Closure with variable number of arguments | Rust Programming Language Community | Page 1

scarlet notch Dec 12, 2022, 9:27 PM

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I have a struct that I want to look a bit like the following:

struct C {
  arg_1: Option<A>,
  arg_2: Option<B>,
  caller: FnMut[...]
}

The caller is supposed to be a closure which can have either arguments

|x| if both arg_1 and arg_2 are None
|x, a/b| depending whether arg_1 or arg_2 are Some with all three arguments if neither are None. Currently looking at this as part of a builder problem - is this possible? If yes, how would that look like?

outer yoke Dec 12, 2022, 11:05 PM

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it is not possible to store all those different functions in one field

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however, if your goal is "in the future, call the function with these arguments" then you can do that by storing a boxed closure with zero arguments, that captures the arguments and the function

scarlet notch Dec 13, 2022, 5:33 AM

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Okay, I see. I still had an idea to implement a marker trait but that’s then still not gonna achieve what I wanted to. Might fall back to several structs then…
There’s also no macro magic with which I could overengineer this, right?

outer yoke Dec 13, 2022, 5:38 AM

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you could define an enum where each variant is a function and the right number of arguments; if you did that, you would have to implement the function call code for each variant separately

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enum C {
    Zero(Box<dyn FnMut(X)>),
    A(Box<dyn FnMut(X, A)>, A),
    B(Box<dyn FnMut(X, B)>, B),
    AB(Box<dyn FnMut(X, A, B)>, A, B),
}

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(or &dyn or whatever flavor of FnMut container you actually need)

scarlet notch Dec 13, 2022, 5:41 AM

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Ah, okay… I can try that route 🙂

#Closure with variable number of arguments