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Comment by throwaway894345 | Hacker News Reader

I really admire Rust iterators, especially compared to C++ iterators, but I lament that there is no easy way (that I know of, anyway) to build an iterator from a closure which yields `Option<T>`s. I don’t like that I have to define a type and implement a Trait (in general working with closures is cumbersome in Rust). That said, this isn’t a major problem either and on balance I’m very happy with Rust iterators.

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Might you be looking for https://doc.rust-lang.org/std/iter/fn.from_fn.html ?

Oh wow, yeah, I think that’s it! Thanks very much!

Thanks for sharing! I wish I'd known about this earlier.

Whoa I did not know about this. This would have saved me so much time during Advent of Code…

> I lament that there is no easy way (that I know of, anyway) to build an iterator from a closure which yields `Option<T>`s.

Could you expand / explain what you actually mean here? Because I can think of multiple interpretations of this prompt which are straight up part of the stdlib, but that I can interpret this description in multiple ways shows the issue.

Let’s say I have a closure `fn() -> Option<T>`—how do I turn that into an Iterator? Maybe it’s possible with the stdlib and I’m just not familiar?

EDIT: I think this is it: https://news.ycombinator.com/item?id=27050861

Let's say std::iter::from_fn didn't exist. Then you'd need to make something that ends the iterator when the function returns none, you could do it like this:

    std::iter::repeat_with(f).take_while(|elt| elt.is_some()).filter_map(|elt| elt)

Where `.filter_map()` removes the Option layer and `repeat_with` creates an initially infinite iterator from your function `f`.

Long time ago, `repeat_with` might not have existed. Maybe you'd do something like this instead for that in Rust 1.0.0: `std::iter::repeat(()).map(|_| f())...`

You prefer to use std::iter::from_fn now that it exists, of course!

To be honest, making a simple iterator adaptor is quite easy too, so you could have a knack at making one you are missing - if you can't find it in std or in the crate itertools already.

It's probably possible to create a wrapper to do that. But if you need a closure, you can also just use a struct implementing the trait to manage the state. It's not perfect, but it's so much nicer than in C++.

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