Help with lifetimes and closure

1

Hello,
I have stuck with this issue where I pass a reference to function(e.g. fn_1) to another function(fn_2) and call fn_1 inside fn_2 with a variable created inside fn_2. The Issue is fn_1 takes references and hence I have to provide lifetimes. But I can't find way to get past this error.

Below is a small snippet shows what I am trying to do.

Playground Link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b3ccadae846a2e5b9553ab55904f1198

Please! If anyone can help me.

Thanks!
Darshan

2

You need higher-ranked trait bounds here, which lets you talk about an arbitrarily-short lifetime:

fn call_fn<'b, F>(v1: &'b Value, mut fn_: F) -> Value
where F: for<'a> FnMut(&'a mut Value, &'b Value) -> &'a Value{
    let mut v2 = Value{v:0};
    let _ = fn_(&mut v2, &v1);
    v2
}

Your original version takes 'a as a parameter from outside. This means it refers to a stack frame outside of call_fn, which prevents any &'a reference pointing to a local variable.

1 Like
3

That is exactly what I was looking for. Thanks!

4

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