From: zverok.offline@... Date: 2019-01-25T16:03:04+00:00 Subject: [ruby-core:91264] [Ruby trunk Feature#15557] A new class that stores a condition and the previous receiver Issue #15557 has been updated by zverok (Victor Shepelev). > `Enumerable#grep` Well, it is not "it accepts callable", it is a part of "it accepts any pattern (including Proc)", and passing Proc into `grep` makes sense only in "polymorphic pattern situation" (we have a pattern variable, which can be Proc, or Regexp, or Range)... I mean, nobody probably writes just ```ruby something.grep(->(x) { x.odd? }) ``` ...because they write ```ruby something.select(&:odd?) ``` My point was, there are not much core APIs that make people used to `some_method(-> { some proc })` ---------------------------------------- Feature #15557: A new class that stores a condition and the previous receiver https://bugs.ruby-lang.org/issues/15557#change-76513 * Author: sawa (Tsuyoshi Sawada) * Status: Open * Priority: Normal * Assignee: * Target version: ---------------------------------------- I often see code like this: ```ruby foo = default_definition foo = some_method(foo) if some_condition(foo) foo = another_method(foo) if another_condition(foo) ... ``` It would be nice if we can write this as a method chain. Since we now have the method `then`, I thought it would be a nice fit to introduce a method called `when`, such that putting it right in front of `then` would execute the `then` method as ordinarily only when the condition is satisfied, and returns the previous receiver otherwise so that the code above can be rewritten as: ```ruby foo = default_definition .when{|foo| some_condition(foo)} .then{|foo| some_method(foo)} .when{|foo| another_condition(foo)} .then{|foo| another_method(foo)} ``` This proposal is also a generalization of what I intended to cover by https://bugs.ruby-lang.org/issues/13807. That is, ```ruby a.some_condition ? a : b ``` would rewritten as: ```ruby a.when(&:some_condition).then{b} ``` The proposal can be implemented by introducing a class called `Condition`, which stores a condition and the previous receiver, and works with `then` in a particular way. ```ruby class Object def when Condition.new(self, yield(self)) end end class Condition def initialize default, condition @default, @condition = default, condition end def then @condition ? yield(@default) : @default end end ``` And additionally, if we introduce a negated method `unless` (or `else`) as follows: ```ruby class Object def unless Condition.new(self, !yield(self)) end end ``` then we can use that for purposes such as validation of a variable as follows: ```ruby bar = gets .unless{|bar| some_validation(bar)} .then{raise "The input is bad."} .unless{|bar| another_validation(bar)} .then{raise "The input is bad in another way."} ``` -- https://bugs.ruby-lang.org/ Unsubscribe: <mailto:ruby-core-request@ruby-lang.org?subject=unsubscribe> <http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>