The parameter ordering of `Option::map_or_else` is unintuitive.

[abonander]

The signature of Option<T>::map_or_else is as follows:

fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, def: D, f: F) -> U

This is highly unintuitive as the two closure parameters are in the reverse order of that suggested by the function's name. This means that anyone who tries to invoke it from memory, including myself earlier today, is going to run into a compiler error on the first try.

Example:

let first_word_letter_count = "hello world".words().next().map_or_else(
    |word| word.len(),
    || 0
);

The above makes sense, logically. If the value is there, map it and return the result. Otherwise (implying a secondary operation), call a closure which will produce a substitute value. Even the documentation supports this logical progression:

Applies a function to the contained value or computes a default.

However, trying to invoke the method this way will result in a compiler error because the no-arg closure is required to be first. The particular reasoning behind this design is not given, but I have found precedent in Haskell's maybe function:

maybe :: b -> (a -> b) -> Maybe a -> b

However, I don't think Haskell's intuition for parameter ordering can extend to Rust because Rust doesn't have currying, partial application, or (global) lazy evaluation. And even in Haskell the ordering isn't necessarily intuitive.

I am aware that this method and Option itself have both been marked as Stable. However, I believe minor unintuities like this add up and ultimately reflect poorly on the user experience of the language as a whole, and should be addressed before Rust hits 1.0.

I would like to note that I am willing to apply the effort to adjust this myself, as it's relatively trivial. However, because it's trivial, I am not sure if it requires an RFC or just a general community agreement.

[reem]

I agree that this ordering is unintuitive and it would be better if the Some case was first.

[frewsxcv]

Yes. I get this wrong every single time. 👍

[emberian]

To be honest I thought the order was always the other way around. Was it changed?

[abonander]

@cmr I thought it was always the other way around too. That's what screwed me up and motivated me to open this issue.

[huonw]

The order is currently chosen to be internally consistent with map_or. map_or takes the "main" closure last because that is syntactically much nicer:

foo.map_or(some_value, |x| {
    bar();
    baz(x);
    qux();
})

vs.

foo.map_or(|x| {
    bar();
    baz(x);
    qux();
}, some_value)

This isn't to say that we shouldn't change it, but we shouldn't consider this in isolation.

[abonander]

@huonw That's understandable but still rather unintuitive. I would prefer to make it look nicer with a minor style tweak:

foo.map_or(|x| {
        bar();
        baz(x);
        qux();
    },
    some_value
)

or even:

foo.map_or(
    |x| {
        bar();
        baz(x);
        qux();
    },
    some_value
)

This does get a little more right-drift but I think it's worth the concession for cleaner, more intuitive code. I use the docs all the time but if I have to consult them to figure out the right argument ordering for something as simple as an operation on a monad, it's going to severely impact my efficiency.

[lilyball]

I don't think I've ever screwed this one up. It just seems natural to me that the default value comes first. Probably because map_or() is "more fundamental" than map_or_else, so to speak (it has the shorter name and comes first in the documentation, and I would not be surprised to learn that it is the more popular of the two methods in actual usage).

[crazymykl]

@kballard, but even map_or has odd ordering. I'd expect the arguments to be, in order: map through this, or give me that.

[Gankra]

So obviously this can't be "fixed" in 1.x, and fixing it in 2.x would be a disaster. So... this bug isn't actionable?

[nagisa]

@gankro I believe deprecating this for an alternative with a name that does not cause wrong associations with parameter ordering might make it actionable. But in general, ditto.

[tshepang]

@gankro why would it be a disaster? People will get a compile error, and the fix is simple.

[frewsxcv]

People will get a compile error, and the fix is simple.

This might be true, but it would be pretty devastating to have virtually all Rust projects break just because some people aren't satisfied with the naming of argument order a method

[tshepang]

@frewsxcv it's the re-ordering of arguments, not the renaming. Besides, people are free to stay with an ancient tool if doing a simple fix is too much effort. Some of these fixes could be automated even, especially simple ones like this.

[Gankra]

@tshepang This is not a good way for a language developer to act. Breaking client code in such a blatant way should not be taken so lightly.

[tshepang]

@gankro I think "fixing mistakes and oversights" is supposed to be part of 2.x. Some (more important) stuff will be broken anyways, so we might as well break the smaller things as well. I think making things more intuitive is worth the pain.