Feature #8804: ONCE syntax

• Status: Assigned
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[ko1 (Koichi Sasada)]

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.

[andhapp (Anuj Dutta)]

Hello,

Do you have any use-cases to share for the new syntax?

Thanks.

Regards

[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

[trans (Thomas Sawyer)]

Also, can it take an optional key argument?

def ONCE(k=nil,&b)
raise unless block_given?
ONCE_MUTEX.synchronize{
key = k || caller(3, 1)[0]

[ko1 (Koichi Sasada)]

(2013/08/20 19:30), trans (Thomas Sawyer) wrote:

Also, can it take an optional key argument?

def ONCE(k=nil,&b)
raise unless block_given?
ONCE_MUTEX.synchronize{
key = k || caller(3, 1)[0]

I assume that ONCE is keyword like BEGIN{} and END{}.
And key will be a special key provided by compiler.

Completely same feature as /#{...}/o.

--
// SASADA Koichi at atdot dot net

[matz (Yukihiro Matsumoto)]

• Status changed from Open to Rejected

I don't see any major use-case for the function.
Reopen if you (or anyone else) have.

Matz.

• Status changed from Rejected to Open

matz: Here's some code I wrote recently that would benefit from ONCE:

https://github.com/charliesome/better_errors/blob/master/lib/better_errors/stack_frame.rb#L30-42

[naruse (Yui NARUSE)]

• Target version deleted (2.6)

[Eregon (Benoit Daloze)]

• Description updated (diff)

[Eregon (Benoit Daloze)]

• Related to Feature #17474: Interpreting constants at compile time added

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Assigned