ref rlln) = *self {
return &*rlln;
}
panic!("LazyList::tail called on an Empty LazyList!!!")
}
#[inline]
pub fn unwrap(self) -> (T, RcLazyListNode<'a, T>) {
// consumes the object
if let Cons(hd, rlln) = self {
return (hd, rlln);
}
panic!("LazyList::unwrap called on an Empty LazyList!!!")
}
#[inline]
fn iter(&self) -> Iter<'a, T> {
Iter(self)
}
pub fn from_iter<I: IntoIterator<Item = T> + 'a>(itrbl: I) -> LazyList<'a, T> {
let itr = itrbl.into_iter();
#[inline(always)]
fn next_iter<'b, R, Itr>(mut iter: Itr) -> LazyList<'b, R>
where R: 'b + Clone,
Itr: 'b + Iterator<Item = R>
{
match iter.next() {
Some(val) => LazyList::cons(val, move || next_iter(iter)),
None => Empty,
}
}
next_iter(itr)
}
}
impl<'a, T: 'a + Clone> Iterator for LazyList<'a, T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
match replace(self, Empty) {
Cons(hd, rlln) => {
let mut newll = (*rlln).clone();
swap(self, &mut newll); // self now contains tail, newll contains the Empty
Some(hd)
}
_ => None,
}
}
}
pub struct Iter<'a, T: 'a + Clone>(*const LazyList<'a, T>);
impl<'a, T: 'a + Clone> Iterator for Iter<'a, T> {
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
unsafe {
if let LazyList::Cons(ref v, ref r) = *self.0 {
self.0 = &***r;
Some(v)
} else {
None
}
}
}
}
impl<'i, 'l, T: 'i + Clone> IntoIterator for &'l LazyList<'i, T> {
type Item = &'i T;
type IntoIter = Iter<'i, T>;
fn into_iter(self) -> Self::IntoIter {
self.iter()
}
}
/*
impl<'a, T: 'a + Clone> FromIterator<T> for LazyList<'a, T> {
fn from_iter<I: IntoIterator<Item = T> + 'a>(itrbl: I) -> LazyList<'a, T> {
let itr = itrbl.into_iter();
#[inline(always)]
fn next_iter<'b, R, Itr>(mut iter: Itr) -> LazyList<'b, R>
where R: 'b + Clone,
Itr: 'b + Iterator<Item = R>
{
match iter.next() {
Some(val) => LazyList::cons(val, move || next_iter(iter)),
None => Empty,
}
}
next_iter(itr)
}
}
*/
}
mod lazylist_sync {
use crate::lazy_sync::Lazy;
use std::sync::Arc as Rc;
use std::mem::{replace, swap};
#[derive(Clone)]
pub enum LazyList<'a, T: 'a + Send + Sync + Clone> {
Empty,
Cons(T, RcLazyListNode<'a, T>),
}
pub use self::LazyList::Empty;
use self::LazyList::Cons;
type RcLazyListNode<'a, T> = Rc<Lazy<'a, LazyList<'a, T>>>;
unsafe impl<'a, T: 'a + Send + Sync + Clone> Send for LazyList<'a, T> {}
unsafe impl<'a, T: 'a + Send + Sync + Clone> Sync for LazyList<'a, T> {}
impl<'a, T: 'a + Send + Sync + Clone> LazyList<'a, T> {
#[inline]
pub fn singleton(v: T) -> LazyList<'a, T> {
Cons(v, Rc::new(Lazy::evaluated(Empty)))
}
#[inline]
pub fn cons<F>(v: T, cntf: F) -> LazyList<'a, T>
where F: 'a + FnOnce() -> LazyList<'a, T>
{
Cons(v, Rc::new(Lazy::new(cntf)))
}
#[inline]
pub fn head<'b>(&'b self) -> &'b T {
if let Cons(ref hd, _) = *self {
return hd;
}
panic!("LazyList::head called on an Empty LazyList!!!")
}
#[inline]
pub fn tail<'b>(&'b self) -> &'b Lazy<'a, LazyList<'a, T>> {
if let Cons(_, ref rlln) = *self {
return &*rlln;
}
panic!("LazyList::tail called on an Empty LazyList!!!")
}
#[inline]
pub fn unwrap(self) -> (T, RcLazyListNode<'a, T>) {
// consumes the object
if let Cons(hd, rlln) = self {
return (hd, rlln);
}
panic!("LazyList::unwrap called on an Empty LazyList!!!")
}
#[inline]
fn iter(&self) -> Iter<'a, T> {
Iter(self)
}
pub fn from_iter<I: IntoIterator<Item = T> + 'a>(itrbl: I) -> LazyList<'a, T> {
let itr = itrbl.into_iter();
#[inline(always)]
fn next_iter<'b, R: 'b + Send + Sync, Itr>(mut iter: Itr) -> LazyList<'b, R>
where R: 'b + Clone,
Itr: 'b + Iterator<Item = R>
{
match iter.next() {
Some(val) => LazyList::cons(val, move || next_iter(iter)),
None => Empty,
}
}
next_iter(itr)
}
}
impl<'a, T: 'a + Send + Sync + Clone> Iterator for LazyList<'a, T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
match replace(self, Empty) {
Cons(hd, rlln) => {
let mut newll = (*rlln).clone();
swap(self, &mut newll); // self now contains tail, newll contains the Empty
Some(hd)
}
_ => None,
}
}
}
pub struct Iter<'a, T: 'a + Send + Sync + Clone>(*const LazyList<'a, T>);
impl<'a, T: 'a + Send + Sync + Clone> Iterator for Iter<'a, T> {
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
unsafe {
if let LazyList::Cons(ref v, ref r) = *self.0 {
self.0 = &***r;
Some(v)
} else {
None
}
}
}
}
impl<'i, 'l, T: 'i + Send + Sync + Clone> IntoIterator for &'l LazyList<'i, T> {
type Item = &'i T;
type IntoIter = Iter<'i, T>;
fn into_iter(self) -> Self::IntoIter {
self.iter()
}
}
/*
impl<'a, T: 'a + Send + Sync + Clone> FromIterator<T> for LazyList<'a, T> {
fn from_iter<I: IntoIterator<Item = T> + 'a>(itrbl: I) -> LazyList<'a, T> {
let itr = itrbl.into_iter();
#[inline(always)]
fn next_iter<'b, R: 'b + Send + Sync, Itr>(mut iter: Itr) -> LazyList<'b, R>
where R: 'b + Clone,
Itr: 'b + Iterator<Item = R>
{
match iter.next() {
Some(val) => LazyList::cons(val, move || next_iter(iter)),
None => Empty,
}
}
next_iter(itr)
}
}
*/
}
use self::lazylist::LazyList;
//use self::lazylist_sync::LazyList; // for slower thread-safe version
fn main() {
fn fib<'a>() -> LazyList<'a, u64> {
fn fibi<'b>(f: u64, s: u64) -> LazyList<'b, u64> {
LazyList::cons(f, move || { let n = &f + &s; fibi(s, n) })
}
fibi(0, 1)
}
let test1 = fib();
for v in test1.take(20) {
print!("{} ", v);
}
println!("");
let test2 = LazyList::from_iter(0..);
for i in (&test2).into_iter().take(15) {
print!("{} ", i)
} // and from_iter() works
println!("");
}
with the output as follows:
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Note that although this shows that functional programming style with LazyList is possible in Rust, it doesn't mean that it should be the preferred style for all use cases, especially if high performance is desired. For instance, if the above fib() function were written to output an iterator directly rather than a LazyList, then it would only take a very few CPU clock cycles per iteration (other than if infinite precision BigUint were used, which are slower) rather than the hundreds of cycles needed per iteration for the LazyList (and many more for the "sync" version).
In general, more imperative implementations, perhaps using Vec<T> if memoization is required, are much more performant than this due to the high overhead of reference counting, the many small allocations/deallocations and the clone/copying required for functional style programming.
