Comment by T-R | Hacker News Reader

Comment by T-R | Hacker News Reader

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I definitely agree for traversals, but Lenses need some sort of primitive support - even in Haskell they're mostly generated with TemplateHaskell, and the language developers have spent a long time trying to make the `record.field` accessor syntax overloadable enough to work with lenses[1][2]. Hopefully someday we'll be free from having to memorize all the lens operators.

Optics are famously abstract in implementation, but I don't think people have trouble applying them - people seem to like JQuery/CSS selectors, and insist on `object.field` syntax; it's kind of wild that no mainstream language has a first-class way to pass around the description of a location in an arbitrary data structure.

[1] https://ghc-proposals.readthedocs.io/en/latest/proposals/002...

[2] https://ghc-proposals.readthedocs.io/en/latest/proposals/015...

Like offsetof[1]?

[1] https://en.cppreference.com/w/cpp/types/offsetof

Optics let you concisely describe the location, but defer the dereferencing, so you could definitely approximate optics, not by passing around pointers you compute with `offsetof`, but passing around functions that use `offsetof` to return memory locations to reference (read/write to). You could certainly write a composition operator for `*(*T) => List<*R>`... Some people have done something like it[1][2]:

    Account acc = getAccount();
    QVERIFY(acc.person.address.house == 20);

    auto houseLens = personL() to addressL() to houseL();
    std::function<int(int)> modifier = [](int old) { return old + 6; };
    
    Account newAcc2 = over(houseLens, newAcc1, modifier);

These also use templating to get something that still feels maybe a little less ergonomic than it could be, though.

[1] https://github.com/graninas/cpp_lenses [2] https://github.com/jonsterling/Lens.hpp

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