I'm trying to make a function to do the following:
accept a closure f
of the form fn(T) -> T
return a closure of the form fn(T, bool) -> T
, that conditionally performs f
depending on the bool parameter.
I come from a haskell-ish background, and in Haskell this would be something like this:
conditionally :: (a -> a) -> a -> Bool -> a
conditionally f x True = f x
conditionally f x False = x
Translating that to something more rust-like:
conditionally :: ((t) -> t) -> ((t, Bool) -> t)
conditionally f = (x, b) -> if b then (f x) else (x)
I tried the following in rust:
fn conditionally<T>(f: &'static (dyn Fn(T) -> T + 'static)) -> Box<dyn Fn(T, bool) -> T> {
Box::new(&|x, b| if b { f(x) } else { x } )
}
and was helpfully told to use the move
keyword to ensure the closure takes ownership of f
. However, the following still doesn't work:
fn conditional<T>(f: &'static (dyn Fn(T) -> T + 'static)) -> Box<dyn Fn(T, bool) -> T> {
Box::new(&move|x, b| if b { f(x) } else { x } )
}
I'm getting the following error (this also appeared prior to adding move
):
error[E0515]: cannot return value referencing temporary value
--> src/main.rs:216:5
|
216 | Box::new(&move|x, b| if b { f(x) } else { x } )
| ^^^^^^^^^^-----------------------------------^^
| | |
| | temporary value created here
| returns a value referencing data owned by the current function
I'm thinking that the 'data owned by the current function' is either the closure I've defined, or the f
that I've moved, but I can't get my head around how it all fits together.
As a smoke check, I made sure I'm able to box simpler closures that I define in the function body, and the following compiles:
fn conditional_increment() -> Box<dyn Fn(i32, bool) -> i32> {
Box::new(&|x, b| if b { x + 1 } else { x } )
}
What am I missing here? Is this possible in rust? I'm also wondering if there's a more specific name for what I'm trying to do than simply higher-order functions, as I'm struggling to find resources on this kind of problem.
Update: I've realised that "currying in rust" would have been a good term to search for. While this isn't an example of currying, it would use the same language features, and would have led me to the answer given by vallentin.