[ruby-core:56756] [ruby-trunk - Feature #8804] ONCE syntax

Issue #8804 has been updated by trans (Thomas Sawyer).


A use case is memoization. Instead of 

  def q
    @q ||= (...some long calc...)
  end

You could do

  def q
    ONCE { ...some long calc... }
  end

----------------------------------------
Feature #8804: ONCE syntax
https://bugs.ruby-lang.org/issues/8804#change-41299

Author: ko1 (Koichi Sasada)
Status: Open
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: syntax
Target version: next minor


How about to introduce ONCE{...} syntax which do block only once and return the first value?

- It is similar of BEGIN{} and END{}.
- General syntax of /reg/o.

## simulation code
ONCE_CACHE = {}
ONCE_MUTEX = Mutex.new
def ONCE(&b)
  raise unless block_given?
  ONCE_MUTEX.synchronize{
    key = caller(3, 1)[0]
    if value = ONCE_CACHE[key]
      value
    else
      ONCE_CACHE[key] = yield
    end
  }
end

3.times{|i|
  p ONCE{
    i #=> every time 0
  }
}
##
## Note that this code doesn't work if two or more ONCE{} are available in one line.



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