Issue #21139 has been updated by nobu (Nobuyoshi Nakada). matz (Yukihiro Matsumoto) wrote in #note-1:

Assignment to `it` should be prohibited in the long run. Temporarily, I vote for Prism behavior.

1. Currently, `it` is an ordinary local variable if assigned syntactically. 2. And a local variable is `nil` before assigned, even in its RHS, at the runtime. The behavior in parse.y obeys above principles. Do you want to change the rule 1, make `it` always special? ---------------------------------------- Bug #21139: Prism and parse.y parses `it = it` differently https://bugs.ruby-lang.org/issues/21139#change-111976 * Author: tompng (tomoya ishida) * Status: Assigned * Assignee: prism * ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ~~~ # ruby --parser=parse.y -e "42.tap { it = it; p it }" nil # ruby --parser=prism -e "42.tap { it = it; p it }" 42 ~~~ -- https://bugs.ruby-lang.org/

Issue #21139 has been updated by matz (Yukihiro Matsumoto). I said I prefer Prism behavior to `parse.y`. But It was due to my misunderstanding. `it = it` should initialize the local variable named `it` and the value should be `nil`, just like other `x = x` assignment in Ruby. In the future, all assignment to `it` could be prohibited, maybe. Matz. ---------------------------------------- Bug #21139: Prism and parse.y parses `it = it` differently https://bugs.ruby-lang.org/issues/21139#change-112308 * Author: tompng (tomoya ishida) * Status: Open * ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ~~~ # ruby --parser=parse.y -e "42.tap { it = it; p it }" nil # ruby --parser=prism -e "42.tap { it = it; p it }" 42 ~~~ ---Files-------------------------------- clipboard-202503081702-idzz2.png (22.6 KB) -- https://bugs.ruby-lang.org/

Issue #21139 has been updated by tenderlovemaking (Aaron Patterson). Can we get some clarification on this? @matz likes the behavior of Prism, but this bug is assigned to the Prism team. It sounds like this should be considered a feature in parse.y. Thanks. -Aaron ---------------------------------------- Bug #21139: Prism and parse.y parses `it = it` differently https://bugs.ruby-lang.org/issues/21139#change-114102 * Author: tompng (tomoya ishida) * Status: Assigned * Assignee: prism * ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ~~~ # ruby --parser=parse.y -e "42.tap { it = it; p it }" nil # ruby --parser=prism -e "42.tap { it = it; p it }" 42 ~~~ ---Files-------------------------------- clipboard-202503081702-idzz2.png (22.6 KB) -- https://bugs.ruby-lang.org/

Issue #21139 has been updated by S_H_ (Shun Hiraoka). I've created a pull request to fix this Prism behavior where `42.tap { it = it; p it }` returns `42` instead of `nil`. https://github.com/ruby/prism/pull/3604 This change makes Prism treat the right-hand side `it` as a local variable in assignment context, matching parse.y's behavior. I'm not entirely sure if there are other edge cases that need to be addressed. I would appreciate any feedback or suggestions. ---------------------------------------- Bug #21139: Prism and parse.y parses `it = it` differently https://bugs.ruby-lang.org/issues/21139#change-114120 * Author: tompng (tomoya ishida) * Status: Feedback * Assignee: prism * ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ~~~ # ruby --parser=parse.y -e "42.tap { it = it; p it }" nil # ruby --parser=prism -e "42.tap { it = it; p it }" 42 ~~~ ---Files-------------------------------- clipboard-202503081702-idzz2.png (22.6 KB) -- https://bugs.ruby-lang.org/

Issue #21139 has been updated by mame (Yusuke Endoh). @vinistock To clarify, do you want to change the scope of all variables, or only for `it`? If you meant all variables, it would create a major incompatibility. In an assignment `foo = expr`, the variable `foo` must be available in `expr`. Changing this so that a variable is only defined lexically after `expr` would break this: ```ruby fact = -> n do # This call would be invalid because 'fact' is not available here fact.call(n - 1) end ``` Changing the scope of only a variable named `it` seems like an ad-hoc rule. The syntax would be very hard to explain. I think it would be far simpler to forbid `it` as a local variable name, though the compatibility implications would need to be carefully considered. ---------------------------------------- Bug #21139: Prism and parse.y parses `it = it` differently https://bugs.ruby-lang.org/issues/21139#change-114175 * Author: tompng (tomoya ishida) * Status: Feedback * Assignee: prism * ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ~~~ # ruby --parser=parse.y -e "42.tap { it = it; p it }" nil # ruby --parser=prism -e "42.tap { it = it; p it }" 42 ~~~ ---Files-------------------------------- clipboard-202503081702-idzz2.png (22.6 KB) -- https://bugs.ruby-lang.org/

Issue #21139 has been updated by jeremyevans0 (Jeremy Evans). vinistock (Vinicius Stock) wrote in #note-15:

The way I reason about this code is as follows:

```ruby 42.tap do it = it # ^ First step: read the value of `it`, which is the block's argument and therefore is equal to 42 # ^ Second step: declare a local variable called `it` and store the value of the right hand side (which is 42) p it # ^ Third step: at this point `it` no longer refers to the special variable `it`, but to a local variable `it`, for which the value is currently 42 end ```

This isn't how Ruby works in general. Ruby is parsed left-to-right. When it comes across a local variable declaration, it registers the local variable. If Ruby comes across that identifier later in the same local variable scope (even the RHS of the same assignment expression), Ruby treats the identifier as a local variable reference instead of a method call. Consider: ```ruby def x = 1 x = x x # => nil ``` `x` is `nil` in the above example because by the time the RHS of the assignment is parsed, `x` is a local variable, so it is not treated as a method call. When parsing `it` instead of a block, `it` is not considered a local variable until it is referenced, and `it` as a user-defined local variable has precedence over `it` as a block parameter. You can tell this from the following code: ```ruby it = 1 proc{it}.call(2) # 1, not 2 ``` Since `it` is declared as a user-defined variable on the LHS of the assignment before `it` is referenced on the RHS of the assignment, `it` is a user-defined local variable and not the implicit block parameter on the RHS on the assignment, and therefore should have value `nil`, just as any user-defined local variable would.

...

I think it would be far simpler to forbid it as a local variable name, though the compatibility implications would need to be carefully considered.

Maybe this is a better path forward as these edge cases can be confusing. That said, I still think that the current behaviour in Prism is more intuitive and consistent.

It think it is inconsistent with the rest of Ruby, and a bug in Prism. Consider the VM instructions: For `it = it` outside of a block, both parse.y and Prism, and for `it = it` inside a block in parse.y: ``` local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1]) [ 1] it@0 0000 getlocal_WC_0 it@0 ( 1)[Li] 0002 leave ``` For `it = it` instead of a block in Prism: ``` local table (size: 2, argc: 1 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1]) [ 2] ?@0<AmbiguousArg>[ 1] it@1 0000 getlocal_WC_0 ?@0 ( 1)[LiBc] 0002 dup 0003 setlocal_WC_0 it@1 ``` You could argue the Prism behavior is more useful. But if you want to make it consistent, you would have to change Ruby so that local variables are not in scope until after they are assigned, instead of after they are declared. That would break a lot of Ruby code. ---------------------------------------- Bug #21139: Prism and parse.y parses `it = it` differently https://bugs.ruby-lang.org/issues/21139#change-114177 * Author: tompng (tomoya ishida) * Status: Feedback * Assignee: prism * ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ~~~ # ruby --parser=parse.y -e "42.tap { it = it; p it }" nil # ruby --parser=prism -e "42.tap { it = it; p it }" 42 ~~~ ---Files-------------------------------- clipboard-202503081702-idzz2.png (22.6 KB) -- https://bugs.ruby-lang.org/

Issue #21139 has been updated by vinistock (Vinicius Stock). Thanks for the context, I understand your point about consistency. So this case ```ruby 42.tap { |x| x = x; p x } ``` Works because by the time we reach `x = x`, the block parameter already declared `x` as a local and so it doesn't end up setting it equal to `nil`. Would it make sense to automatically declare `it` as a local inside the block? Or another question, do we agree that these two should return the same result? ```ruby 42.tap { it = it; p it } 42.tap { |x| x = x; p x } ``` ---------------------------------------- Bug #21139: Prism and parse.y parses `it = it` differently https://bugs.ruby-lang.org/issues/21139#change-114179 * Author: tompng (tomoya ishida) * Status: Feedback * Assignee: prism * ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ~~~ # ruby --parser=parse.y -e "42.tap { it = it; p it }" nil # ruby --parser=prism -e "42.tap { it = it; p it }" 42 ~~~ ---Files-------------------------------- clipboard-202503081702-idzz2.png (22.6 KB) -- https://bugs.ruby-lang.org/

Issue #21139 has been updated by jeremyevans0 (Jeremy Evans). vinistock (Vinicius Stock) wrote in #note-18:

Thanks for the context, I understand your point about consistency. So this case

```ruby 42.tap { |x| x = x; p x } ```

Works because by the time we reach `x = x`, the block parameter already declared `x` as a local and so it doesn't end up setting it equal to `nil`. Would it make sense to automatically declare `it` as a local inside the block?

Or another question, do we agree that these two should return the same result?

```ruby 42.tap { it = it; p it } 42.tap { |x| x = x; p x } ```

No. `it` is not a local variable until it is referenced, so the first example should always print `nil` (unless `it` was already a local variable in higher scope). Everywhere else in Ruby: ```ruby lvar = lvar ``` Will result in `lvar` being nil if was not already a local variable in scope. To think that `42.tap { it = it; p it }` should print 42 is to believe that `it` is a local variable before it is referenced. However, if `it` is never referenced, then `it` is never a local variable, and the block does not accept an argument: ```ruby proc{}.arity # => 0 proc{it}.arity # => 1 ``` You could argue that `it` is a local variable from the start of the block instead of where `it` is first referenced in the block. However, that's not how local variables work in general. Every other place in Ruby where a local variable is declared (referenced in the LHS of an assignment), it is only considered declared after the reference, not before the reference: ``` def x = 1 def y x # => 1 x = x x # => nil end ``` If `it` should be considered a local variable from the start of the block if it is referenced anywhere in the block, then in the above example, the first reference to `x` inside `y` should be a local variable reference instead of a method call, even though the `x` local variable is not declared until afterward. In general, you shouldn't consider `it` to be a local variable. If `it` was a local variable, this would work: ```ruby proc{eval("it")}.call(1) # NameError ``` But it doesn't work, even if `it` is already referenced: ```ruby proc{it; eval("it")}.call(1) # NameError ``` If at the point of reference, `it` is already a local variable, Ruby treats it as it would any local variable. If `it` is not already a local variable, then Ruby treats it as the implicit block parameter. Inside a block, for `it = it`, the RHS `it` was previously declared as a local variable by the LHS, so it is treated as a local variable instead of as the implicit block parameter. AMomchilov (Alexander Momchilov) wrote in #note-19:

@jeremyevans0 I see your point about how ivars take precedence over methods of the same name, but I think `it` is more like a block argument (well, a local variable) than a method, so I find this inconsistently really surprising:

``` ruby --parser=parse.y -e "42.tap { it = it; p it }" # => nil ruby --parser=parse.y -e "42.tap { |it| it = it; p it }" # => 42 ```

Since `it` is just a shorthand for "the first block argument," then I think people would expect that it works _as if_ you declared `|it|` yourself.

They may. However, doing so is inconsistent with how local variables are designed in Ruby. Personally, I think consistency is much more important in this case. I cannot think of a good reason to define an `it` local variable while also using `it` as the implicit block parameter. If you are only using `it` in the same block scope, no local variable is necessary. If you are setting `it` to a local variable so it is usable inside a nested block, you should pick a different variable name, as using `it` is more likely to cause confusion. ---------------------------------------- Bug #21139: Prism and parse.y parses `it = it` differently https://bugs.ruby-lang.org/issues/21139#change-114181 * Author: tompng (tomoya ishida) * Status: Feedback * Assignee: prism * ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ~~~ # ruby --parser=parse.y -e "42.tap { it = it; p it }" nil # ruby --parser=prism -e "42.tap { it = it; p it }" 42 ~~~ ---Files-------------------------------- clipboard-202503081702-idzz2.png (22.6 KB) -- https://bugs.ruby-lang.org/