feat(tazjin/generator-example): add an example for genawaiter crate

This is an experiment for tvix-eval.

Change-Id: Ic752b5b125cefefeb1343e38a70beb364478e6eb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8131
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

users/tazjin/generator-example/src/main.rs

@@ -0,0 +1,115 @@
+use genawaiter::rc::{Co, Gen};
+use std::cell::RefCell;
+use std::future::Future;
+use std::pin::Pin;
+use std::rc::Rc;
+
+#[derive(Debug)]
+enum ValueRepr {
+    Int(i64),
+    Thunk((i64, i64)),
+}
+
+#[derive(Clone, Debug)]
+struct Value(Rc<RefCell<ValueRepr>>);
+
+impl Value {
+    fn force(&self) {
+        let mut inner = self.0.borrow_mut();
+        match *inner {
+            ValueRepr::Int(_) => return,
+            ValueRepr::Thunk((a, b)) => {
+                *inner = ValueRepr::Int(a + b);
+            }
+        }
+    }
+
+    fn is_forced(&self) -> bool {
+        matches!(*self.0.borrow(), ValueRepr::Int(_))
+    }
+
+    fn int(&self) -> i64 {
+        match *self.0.borrow() {
+            ValueRepr::Int(i) => i,
+            ValueRepr::Thunk(_) => panic!("unforced thunk!"),
+        }
+    }
+}
+
+impl From<i64> for Value {
+    fn from(value: i64) -> Self {
+        Value(Rc::new(RefCell::new(ValueRepr::Int(value))))
+    }
+}
+
+impl From<(i64, i64)> for Value {
+    fn from(value: (i64, i64)) -> Self {
+        Value(Rc::new(RefCell::new(ValueRepr::Thunk(value))))
+    }
+}
+
+async fn list_maker(values: Vec<Value>, co: Co<Value>) -> Vec<i64> {
+    let mut output: Vec<i64> = vec![];
+
+    for value in values {
+        if !value.is_forced() {
+            co.yield_(value.clone()).await;
+        }
+
+        output.push(value.int());
+    }
+
+    output
+}
+
+async fn list_reverser(values: Vec<Value>, co: Co<Value>) -> Vec<i64> {
+    let mut output = list_maker(values, co).await;
+    output.reverse();
+    output
+}
+
+struct Frame {
+    gen: Gen<Value, (), Pin<Box<dyn Future<Output = Vec<i64>>>>>,
+}
+
+fn pin_future(
+    f: impl Future<Output = Vec<i64>> + 'static,
+) -> Pin<Box<dyn Future<Output = Vec<i64>>>> {
+    Box::pin(f)
+}
+
+fn main() {
+    let mut frames: Vec<Frame> = vec![];
+
+    let values: Vec<Value> = vec![
+        42.into(),
+        (12, 54).into(),
+        4.into(),
+        (40, 2).into(),
+        2.into(),
+    ];
+    let second = values.clone();
+
+    frames.push(Frame {
+        gen: Gen::new(|co| pin_future(list_maker(values, co))),
+    });
+
+    frames.push(Frame {
+        gen: Gen::new(|co| pin_future(list_reverser(second, co))),
+    });
+
+    for (idx, mut frame) in frames.into_iter().enumerate() {
+        loop {
+            match frame.gen.resume() {
+                genawaiter::GeneratorState::Yielded(val) => {
+                    println!("yielded {:?} in frame {}", val, idx);
+                    val.force();
+                }
+                genawaiter::GeneratorState::Complete(list) => {
+                    println!("result {}: {:?}", idx, list);
+                    break;
+                }
+            }
+        }
+    }
+}