> The ExpressionProblem is a specific example of a larger class of problems known generally as Cross Cutting Concerns [...] Many languages include designs to help solve the ExpressionProblem. Open functions (functions that can be extended with new pattern-matches), open data-types (data types that can be extended with new patterns), and MultiMethods ('open' specialized polymorphic functions with 'open' set of classes), and PredicateDispatching, are all viable approaches.

Working on GPU code, I feel like I'm always stuck in this problem of 'cross-cutting concerns'. I remember the guy who hired me years ago said during the interview, "we [programmers] have solved the problem of writing code for functionality, but we haven't solved the problem of writing code for performance." And this is what he meant, that cross-cutting concerns are ever present, and a new feature in the code can introduce the need for a complete refactor, or a major top-to-bottom flip in your data structures, etc.

Which languages & language features are good at both solving the expression problem functionally, and can do this with the highest priority being performance?

I also wouldn't mind hearing advice about how to evaluate when to use such features, because most often when I bump into this problem, it's not really due to lack of language features, it's due to having chosen not to use them from the start because there's some development time overhead, and I decided I most likely wouldn't use it, then over time painted myself into a corner.