[ruby-core:93157] [Ruby trunk Feature#15897] `it` as a default block parameter

Issue #15897 has been updated by janosch-x (Janosch M=FCller).


Kotlin has implemented `it` like this ([docs](https://kotlinlang.org/docs/r=
eference/lambdas.html#it-implicit-name-of-a-single-parameter)).

From purely personal experience, after doing just a little bit of Kotlin, I=
 often feel a temptation to use `it` when writing Ruby, just to notice that=
 I can't. I found `it` in Kotlin natural, easy to get used to, and easy to =
parse visually and understand when re-reading my code after a while.

----------------------------------------
Feature #15897: `it` as a default block parameter
https://bugs.ruby-lang.org/issues/15897#change-78600

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: =

----------------------------------------
How about considering "it" as a keyword for the block parameter only if it =
is the form of a local varaible reference and if there is no variable named=
 "it"?

```
[1, 2, 3].map { it.to_s } #=3D> ["1", "2", "3"]
```

If you are familiar with Ruby's parser, this explanation is more useful: NO=
DE_VCALL to "it" is considered as a keyword.

Examples:

```
public def it(x =3D "X")
  x
end

[1, 2, 3].map { it.to_s }    #=3D> ["1", "2", "3"]
[1, 2, 3].map { self.it }    #=3D> ["X", "X", "X"] # a method call because =
of a receiver
[1, 2, 3].map { it() }       #=3D> ["X", "X", "X"] # a method call because =
of parentheses
[1, 2, 3].map { it "Y" }     #=3D> ["Y", "Y", "Y"] # a method call because =
of an argument
[1, 2, 3].map { it=3D"Y"; it } #=3D> ["Y", "Y", "Y"] # there is a variable =
named "it" in this scope

it =3D "Z"
[1, 2, 3].map { it.to_s }    #=3D> ["Z", "Z", "Z"] # there is a variable na=
med "it" in this scope
```

Pros:
* it is the best word for the feature (according to @matsuda)
* it is reasonably compatible; RSpec won't break because their "it" require=
s an argument

Cons:
* it actually brings incompatibility in some cases
* it is somewhat fragile; "it" may refer a wrong variable
* it makes the language semantics dirty

Fortunately, it is easy to fix the incompatible programs: just replace `it`=
 with `it()`.  (Off topic: it is similar to `super()`.)
Just inserting an assignment to a variable "it" may affect another code.  T=
his is a bad news, but, IMO, a variable named "it" is not so often used.  I=
f this proposal is accepted, I guess people will gradually avoid the variab=
le name "it" (like "p").
The dirtiness is the most serious problem for me.  Thus, I don't like my ow=
n proposal so much, honestly.  But it would be much better than Perlish `@1=
`.  (Note: I don't propose the removal of `@1` in this ticket.  It is anoth=
er topic.)  In any way, I'd like to hear your opinions.


An experimental patch is attached.  The idea is inspired by @jeremyevans0's=
 [proposal of `@`](https://bugs.ruby-lang.org/issues/15723#note-98).


P.S. It would be easy to use `_` instead of `it`.  I'm unsure which is pref=
erable.

---Files--------------------------------
its.patch (4.92 KB)


-- =

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