# Lazy
> Lazy evaluation in Rust.
## Example
```rust
fn expensive() -> i32 {
println!("I am only evaluated once!"); 7
}
fn main() {
let a = lazy!(expensive());
// Thunks are just smart pointers!
assert_eq!(*a, 7); // "I am only evaluated once." is printed here
let b = [*a, *a]; // Nothing is printed.
assert_eq!(b, [7, 7]);
}
```
## API
> `lazy!($expr)`
Expands to `Thunk::new(|| { $expr })`
> `Thunk::new(|| -> T)`
Takes an FnOnce closure, creates a delayed computation.
> `Thunk::force()`
Forces the evaluation of the thunk so subsequent accesses are cheap. Values are
stored unboxed.
> `Thunk::unwrap()`
Consumes and forces the evaluation of the thunk and returns the contained
value.
> `Thunk::deref()` / `Thunk::deref_mut()`
Gets the value out of the thunk by evaluating the closure or grabbing it
from the cache. Allows you to call methods on the thunk as if it was
an instance of the contained valued through auto-deref.
There is also an equivalent API for `SyncThunk`, which is `Send + Sync` and
usable for safe, concurrent laziness, except that they are created using
`sync_lazy!` or by doing `use lazy::SyncThunk as Thunk` and using `lazy!`.