Documenting this here and will open a PR for it.

Metadata

• Ruby version: 2.6
• @prettier/plugin-ruby version: 0.20.1

Input

  scope :sandbox, ->(true_or_false = !Rails.env.production?) { where(sandbox: true_or_false) }

Current output

  scope :sandbox,
        lambda { |true_or_false = !Rails.env.production?|
          where(sandbox: true_or_false)
        }

Expected output

  scope :sandbox,
        ->(true_or_false = !Rails.env.production?) {
          where(sandbox: true_or_false)
        }

Reasoning

The current output is invalid ruby syntax. I did my best to read through the ruby grammar (parse.y) and determine why a unary (!) would be valid in ->(...), but not in do |...| and I was grasping at straws a bit. I'm not experienced enough with the grammar there to determine if there's a bug in the parser and it's picking the wrong matcher for args in do |...| where a unary isn't allowed, but nonetheless it breaks.

Example:

>> ->(active = !dead?) { '' }
=> #<Proc:0x00007f968403ac10@(irb):1 (lambda)>

>> lambda { |active = !dead?| '' }
Traceback (most recent call last):
        3: from /Users/jbielick/.rubies/ruby-2.6.6/bin/irb:23:in `<main>'
        2: from /Users/jbielick/.rubies/ruby-2.6.6/bin/irb:23:in `load'
        1: from /Users/jbielick/.rubies/ruby-2.6.6/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
SyntaxError ((irb):2: syntax error, unexpected '!')
lambda { |active = !dead?| '' }
                   ^
(irb):2: syntax error, unexpected '}', expecting end-of-input
lambda { |active = !dead?| '' }

Without determining exactly what the parsing grammar allow and disallows, I found it difficult to propose a change that would conditionally use -> vs. lambda { when a particular arg pattern was detected. It was my assumption that detection of exceptions where -> must be used was futile. As a result, I think the best course of action is to use -> for everything, since the args patterns it allows are a superset of do |...| (block params) and it is likely unsafe to transform a ->(...) into lambda { |...|.