1 files changed, 74 insertions, 0 deletions
diff --git a/src/lazy.rs b/src/lazy.rs
new file mode 100644
index 0000000..b4f801b
--- /dev/null
+++ b/src/lazy.rs
@@ -0,0 +1,74 @@
+#![macro_escape]
+
+use std::ptr;
+use std::cell::UnsafeCell;
+use std::ops::Deref;
+use std::boxed::FnBox;
+
+pub enum State<V> {
+    Evaluated(V),
+    Evaluating,
+    Unevaluated(Box<FnBox() -> V>)
+}
+
+pub struct Lazy<V> {
+    state: UnsafeCell<State<V>>,
+}
+
+impl<V> Lazy<V> {
+    pub fn new(f: Box<FnBox() -> V>) -> Lazy<V> {
+        Lazy { state: UnsafeCell::new(State::Unevaluated(f)) }
+    }
+
+    pub fn ready(v: V) -> Lazy<V> {
+        Lazy { state: UnsafeCell::new(State::Evaluated(v)) }
+    }
+
+    pub fn force(&mut self) {
+        self.value();
+    }
+
+    pub fn unwrap(self) -> V {
+        unsafe {
+            match self.state.into_inner() {
+                State::Unevaluated(f) => f(),
+                State::Evaluating => panic!("Illegal state, can't call unwrap during evaluation"),
+                State::Evaluated(v) => v
+            }
+        }
+    }
+
+    pub fn value(&self) -> &V {
+        unsafe {
+            let state = self.state.get();
+            match *state {
+                State::Evaluated(ref v) => v,
+                State::Evaluating => panic!("Illegal state, can't call value during evaluation"),
+                State::Unevaluated(_) => {
+                    if let State::Unevaluated(f) = ptr::replace(state, State::Evaluating) {
+                        ptr::replace(state, State::Evaluated(f()));
+                    }
+                    if let State::Evaluated(ref v) = *state { return v }
+                    unreachable!()
+                }
+            }
+        }
+    }
+}
+
+impl<V> Deref for Lazy<V> {
+    type Target = V;
+    fn deref(&self) -> &V { self.value() }
+}
+
+#[macro_export]
+macro_rules! lazy {
+    (@as_expr $val: expr) => { $val };
+    ($($val: tt)*) => { Lazy::new(Box::new(move || lazy![@as_expr { $($val)* }])) };
+}
+
+#[macro_export]
+macro_rules! eager {
+    (@as_expr $val: expr) => { $val };
+    ($($val: tt)*) => { Lazy::<_>::ready(eager![@as_expr { $($val)* }]) };
+}