[ruby-core:74993] [Ruby trunk Bug#11816] Partial safe navigation operator

Issue #11816 has been updated by Joan Blackmoore.


@Matthew

Thought about it again and would agree with the last paragraph. Direct substitution is not appropriate here, despite it sounds logical.
The *&.* operator is a strange beast as other general rules also won't apply, like (optional) preceding dot, ie. *object.&.hash* is also a syntax error.

Btw. by playing with different &. contained expressions, I've discovered a possible bug:

This runs ok
`a = nil
a&.foo &&= false  # nil
`

Following however freezes VM and need to be SIGKILLed
`nil&.foo &&= false
`

Even at bytecode compilation
`RubyVM::InstructionSequence.compile('nil&.foo ||= 42')
`

tail of strace output
lstat("/usr", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/usr/lib64", {st_mode=S_IFDIR|0755, st_size=139264, ...}) = 0
lstat("/usr/lib64/ruby", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/usr/lib64/ruby/2.3.0", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/usr/lib64/ruby/2.3.0/unicode_normalize.rb", {st_mode=S_IFREG|0644, st_size=3265, ...}) = 0


Tested with
`ruby -v
ruby 2.3.0p75 (2016-04-07 revision 54505) [x86_64-linux]
`

command used
`strace ruby --disable=gems,rubyopt -e 'nil&.foo &&= false'`


----------------------------------------
Bug #11816: Partial safe navigation operator
https://bugs.ruby-lang.org/issues/11816#change-58120

* Author: Marc-Andre Lafortune
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
* ruby -v: preview 2
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
----------------------------------------
I'm extremely surprised (and disappointed) that, currently:

```ruby
x = nil
x&.foo.bar # => NoMethodError: undefined method `bar' for nil:NilClass
```

To make it safe, you have to write `x&.foo&.bar`. But if `foo` is never supposed to return `nil`, then that code isn't "fail early" in case it actually does. `nil&.foo.bar` is more expressive, simpler and is perfect if you want to an error if `foo` returned `nil`. To actually get what you want, you have to resort using the old form `x && x.foo.bar`...

In CoffeeScript, you can write `x()?.foo.bar` and it will work well, since it gets compiled to

```js
if ((_ref = x()) != null) {
  _ref.foo.bar;
}
```

All the discussion in #11537 focuses on `x&.foo&.bar`, so I have to ask:

Matz, what is your understanding of `x&.foo.bar`?

I feel the current implementation is not useful and should be changed to what I had in mind. I can't see any legitimate use of `x&.foo.bar` currently.




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