Forum: Ruby named method arguments with defaults

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unknown (Guest)
on 2006-03-11 09:49
(Received via mailing list)
Hello all,

What is the current idiomatic approach to pass named arguments to a
method in Ruby ?

In Perl, I can do:

sub method
{
  # first the defaults are specified, then @_ is appended and overrides
  # the defaults, using Perl's array-folding-into-hash feature
  #
  my %args = ("size" => 45,
                      "length" => 12,
                      @_);

  my $the_size = $args{"size"};
}

# now, a call: 44 will override the default size 45, but length will
remain 12
#
method("size" => 44);

I'm sure Ruby has a nearly similar approach to achieve this, I just not
skilled enough (yet) to know how. Please advise.

Thanks in advance
Eli
Jules J. (Guest)
on 2006-03-11 11:23
(Received via mailing list)
You can use merge:

def a_method(*options)
  options = {:size => 45, :length => 12}.merge(options)

  # do stuff
end

a_method(:size => 44)

*should* work.

On Saturday, March 11, 2006, at 4:48 PM,  wrote:
>  # the defaults, using Perl's array-folding-into-hash feature
>#
>method("size" => 44);
>
>I'm sure Ruby has a nearly similar approach to achieve this, I just not
>skilled enough (yet) to know how. Please advise.
>
>Thanks in advance
>Eli
>
>


Jules
unknown (Guest)
on 2006-03-11 11:50
(Received via mailing list)
Jules J. wrote:
> *should* work.
>

This works when I remove the asterisk from the (*options).
Makes sense, I guess, since when I call:

a_method(:size => 21, :weight => 22)

With the asterisk specified, the arguments are folded into an array,
and merge can't take an array as an argument when called on a Hash.
Without the asterisk, Ruby's "special features" takes the arguments I
passed to a_method as a Hash, since nothing came after the last x => y
assignment, and then merge works.

Did I decipher this correctly ?

In any case, your proposal (without the asterisk) works nicely. In the
1st edition of "Programming Ruby" the authors noted that by Ruby 1.8,
named arguments will be built into the language. I assume this hasn't
happened yet ?

Thanks
Eli
Jules J. (Guest)
on 2006-03-11 12:39
(Received via mailing list)
Language support for named arguments will be in Ruby 2.0 (hopefully :P)

On Saturday, March 11, 2006, at 6:48 PM,  wrote:
>> a_method(:size => 44)
>and merge can't take an array as an argument when called on a Hash.
>
>Thanks
>Eli
>
>


Jules
Gene T. (Guest)
on 2006-03-11 17:39
(Received via mailing list)
Jules J. wrote:
> Language support for named arguments will be in Ruby 2.0 (hopefully :P)
>

There's a discussion of how to simulate keyword args in the draft Ruby
Cookbook, based on a KeywordProcessor module by Gavin S. which i
can't seem to google for.
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