Draft RFC: variadic generics

[rust-highfive]

Issue by eddyb
Monday Oct 28, 2013 at 17:39 GMT

For earlier discussion, see rust-lang/rust#10124

This issue was labelled with: B-RFC in the Rust repository

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The Problem

• fn types need a reform, and being able to define a trait with a variable number of type parameters would help
• working with functions which have a variable number of arguments is impossible right now (e.g. a generic bind method, f.bind(a, b)(c) == f(a, b, c)) and defining such functions may only be done (in a limited fashion) with macros
• tuples also have similar restrictions right now, there is no way to define a function which takes any tuple and returns the first element, for example

The Solution: Part One

C++11 sets a decent precedent, with its variadic templates, which can be used to define type-safe variadic functions, among other things.
I propose a similar syntax, a trailing ..T in generic formal type parameters:

type Tuple<..T> = (..T); // the parens here are the same as the ones around a tuple.
// use => expansion
Tuple<> => ()
Tuple<int> => (int,)
Tuple<A, B, C> => (A, B, C)

The simple example above only uses ..T, but not T itself.
The question which arises is this: what is T? C++11 has a special case for variadic parameter packs, but we can do better.
We have tuples. We can use them to store the actual variadic generic type parameters:

// Completely equivalent definition:
type Tuple<..T> = T;
// Detailed expansion of (..T) from above:
Tuple<> => (..()) => ()
Tuple<int> => (..(int,)) => (int,)
Tuple<A, B, C> => (..(A, B, C)) => (A, B, C)

The Solution: Part Two

Now that we know the answer is "tuples", everything else is about extending them.
The prefix .. operator would expand a tuple type:

// in another tuple type:
type AppendTuple<T, U> = (..T, U);
type PrependTuple<T, U> = (U, ..T);
AppendTuple<PrependTuple<Tuple<B>, A>, C> => (A, B, C)
// in a fn type's parameter list:
type VoidFn<..Args> = fn(..Args);
VoidFn<A, B, C> => fn(A, B, C);
// in a variadic generic parameter list:
Tuple<..(A, B, C), ..(D, E, F)> => (A, B, C, D, E, F)

Then we can do the same thing with values:

// in another tuple:
(..(true, false), ..(1, "foo"), "bar") => (true, false, 1, "foo", "bar")
// in a function call:
f(..(a, b), ..(c, d, e), f) => f(a, b, c, d, e, f)

There's only one piece missing: we're still not able to define a function which takes a variable number of arguments.
For this, I propose ..x: T (where T is a tuple type) in a pattern, which can be used to "capture" multiple arguments when used in fn formal arguments:

// Destructure a tuple.
let (head, ..tail) = foo;
// This function has the type fn(int, A, B, C), and can be called as such:
fn bar(_: int, ..x: (A, B, C)) -> R {...}

A type bound for ..T (i.e. impl<..T: Trait>) could mean that the tuple T has to satisfy the bound (or each type in T, but that's generally less useful).

Examples:

// The highly anticipated Fn trait.
trait Fn<Ret, ..Args> {
    fn call(self, .._: Args) -> Ret;
}
impl Fn<Ret, ..Args> for fn(..Args) -> Ret {
    fn call(self, ..args: Args) -> Ret {
        self(..args)
    }
}
impl Fn<Ret, ..Args> for |..Args| -> Ret {
    fn call(self, ..args: Args) -> Ret {
        self(..args)
    }
}

// Cloning tuples (all cons-list-like algorithms can be implemented this way).
impl<Head, ..Tail: Clone> Clone for (Head, ..Tail) {
    fn clone(&self @ &(ref head, ..ref tail)) -> (Head, ..Tail) {
        (head.clone(), ..tail.clone())
    }
}
impl Clone for () {
    fn clone(&self) -> () {
        ()
    }
}

// Restricting all types in a variadic tuple to one type.
trait ArrayLikeTuple<T> {
    fn len() -> uint;
}
impl<T> ArrayLikeTuple<T> for () {
    fn len() -> uint {0}
}
impl<T, ..Tail: ArrayLikeTuple<T>> ArrayLikeTuple<T> for (T, ..Tail) {
    fn len() -> uint {
        1 + Tail::len()
    }
}

// And using that trait to write variadic container constructors.
impl<T> Vec<T> {
    fn new<..Args: ArrayLikeTuple<T>>(..args: Args) -> Vec<T> {
        let v: Vec<T> = Vec::with_capacity(Args::len());
        // Use an unsafe move_iter-like pattern to move all the args
        // directly into the newly allocated vector. 
        // Alternatively, implement &move [T] and move_iter on that.
    }
}

// Zipping tuples, using a `Tuple` kind and associated items.
trait TupleZip<Other>: Tuple {
    type Result: Tuple;
    fn zip(self, other: Other) -> Result;
}
impl TupleZip<()> for () {
    type Result = ();
    fn zip(self, _: ()) -> () {}
}
impl<A, ATail: TupleZip<BTail>, B, BTail: Tuple> TupleZip<(B, ..BTail)> for (A, ..ATail) {
    type Result = ((A, B), ..ATail::Result);
    fn zip(self @ (a, ..a_tail), (b, ..b_tail): (B, ..BTail)) -> Result {
        ((a, b), ..a_tail.zip(b_tail))
    }
}

Todo

• better formal descriptions
• figure out the details of pattern matching
• more examples

[eddyb]

There is one issue with my initial approach: a reference to a "subtuple" of a larger tuple could have different padding than the tuple type made out of those fields.

One solution I may have is a &(ref head, ..ref tail) pattern turning &(A, B, C, D) into &A and (&B, &C, &D), i.e. a subtuple of references instead of a reference of a subtuple.
But I have no idea how inference or generics would work with it.

[alexchandel]

This is probably a prerequisite for taking the Cartesian product of iterators without a macro, like in rust-itertools/iproduct! and rust-itertools/itertools#10.

[Diggsey]

This should be expanded to also support using the ".." operator on fixed size arrays (behaves equivalent to a tuple where the elements are all the same type and the arity matches the length of the array).

In combination with #174 this would make it much easier to deal with variadic arguments of the same type. Something which even C++ is still missing.

[alexchandel]

This would need to be compatible with range syntax.

[gnzlbg]

How would one implement the cartesian product of two type tuples with this?

[eddyb]

@gnzlbg what exactly do you have in mind, Product<(A, B), (X, Y)> = ((A, X), (A, Y), (B, X), (B, Y))?

[gnzlbg]

@eddyb yes, but with variadics.

[goertzenator]

Having wrestled with c++11 templates and Rust macros, I really like this RFC. I hope it comes to fruition sooner rather than later.

[oblitum]

Me too, I really wished this to come before 1.0 and solved tuple impl etc.

[dgrunwald]

Unfortunately this syntax doesn't work, because the .. operator is already taken by range syntax. We could avoid this problem by using triple dots like C++. Which are also already taken (by inclusive range syntax), but only as infix operator and only in pattern syntax (though there were some requests to allow triple dots in expressions, too).

For the general idea: 👍, I was imagining variadic generics using pretty much the same approach. However, the devil is in the details that aren't yet worked out. C++ resolves expressions involving parameter pack expansion only after monomorphization; but I assume Rust would want to type-check the generic code? Wouldn't that require restrictions on where the unpacking operator is used? Also, consuming variadics in C++ usually involves a recursive function using template specialization. With this proposal, it seems like you'd need to create a trait for every variadic function, so that you can "specialize" using trait impls as in the Clone example?

Also, the ... operator in C++ allows "unpacking" complex expressions, not just directly unpacking the parameter pack. For example, in C++11 I can do:

template <typename... T> void consumer(T... t) { ... }

template <typename... T> void adapter(T... t)
{
   consumer(t.adapt()...);
   // expands to:
   // consumer(t1.adapt(), t2.adapt(), t3.adapt(), ...);
}

What's the easiest way to write the 3-line adapter function in Rust with this RFC? Do I have to write recursive Adapt implementations like the Clone impls in the example?

As for Vec::new: I think in cases where we don't actually need a type parameter pack, but just a variadic function with a single parameter type, it might be better to support something like this:
fn new(..elements: [T]) -> Vec<T> { ... }
This requires being able to pass unsized types as parameters - but that seems doable (and might already be planned?).

[alexchandel]

Why not the dereference operator?

type AppendTuple<T, U> = (*T, U);
f(*(a, b), *(c, d, e), f) => f(a, b, c, d, e, f);

[gnzlbg]

@dgrunwald :

Also, consuming variadics in C++ usually involves a recursive function using template specialization.

In C++1y there are fold-expressions that allow you to do a lot of things without recursion:

template<typename... Args>
bool all(Args... args) { return (... && args); }

bool b = all(true, true, true, false);

but I assume Rust would want to type-check the generic code? Wouldn't that require restrictions on where the unpacking operator is used?

You can easily type check that all parameters in the parameter pack implement a given trait. If one needs more complicated type-checking a variadic function is likely not the best solution.