[ruby-core:105110] [Ruby master Feature#18135] Introduce Enumerable#detect_only

Issue #18135 has been updated by duerst (Martin D=FCrst).


meisel (Michael Eisel) wrote in #note-2:
> Also, another alternative could be `Enumerable#take_only`, where it takes=
 the only element from the enumerable or else fails. So instead of:
> ```
> array.detect_only { |elem| some_method(elem) }
> ```
> =

> We'd have:
> =

> ```
> array.select { |elem| some_method(elem) }.take_only
> ```

It's a good idea to separate things out, but this could be highly inefficie=
nt (checking and selecting lots of elements only to find out later that the=
re's more than one).


----------------------------------------
Feature #18135: Introduce Enumerable#detect_only
https://bugs.ruby-lang.org/issues/18135#change-93520

* Author: meisel (Michael Eisel)
* Status: Open
* Priority: Normal
----------------------------------------
It can be important to get the only element for which a block returns true,=
 and to assert that this is the only element as such. For example, this can=
 be a very helpful sanity check when one is working with data that's outsid=
e of their control and is not perfectly understood. They may have a guess a=
s to how to get a specific element matching some criteria, but if they used=
 Enumerable#detect might be hiding the fact that they have written an incor=
rect block and that there's in fact more than one element that matches it. =
It could also be a parameter on Typically, I'd do it like this:

```
matches =3D array.select { |elem| some_method(elem) }
raise if matches.size !=3D 0
match =3D matches.first
```
Here, it would be shortened to:
```
match =3D array.detect_only { |elem| some_method(elem) }
```

It could also be a parameter on Enumerable#detect instead of a separate met=
hod.



-- =

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