Following last time, where we saw that, given parameterision over traits (rather than just types), we could implement functors and monads in Rust that supported existing “monad-like” traits like Iterator and Future, I thought it would be interesting to tackle another one of the arguments against monads in Rust.

I view this one more as an aside than a central point in the argument, but it’s worth addressing anyway (it’s also easier to solve than the general case of higher-kinded types (HKTs), which makes it attractive to cherry-pick).

What’s the point in question?

Rust’s imperative control flow statements like return and break inside of do notation also doesn’t make sense, because we do not have TCP preserving closures.

(Source.)

Let me clarify the objection that’s being made here.

do notation” refers to Haskell’s do notation, which is a special syntax for manipulating monads without necessitating long sequences of nested functions (we’ll see an example of this shortly).

“TCP” is an obscure initialism1, but people seem to use it in this sense: a language follows “TCP” if any expression expr is equivalent to (|| expr)()2. In general in Rust this is true, but it breaks down when expr contains control flow. Take the following program:

// This function returns 5.
fn early_return() -> u8 {
return 5;
0
}

If we enclose return 5; in a closure, we get a different result:

// This function returns 0.
fn abstracted_early_return() -> u8 {
(|| return 5)();
0
}

Why is this a problem? We’ll need to take a look at an example of do notation to make this clearer. I’m going to use a hypothetical syntax that, while not being too pretty, should at least be functional for our examples.

(Rather than write out the fully explicit syntax as in the last post, I’m going to assume the function traits are implicit and use the shorthand impl (impl Trait) for a type that implements a trait that implements a trait-on-traits. Obviously, if higher-order traits were to exist in Rust, we’d need a nicer syntax for them.)

Take the following snippet:

// monadA has type impl (impl Monad<A>).
// monadB has type impl (impl Monad<B>).
// The type annotations aren't necessary,
// but hopefully make things clearer.
//
// let! takes a monad and binds (using
// Monad::bind) its "inner value" to a
// new variable.
// return! takes a value and wraps it in
// a monad (using Monad::unit).
return! (a, b);
};
// monadAB has type impl (impl Monad<(A, B)>).

This is intuitively equivalent to:

(If you’re familiar with Haskell, let a! = monadA; corresponds to a <- monadA and return! expr; corresponds to return expr.)

You can see that the do notation version is a lot more readable. With more complex sequences of binds and units, the difference becomes even more pronounced.

However, there’s a problem with desugaring this in the obvious way shown above. We’d like to be able enclose any normal Rust block inside a do!. However, let’s take a look at what happens when we introduce control flow.

do! {
// This loop is a little pointless, but
// it gives us something to break from.
loop {
break;
println!("{} {}", a, b);
}
}

This naïvely desugars to:

loop {
break; // Uh oh...
println!("{} {}", a, b);
})
})
}

There’s an obvious problem: we expect break to break out of the loop, but since it’s now inside a closure, it’s not going to work (in fact, it won’t even compile). This is the problem withoutboats is referring to when they say that return and break don’t make sense in do notation.

We could simply forbid control flow expressions in do!, but this is very much an artificial (and to the user, a seemingly-arbitrary) solution and limits the general applicability and usefulness of do notation in Rust. Fortunately, however, there’s a solution.

## A better desugaring for do!

We’ll use a less specific example for the proposed desugaring so that it’s slightly clearer.

Take the do notation below:

do! {
expr1;
let! a = expr2;
expr3;
let! b = expr4;
expr5;
expr6(a, b);
return! expr7;
}

As a reminder, this is the naïve desugaring:

expr1;
expr2.bind(|a| {
expr3;
expr4.bind(|b| {
expr5;
expr6(a, b);
})
})

As we saw, this isn’t good enough, so instead, we’re going to desugar it like this3:

surface! {
expr1;
bubble! expr2.bind(|a| {
expr3;
bubble! expr4.bind(|b| {
expr5;
expr6(a, b);
})
})
}

where bubble! expr desugars to:

match expr {
ControlFlow::Return(_) => return expr,
ControlFlow::Break(Some(_)) => break expr,
ControlFlow::Break(None) => break,
ControlFlow::Continue => continue,
ControlFlow::Value(_) => expr,
// We'd also want Yield here eventually,
// but that comes with its own problems,
// which is a story for another time.
}

We’ve got a couple of options for surface!, depending on whether or not we want control flow to be able to bubble up out of do! (for instance, whether a return inside do! will return out of the function enclosing the do! or not).

If we do, surface! block desugars to:

match (|| block)() {
ControlFlow::Return(t) => return t,
ControlFlow::Break(Some(t)) => break t,
ControlFlow::Break(None) => break,
ControlFlow::Continue => continue,
ControlFlow::Value(t) => t,
}

If we want to forbid control flow at the top level, we need some custom error handling, but it’s straightforward, technically, to implement.

The enum ControlFlow is defined:

enum ControlFlow<T> {
Return(T),
Break(Option<T>),
Continue,
// Value simply means we're forwarding
// the value without effecting any control
// flow.
Value(T),
}

Essentially, ControlFlow reifies the control flow. By capturing the control flow inside a closure and forwarding it on immediately outside the closure, we can pretend that the control flow escapes the closure directly, which is precisely what we want for do notation.

## Final notes

As a serious proposal for a do notation desugaring in Rust, this has its flaws. Simulating the control flow in this way increases branching and it is thus likely not to have ideal performance. If we actually did want some form of do notation, we’d probably prefer to handle control flow more directly in the compiler, rather than simulating it using ControlFlow. But hopefully this demonstrates that it’s not necessary to have special handling to support monadic do in Rust and at least it’s not an issue with control flow that makes do notation implausible.

The issue with control flow in do notation is not the only one that was raised (see this tweet and this one for the others), so we don’t have a full solution yet, but by tackling the difficulties one at a time we can get closer to a point at which we understand where the difficulty with these abstractions in Rust really lie.

1. It stands for “Tennent’s Correspondence Principle”, if you’re wondering, but it’s not a standard term in programming language design. As far as I can tell was first mentioned in this old Rust internals post. This Stack Exchange post gives a bit more context.

2. This is simply $$\eta$$-equivalence for functions whose domain is the unit type (). (In general, $$\eta$$-equivalence for functions doesn’t hold for call-by-value languages with effects, but it’s plausible in this special case it could hold, as () has no effect.)

3. surface! and bubble! here aren’t actually macros: in actuality, we would probably define the desugaring in a single step, which would render them unnecessary, but they help illustrate what’s going on.