Accept closure of variable arity as argument

1

I have a function and I'd like to accept closures with different sets of arguments and return value types. For example would it be possible to create a trait Func.

trait Func {
    fn func(&self, input: Input) -> Output;
}

which could be implemented for a set of functions:

Fn(A, B, C, ..) -> R

Where

  • A, B, C, ... each implement FuncArg
  • R implements FuncReturn

So these functions would all have arguments implementing a trait FuncArg and a return value implementing FuncReturn where these traits convert the concrete Input type to a set of impl FuncArg and a concrete Output type from the impl FuncReturn.

Is this kind of API possible in Rust? Could anyone point me to some example code where anything similar has been done before?

2

There's no direct support for this, yet (look for "variadic generics" if you want to know the status of this feature). In the meantime, you can just accept a single-argument closure and pass a tuple.

4 Likes
3

Okay so I managed to get something working. However, it only works for owned arguments. Is there a way for me to get the lifetimes to work?

Playground

error: implementation of `Function` is not general enough
  --> src/lib.rs:16:5
   |
16 |     takes_any_function(f);
   |     ^^^^^^^^^^^^^^^^^^ implementation of `Function` is not general enough
4

This is a well-known limitation of closures: they cannot satisfy a for<'a> Trait<'a> bound where Trait<'a> requires Fn(&'a T). Here, the compiler can infer that the type of f implements Function<'a, String, (&'a str,)> for any particular lifetime 'a. This allows the explicit Function::call_func(&f, args) in main() to work with no issues. However, f cannot be passed to takes_any_function(), since f's type does not implement for<'a> Function<'a, String, (&'a str,)>. Currently, the only workaround is to use a lifetime-generic fn item instead of a closure. See also RFC 3216 for a proposed language solution to this issue.

5

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.