The so-called “unboxed closure” implementation in Rust has reached the point where it is time to start using it in the standard library. As a starting point, I have a pull request that removes proc from the language. I started on this because I thought it’d be easier than replacing closures, but it turns out that there are a few subtle points to this transition.

I am writing this blog post to explain what changes are in store and give guidance on how people can port existing code to stop using proc. This post is basically targeted Rust devs who want to adapt existing code, though it also covers the closure design in general.

To some extent, the advice in this post is a snapshot of the current Rust master. Some of it is specifically targeting temporary limitations in the compiler that we aim to lift by 1.0 or shortly thereafter. I have tried to mention when that is the case.

The new closure design in a nutshell

For those who haven’t been following, Rust is moving to a powerful new closure design (sometimes called unboxed closures). This part of the post covers the highlight of the new design. If you’re already familiar, you may wish to skip ahead to the “Transitioning away from proc” section.

The basic idea of the new design is to unify closures and traits. The first part of the design is that function calls become an overloadable operator. There are three possible traits that one can use to overload ():

trait Fn<A,R> { fn call(&self, args: A) -> R };
trait FnMut<A,R> { fn call_mut(&mut self, args: A) -> R };
trait FnOnce<A,R> { fn call_once(self, args: A) -> R };

As you can see, these traits differ only in their “self” parameter. In fact, they correspond directly to the three “modes” of Rust operation:

  • The Fn trait is analogous to a “shared reference” – it means that the closure can be aliased and called freely, but in turn the closure cannot mutate its environment.
  • The FnMut trait is analogous to a “mutable reference” – it means that the closure cannot be aliased, but in turn the closure is permitted to mutate its environment. This is how || closures work in the language today.
  • The FnOnce trait is analogous to “ownership” – it means that the closure can only be called once. This allows the closure to move out of its environment. This is how proc closures work today.

Enabling static dispatch

One downside of the older Rust closure design is that closures and procs always implied virtual dispatch. In the case of procs, there was also an implied allocation. By using traits, the newer design allows the user to choose between static and virtual dispatch. Generic types use static dispatch but require monomorphization, and object types use dynamic dispatch and hence avoid monomorphization and grant somewhat more flexibility.

As an example, whereas before I might write a function that takes a closure argument as follows:

fn foo(hashfn: |&String| -> uint) {
    let x = format!("Foo");
    let hash = hashfn(&x);

I can now choose to write that function in one of two ways. I can use a generic type parameter to avoid virtual dispatch:

fn foo<F>(hashfn: F)
    where F : FnMut(&String) -> uint
    let x = format!("Foo");
    let hash = hashfn(&x);

Note that we write the type parameters to FnMut using parentheses syntax (FnMut(&String) -> uint). This is a convenient syntactic sugar that winds up mapping to a traditional trait reference (currently, for<'a> FnMut<(&'a String,), uint>). At the moment, though, you are required to use the parentheses form, because we wish to retain the liberty to change precisely how the Fn trait type parameters work.

A caller of foo() might write:

let some_salt: String = ...;
foo(|str| myhashfn(str.as_slice(), &some_salt))

You can see that the || expression still denotes a closure. In fact, the best way to think of it is that a || expression generates a fresh structure that has one field for each of the variables it touches. It is as if the user wrote:

let some_salt: String = ...;
let closure = ClosureEnvironment { some_salt: &some_salt };

where ClosureEnvironment is a struct like the following:

struct ClosureEnvironment<'env> {
    some_salt: &'env String

impl<'env,'arg> FnMut(&'arg String) -> uint for ClosureEnvironment<'env> {
    fn call_mut(&mut self, (str,): (&'arg String,)) -> uint {
        myhashfn(str.as_slice(), &self.some_salt)

Obviously the || form is quite a bit shorter.

Using object types to get virtual dispatch

The downside of using generic type parameters for closures is that you will get a distinct copy of the fn being called for every callsite. This is a great boon to inlining (at least sometimes), but it can also lead to a lot of code bloat. It’s also often just not practical: many times we want to combine different kinds of closures together into a single vector. None of these concerns are specific to closures. The same things arise when using traits in general. The nice thing about the new closure design is that it lets us use the same tool – object types – in both cases.

If I wanted to write my foo() function to avoid monomorphization, I might change it from:

fn foo<F>(hashfn: F)
    where F : FnMut(&String) -> uint


fn foo(hashfn: &mut FnMut(&String) -> uint) {

Note that the argument is now a &mut FnMut(&String) -> uint, rather than being of some type F where F : FnMut(&String) -> uint.

One downside of changing the signature of foo() as I showed is that the caller has to change as well. Instead of writing:

foo(|str| ...)

the caller must now write:

foo(&mut |str| ...)

Therefore, what I expect to be a very common pattern is to have a “wrapper” that is generic which calls into a non-generic inner function:

fn foo<F>(hashfn: F)
    where F : FnMut(&String) -> uint
    foo_obj(&mut hashfn)

fn foo_obj(hashfn: &mut FnMut(&String) -> uint)

This way, the caller does not have to change, and only this outer wrapper is monomorphized, and it will likely be inlined away, and the “guts” of the function remain using virtual dispatch.

In the future, I’d like to make it possible to pass object types (and other “unsized” types) by value, so that one could write a function that just takes a FnMut() and not a &mut FnMut():

fn foo(hashfn: FnMut(&String) -> uint) {

Among other things, this makes it possible to transition simply between static and virtual dispatch without altering callers and without creating a wrapper fn. However, it would compile down to roughly the same thing as the wrapper fn in the end, though with guaranteed inlining. This change requires somewhat more design and will almost surely not occur by 1.0, however.

Specifying the closure type explicitly

We just said that every closure expression like || expr generates a fresh type that implements one of the three traits (Fn, FnMut, or FnOnce). But how does the compiler decide which of the three traits to use?

Currently, the compiler is able to do this inference based on the surrouding context – basically, the closure was an argument to a function, and that function requested a specific kind of closure, so the compiler assumes that’s the one you want. (In our example, the function foo() required an argument of type F where F implements FnMut.) In the future, I hope to improve the inference to a more general scheme.

Because the current inference scheme is limited, you will sometimes need to specify which of the three fn traits you want explicitly. (Some people also just prefer to do that.) The current syntax is to use a leading &:, &mut:, or :, kind of like an “anonymous parameter”:

// Explicitly create a `Fn` closure which cannot mutate its
// environment. Even though `foo()` requested `FnMut`, this closure
// can still be used, because a `Fn` closure is more general
// than `FnMut`.
foo(|&:| { ... })

// Explicitly create a `FnMut` closure. This is what the
// inference would select anyway.
foo(|&mut:| { ... })

// Explicitly create a `FnOnce` closure. This would yield an
// error, because `foo` requires a closure it can call multiple
// times in a row, but it is being given a closure that can be
// called exactly once.
foo(|:| { ... }) // (ERROR)

The main time you need to use an explicit fn type annotation is when there is no context. For example, if you were just to create a closure and assign it to a local variable, then a fn type annotation is required:

let c = |&mut:| { ... };

Caveat: It is still possible we’ll change the &:/&mut:/: syntax before 1.0; if we can improve inference enough, we might even get rid of it altogether.

Moving vs non-moving closures

There is one final aspect of closures that is worth covering. We gave the example of a closure |str| myhashfn(str.as_slice(), &some_salt) that expands to something like:

struct ClosureEnvironment<'env> {
    some_salt: &'env String

Note that the variable some_salt that is used from the surrounding environment is borrowed (that is, the struct stores a reference to the string, not the string itself). This is frequently what you want, because it means that the closure just references things from the enclosing stack frame. This also allows closures to modify local variables in place.

However, capturing upvars by reference has the downside that the closure is tied to the stack frame that created it. This is a problem if you would like to return the closure, or use it to spawn another thread, etc.

For this reason, closures can also take ownership of the things that they close over. This is indicated by using the move keyword before the closure itself (because the closure “moves” things out of the surrounding environment and into the closure). Hence if we change that same closure expression we saw before to use move:

move |str| myhashfn(str.as_slice(), &some_salt)

then it would generate a closure type where the some_salt variable is owned, rather than being a reference:

struct ClosureEnvironment {
    some_salt: String

This is the same behavior that proc has. Hence, whenever we replace a proc expression, we generally want a moving closure.

Currently we never infer whether a closure should be move or not. In the future, we may be able to infer the move keyword in some cases, but it will never be 100% (specifically, it should be possible to infer that the closure passed to spawn should always take ownership of its environment, since it must meet the 'static bound, which is not possible any other way).

Transitioning away from proc

This section covers what you need to do to modify code that was using proc so that it works once proc is removed.

Transitioning away from proc for library users

For users of the standard library, the transition away from proc is fairly straightforward. Mostly it means that code which used to write proc() { ... } to create a “procedure” should now use move|| { ... }, to create a “moving closure”. The idea of a moving closure is that it is a closure which takes ownership of the variables in its environment. (Eventually, we expect to be able to infer whether or not a closure must be moving in many, though not all, cases, but for now you must write it explicitly.)

Hence converting calls to libstd APIs is mostly a matter of search-and-replace:

Thread::spawn(proc() { ... }) // becomes:
Thread::spawn(move|| { ... })

task::try(proc() { ... }) // becomes:
task::try(move|| { ... })

One non-obvious case is when you are creating a “free-standing” proc:

let x = proc() { ... };

In that case, if you simply write move||, you will get some strange errors:

let x = move|| { ... };

The problem is that, as discussed before, the compiler needs context to determine what sort of closure you want (that is, Fn vs FnMut vs FnOnce). Therefore it is necessary to explicitly declare the sort of closure using the : syntax:

let x = proc() { ... }; // becomes:
let x = move|:| { ... };

Note also that it is precisely when there is no context that you must also specify the types of any parameters. Hence something like:

let x = proc(x:int) foo(x * 2, y);
//      ~~~~ ~~~~~
//       |     |
//       |     |
//       |     |
//       |   No context, specify type of parameters.
//       |
//      proc always owns variables it touches (e.g., `y`)

might become:

let x = move|: x:int| foo(x * 2, y);
//      ~~~~ ^ ~~~~~
//       |   |   |
//       |   |  No context, specify type of parameters.
//       |   |
//       |   No context, also specify FnOnce.
//       |
//     `move` keyword means that closure owns `y`

Transitioning away from proc for library authors

The transition story for a library author is somewhat more complicated. The complication is that the equivalent of a type like proc():Send ought to be Box<FnOnce() + Send> – that is, a boxed FnOnce object that is also sendable. However, we don’t currently have support for invoking fn(self) methods through an object, which means that if you have a Box<FnOnce()> object, you can’t call it’s call_once method (put another way, the FnOnce trait is not object safe). We plan to fix this – possibly by 1.0, but possibly shortly thereafter – but in the interim, there are workarounds you can use.

In the standard library, we use a trait called Invoke (and, for convenience, a type called Thunk). You’ll note that although these two types are publicly available (under std::thunk), these types do not appear in the public interface any other stable APIs. That is, Thunk and Invoke are essentially implementation details that end users do not have to know about. We recommend you follow the same practice. This is for two reasons:

  1. It generally makes for a better API. People would rather write Thread::spawn(move|| ...) and not Thread::spawn(Thunk::new(move|| ...)) (etc).
  2. Eventually, once Box<FnOnce()> works properly, Thunk and Invoke may be come deprecated. If this were to happen, your public API would be unaffected.

Basically, the idea is to follow the “thin wrapper” pattern that I showed earlier for hiding virtual dispatch. If you recall, I gave the example of a function foo that wished to use virtual dispatch internally but to hide that fact from its clients. It did do by creating a thin wrapper API that just called into another API, performing the object coercion:

fn foo<F>(hashfn: F)
    where F : FnMut(&String) -> uint
    foo_obj(&mut hashfn)

fn foo_obj(hashfn: &mut FnMut(&String) -> uint)

The idea with Invoke is similar. The public APIs are generic APIs that accept any FnOnce value. These just turnaround and wrap that value up into an object. Here the problem is that while we would probably prefer to use a Box<FnOnce()> object, we can’t because FnOnce is not (currently) object-safe. Therefore, we use the trait Invoke (I’ll show you how Invoke is defined shortly, just let me finish this example):

pub fn spawn<F>(taskbody: F)
    where F : FnOnce(), F : Send
    spawn_inner(box taskbody)

fn spawn_inner(taskbody: Box<Invoke+Send>)

The Invoke trait in the standard library is defined as:

trait Invoke<A=(),R=()> {
    fn invoke(self: Box<Self>, arg: A) -> R;

This is basically the same as FnOnce, except that the self type is Box<Self>, and not Self. This means that Invoke requires allocation to use; it is really tailed for object types, unlike FnOnce.

Finally, we can provide a bridge impl for the Invoke trait as follows:

impl<A,R,F> Invoke<A,R> for F
    where F : FnOnce(A) -> R
    fn invoke(self: Box<F>, arg: A) -> R {
        let f = *self;

This impl allows any type that implements FnOnce to use the Invoke trait.

High-level summary

Here are the points I want you to take away from this post:

  1. As a library consumer, the latest changes mostly just mean replacing proc() with move|| (sometimes move|:| if there is no surrounding context).
  2. As a library author, your public interface should be generic with respect to one of the Fn traits. You can then convert to an object internally to use virtual dispatch.
  3. Because Box<FnOnce()> doesn’t currently work, library authors may want to use another trait internally, such as std::thunk::Invoke.

I also want to emphasize that a lot of the nitty gritty details in this post are transitionary. Eventually, I believe we can reach a point where:

  1. It is never (or virtually never) necessary to explicitly declare Fn vs FnMut vs FnOnce explicitly.
  2. We can frequently (though not always) infer the keyword move.
  3. Box<FnOnce()> works, so Invoke and friends are not needed.
  4. The choice between static and virtual dispatch can be changed without affecting users and without requiring wrapper functions.

I expect the improvements in inference before 1.0. Fixing the final two points is harder and so we will have to see where it falls on the schedule, but if it cannot be done for 1.0 then I would expect to see those changes shortly thereafter.