Can't Have Mutable reference of struct instead of &self in cost function of trait CostFunction.

[ramtinkh]

I'm trying to have an element in my struct that changes during Nelder Mead optimization. But despite trait CostFunction I'm not allowed to have mutable reference &mut self in cost function.
What do you recommend me to do?

[stefan-k]

You could try interior mutability using RefCell: https://doc.rust-lang.org/book/ch15-05-interior-mutability.html

[ramtinkh]

I used it but it didn't completely solve my error (Somewhere else another error occured).
I was wondering if there was a reason you have not used &mut self in the trait CostFunction? Or you did it just to avoid possible errors and reach a stable version ASAP?

[stefan-k]

The downsides of using &mut self overweight the advantages. Handling gets much more cumbersome and restrictive due to requiring unique ownership of the problem when evaluating the cost function. If interior mutability is needed, RefCell should to the trick.