From: Alexey Muranov <muranov@...>
Date: 2011-11-03T20:44:01+09:00
Subject: [ruby-dev:44795] [ruby-trunk - Feature #5553] A method for Hash that works differently depending on whether a key exists


Issue #5553 has been updated by Alexey Muranov.


I see.  But then you may also want to pass a second procedure which is run if the key does not exist, and to have both the key and the value passed to the procedures, that is to introduce a method of the form #fetch_and_use(key, proc_if_key_found, proc_if_not), but this will cost having to store the procedures in variables for a one-time use.  Also I am afraid that optimizing like this for all possible use cases may require introducing many new methods.

On the other hand, if you want to use a hash in this way with same procedures multiple times, then what about adding a method `Hash#on_found_proc=` analogous to  `Hash#default_proc=` to be able to do like this:

a = {morning: "おはよう", daytime: "こんにちは", evening: "こんばんは", nothing: nil}
a.default =  "どうも"
a.on_found_proc = proc do |hash,key,value|
  "#{value}世界!"
end
a[:morning]  # => "おはよう世界!"
a[:night]       # => "どうも"

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Feature #5553: A method for Hash that works differently depending on whether a key exists
http://redmine.ruby-lang.org/issues/5553

Author: Tsuyoshi Sawada
Status: Feedback
Priority: Normal
Assignee: 
Category: 
Target version: 


A method Hash#if_key(key, [default], &pr) which works like the following will be often used, and is useful.

    a = {morning: "おはよう", daytime: "こんにちは", evening: "こんばんは", nothing: nil}
    a.if_key(:morning){|str| "#{str}世界!"} #=> "おはよう世界!"
    a.if_key(:nothing){|str| "#{str}世界!"} #=> "世界!"
    a.if_key(:midnight){|str| "#{str}世界!"} #=> nil
    a.if_key(:nothing, "どうも"){|str| "#{str}世界!"} #=> "どうも"

That is, when `key' exists, then the corresponding value will be passed to `pr'. Otherwise, the given `default' or the implicit default will be returned.




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