[codebje]

No language will tell you at compile time whether an arbitrary list filter operation will have an effect or not. If it can, it's because the list filter isn't arbitrary, it's probably always a no-op.

The generalisation is that you have some type parent with a way to select a type child, plus a function that modifies a child, you want to lift your child -> child function to a parent -> parent function.

Optics help a lot with this class of problem.

Because you're in the world of persistent data structures, and you're doing a modify operation, there is no avoiding the 'modify hashtable' part of 'modify an element within a hashtable'. You always create a 'new' hashtable.

If it's common that your update operation has no effect it may be worth switching to a function like child -> Modified child for Modified a = NoOp | Mutated a, so the hashtable modify function can determine if it can skip its own update. A list filter is a trivial thing to write, and one that can return NoOp if no elements are filtered is no less efficient.