[ruby-core:122871] [Ruby Bug#21139] Prism and parse.y parses `it = it` differently
From:
"vinistock (Vinicius Stock) via ruby-core" <ruby-core@...>
Date:
2025-07-28 14:01:50 UTC
List:
ruby-core #122871
Issue #21139 has been updated by vinistock (Vinicius Stock).
Bringing my comment from the [pull request](https://github.com/ruby/prism/pull/3604) here. Reviewed with [@AMomchilov](https://bugs.ruby-lang.org/users/51722)
We wonder if Prism's current behaviour is more intuitive for developers. In this example
```ruby
42.tap { it = it; p it }
```
The proposed parse.y behaviour would treat `it = it` as if it were `it = nil` (reading from a non-intiailized local variable to that same local variable). We would argue it's more intuitive if it prints 42, because `it = it` is creating a new local variable from the result of reading the `it` parameter (before it gets "shadowed" by a local variable of the same name), which is 42. If developers want the the `nil` behaviour, they can explicitly write `it = nil` instead.
----------------------------------------
Bug #21139: Prism and parse.y parses `it = it` differently
https://bugs.ruby-lang.org/issues/21139#change-114174
* 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)
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