infinite recursion / hang on Lens.recur about lens HOT 4 CLOSED

That's just because Map.get(nil, :a) is nil, so you can indeed recurse infinitely deep into a nil like that. I wrote most of this library a while back, can't remember if that's how it always was in Elixir, or if it changed... It's not quite clear to me if a proper fix exists that doesn't break something else. For example, we could make it so that Lens.key(:a) gives no results when applied to a nil, but that breaks the simple relationship that Lens.to_list(Lens.key(k), m) == [m[k]].

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Ah! Scratch that, I forgot how my own library works! There's key? for just this purpose. You can do:

Lens.recur(Lens.key?(:a)) |> Lens.to_list(%{a: %{a: %{b: :c}}})
# => [%{b: :c}, %{a: %{b: :c}}]

which, I think, is what you wanted.

from lens.

Thanks @obrok - that gets me over that hump, maybe for beginners the infinite recursion might be a bit non obvious, I've been having great fun playing with your library so thanks for creating it and thanks for the guidance 🙇

from lens.

Thank you for your kind words! I'd say Lens.recur is a little bit more advanced (although very useful indeed) in that it's difficult to debug when things go wrong. If you'd like to make a PR adding a FAQ section to the README describing this problem and the solution, I'd be happy to merge that.

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