From: Adam Prescott <mentionuse@...>
Date: 2010-10-08T18:20:09+09:00
Subject: Re: named parameters in ruby?

> def do_some(v={})
>  a, b, c = v[:a], v[:b], v[:c]
> end

One thing to remember about this idiom is what happens when you try
and default the values:

(see http://gist.github.com/616543 for the below code in a prettier format)

def foo(hash = { :a => 1, :b => 2 })
  a = hash[:a]
  b = hash[:b]
  puts a.inspect
  puts b.inspect
end

foo

#=>
# 1
# 2

foo(:a => 10)

#=>
# 10
# nil

So you need something like

def foo(hash = {})
  hash = { :a => 1, :b => 2 }.merge(hash)
  a = hash[:a]
  b = hash[:b]
  puts a.inspect
  puts b.inspect
end

foo

#=>
# 1
# 2

foo(:a => 10)

#=>
# 10
# 2

That whole requirement of doing merge is a little messy, I suppose;
one reason that proper named/keyword arguments would be nice. Not too
much of a problem, though, really.


On Thu, Oct 7, 2010 at 9:17 PM, Brian Candler <b.candler@pobox.com> wrote:
> xiMera wrote:
>> A.new
>> a.set_vars( :a => 1. :b =>2 )
>> do_some a
>
> I think you mean:
>
>  a = A.new
>  a.set_vars ( :a => 1, :b => 2)
>  do_some a
>
> How is this better than
>
>  do_some :a => 1, :b => 2
>
> ?
>
> Note that if you are using ruby 1.9, then you can also write
>
>  do_some a: 1, b: 2
>
> which (from the caller's point of view) is getting pretty close to named
> parameters.
>
> At the callee's side, multiple assignment keeps things pretty clean:
>
> def do_some(v={})
>  a, b, c = v[:a], v[:b], v[:c]
> end
>
> Or you can use values_at, which is a bit longer in this case.
>
>  a, b, c = v.values_at(:a, :b, :c)
>
> Note that ruby needs to know at *parse* time that a,b and c are local
> variables, so it can reserve slots for them in the activation record,
> and this assignment achieves that.
>
> Any solution using eval to create local variables would be very
> inefficient, if it could be made to work at all.
>
> irb(main):010:0> def foo
> irb(main):011:1> eval("x = 5")
> irb(main):012:1> puts x
> irb(main):013:1> end
> => nil
> irb(main):014:0> foo
> NameError: undefined local variable or method `x' for main:Object
>  from (irb):12:in `foo'
>  from (irb):14
>  from :0
> --
> Posted via http://www.ruby-forum.com/.
>
>