From: zverok.offline@gmail.com
To: ruby-core@ruby-lang.org
Subject: [ruby-core:91225] [Ruby trunk Feature#15557] A new class that stores a condition and the previous receiver
Date: Wed, 23 Jan 2019 10:21:15 +0000 (UTC) [thread overview]
Message-ID: <redmine.journal-76469.20190123102112.47f3b9fde73f8944@ruby-lang.org> (raw)
In-Reply-To: redmine.issue-15557.20190123091554@ruby-lang.org
Issue #15557 has been updated by zverok (Victor Shepelev).
I played a bit with idea similar to @nobu's, with this syntax:
```ruby
then_if(condition) { action }
then_if(-> { condition }) { action }
```
Explanations:
* boolean vs callable: sometimes, static conditions are useful, sligtly realistic example:
```ruby
def fetch(url, parse: false)
get(url).body
.then_if(parse) { |res| JSON.parse(res) }
# or, simpler:
.then_if(parse, &JSON.:parse)
end
```
* name: "it is like `then`, but conditional"
I experimented a bit with syntax like @sawa's initial proposal, and believe that it _could_ work too (but method's name should probably be `if` ;)), with the same adjustment that it can receive pre-calculated condition, too. The problem with it, though, is the introduction of the new abstraction/"mode" for the sake of nice syntax, e.g. `something.if(&:cond?)` on itself doesn't produce an easily explainable/reusable object.
The problem with @nobu's one, though, that it is pretty rarely seen in core Ruby to pass callable object instead of just a block (though, it exists, like @nobu pointed in another ticket, for example in `Enumerator::new`)
----------------------------------------
Feature #15557: A new class that stores a condition and the previous receiver
https://bugs.ruby-lang.org/issues/15557#change-76469
* 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/
next prev parent reply other threads:[~2019-01-23 10:21 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <redmine.issue-15557.20190123091554@ruby-lang.org>
2019-01-23 9:15 ` [ruby-core:91220] [Ruby trunk Feature#15557] A new class that stores a condition and the previous receiver sawadatsuyoshi
2019-01-23 9:27 ` [ruby-core:91223] " nobu
2019-01-23 10:21 ` zverok.offline [this message]
2019-01-23 10:24 ` [ruby-core:91226] " shevegen
2019-01-25 15:47 ` [ruby-core:91263] " nobu
2019-01-25 16:03 ` [ruby-core:91264] " zverok.offline
2019-01-27 5:22 ` [ruby-core:91300] " nobu
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