[pasteldream]

I think we’re talking past each other.

Lenses serve many purposes. All I’m saying is that in practice, the most common role they fulfil is to act as a counterpart for mutable references in contexts where you want or need immutability.

Can the use of lenses make a program more “composable”? Maybe, but if you have an example of a program taking advantage of that flexibility I’d like to see it.

[ssivark]

Do check out the links in my original comment above, which explain that the whole motivation behind all this (of which lenses are just a small part) is modularity. Modularity and composability are two sides of the same coin---being able to construct a complex whole by combining simple parts---depending on whether you view it top-down or bottom-up.

Suppose you refactor a field `T.e.a.d` to `T.e.b.d`, for whatever reasons. How many places in your codebase will you have to edit, to complete this change?

Dot access exposes to the outside world the implementation details of where `d` lives, while lenses allow you to abstract that as yet another function application (of a "deep-getter" function) so your code becomes extremely modular and flexible. A good language implementation then hopefully allows you to use this abstraction/indirection without a significant performance penalty.

[binary132]

void set_d(T*);