From: duerst@...
Date: 2018-09-03T02:35:33+00:00
Subject: [ruby-core:88813] [Ruby trunk Feature#14183] "Real" keyword argument

Issue #14183 has been updated by duerst (Martin D��rst).


jeremyevans0 (Jeremy Evans) wrote:
> mame (Yusuke Endoh) wrote:

> > This change seems to remind you the breaking change of character encoding in 1.9/2.0, but it was much worse than this change because the previous one was not trivial "where to fix".  The site where an error occurred was often different to the site where a wrong encoding string was created.
> 
> I disagree.  I migrated many applications and libraries from Ruby 1.8 to Ruby 1.9 (and later to Ruby 2.6).  The changes for Ruby 1.8 -> 1.9 were minimal in comparison with the impact of this change, in terms of the amount of code that needed to be modified.

I think the amount of changes from Ruby 1.8 to Ruby 1.9 depended a lot on what kind of processing your application did, and what kind of data was involved. If you mostly just worked with US-ASCII data, the changes needed were minimal. For other data, in particular also for Japanese data, some kinds of processing may have been heavily affected.

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-73858

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: Next Major
----------------------------------------
In RubyWorld Conference 2017 and RubyConf 2017, Matz officially said that Ruby 3.0 will have "real" keyword arguments.  AFAIK there is no ticket about it, so I'm creating this (based on my understanding).

In Ruby 2, the keyword argument is a normal argument that is a Hash object (whose keys are all symbols) and is passed as the last argument.  This design is chosen because of compatibility, but it is fairly complex, and has been a source of many corner cases where the behavior is not intuitive.  (Some related tickets: #8040, #8316, #9898, #10856, #11236, #11967, #12104, #12717, #12821, #13336, #13647, #14130)

In Ruby 3, a keyword argument will be completely separated from normal arguments.  (Like a block parameter that is also completely separated from normal arguments.)
This change will break compatibility; if you want to pass or accept keyword argument, you always need to use bare `sym: val` or double-splat `**` syntax:

```
# The following calls pass keyword arguments
foo(..., key: val)
foo(..., **hsh)
foo(..., key: val, **hsh)

# The following calls pass **normal** arguments
foo(..., {key: val})
foo(..., hsh)
foo(..., {key: val, **hsh})

# The following method definitions accept keyword argument
def foo(..., key: val)
end
def foo(..., **hsh)
end

# The following method definitions accept **normal** argument
def foo(..., hsh)
end
```

In other words, the following programs WILL NOT work:

```
# This will cause an ArgumentError because the method foo does not accept keyword argument
def foo(a, b, c, hsh)
  p hsh[:key]
end
foo(1, 2, 3, key: 42)

# The following will work; you need to use keyword rest operator explicitly
def foo(a, b, c, **hsh)
  p hsh[:key]
end
foo(1, 2, 3, key: 42)

# This will cause an ArgumentError because the method call does not pass keyword argument
def foo(a, b, c, key: 1)
end
h = {key: 42}
foo(1, 2, 3, h)

# The following will work; you need to use keyword rest operator explicitly
def foo(a, b, c, key: 1)
end
h = {key: 42}
foo(1, 2, 3, **h)
```

I think here is a transition path:

* Ruby 2.6 (or 2.7?) will output a warning when a normal argument is interpreted as keyword argument, or vice versa.
* Ruby 3.0 will use the new semantics.



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