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In C++23, monadic operations in std::optional was adopted, and later on std::expected. Why were monadic operations like and_then, transform, and or_else not included in the main paper?

Desmond Gold
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1 Answers1

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Update: std::expected in C++23 will have monadic operations.

The std::expected proposal is very old. The adopted revision is P0323R12, which already suggests a long life, but that paper even predates the P-numbering system and started out as N4015, dated May 2014.

I bring this up because std::expected, even by itself, has taken a very long time to wind its way through the process. In contrast, the monadic operations for std::optional paper only first appeared in October 2017. And since then it was just a long slog just to get std::expected at all.

There is a paper proposing monadic operations for std::expected, that's P2505R0, but at this point it's very incomplete and needs a lot of work. The design window for new C++23 features is closed, but I wouldn't be surprised if there were an NB comment requesting it in later anyway.

At least we finally have std::expected.

Coming in here a few months later, there was an NB comment requesting the monadic operators for std::expected and P2505R5 was adopted for C++23 in Kona in November 2022.

Barry
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    For what it's worth, [P2505 seems to be on-track for C++23](https://github.com/cplusplus/papers/issues/1161). At the very least, LWG seems to be open to considering it for C++23. – Nicol Bolas Sep 25 '22 at 22:28
  • It looks like there's no `.transform_error`. That seems desirable. Or does `.or_else` handle that? – Ben Jan 26 '23 at 22:57
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    @Ben There is a `transform_error` now. Like I said, R0 of that paper was very incomplete. – Barry Jan 27 '23 at 02:26