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Kai Middleton

2011-Feb-15 01:18 UTC

An idea for new syntactic sugar in Ruby

I was looking at this bit of code within Rails:

# File actionpack/lib/action_controller/metal/flash.rb, line 12
def redirect_to(options = {}, response_status_and_flash = {}) #:doc:
  if alert = response_status_and_flash.delete(:alert)
    flash[:alert] = alert
  end

  if notice = response_status_and_flash.delete(:notice)
    flash[:notice] = notice
  end

  if other_flashes = response_status_and_flash.delete(:flash)
    flash.update(other_flashes)
  end

  super(options, response_status_and_flash)
end

And I thought: ouch, aren''t those supposed to be double-equals?  Well,
no, it''s an assignment within an if statement.  It saves a line of
code per usage, thus three lines of code in that method.

But I thought: wow, that sure violates the principle of least
surprise.  Perhaps it would be nice if there were a way to reference
the most recent if-expression.  I recall how a lone underscore in irb
will contain the value of whatever was last evaluated.  So perhaps
something like this would be nice:

  if response_status_and_flash.delete(:notice)
    flash[:notice] = _
  end

instead of what we see above:

  if notice = response_status_and_flash.delete(:notice)
    flash[:notice] = notice
  end

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Philip Hallstrom

2011-Feb-15 01:45 UTC

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Re: An idea for new syntactic sugar in Ruby

> I was looking at this bit of code within Rails:
> 
> # File actionpack/lib/action_controller/metal/flash.rb, line 12
> def redirect_to(options = {}, response_status_and_flash = {}) #:doc:
>  if alert = response_status_and_flash.delete(:alert)
>    flash[:alert] = alert
>  end
> 
>  if notice = response_status_and_flash.delete(:notice)
>    flash[:notice] = notice
>  end
> 
>  if other_flashes = response_status_and_flash.delete(:flash)
>    flash.update(other_flashes)
>  end
> 
>  super(options, response_status_and_flash)
> end
> 
> And I thought: ouch, aren''t those supposed to be double-equals? 
Well,
> no, it''s an assignment within an if statement.  It saves a line of
> code per usage, thus three lines of code in that method.
> 
> But I thought: wow, that sure violates the principle of least
> surprise.  Perhaps it would be nice if there were a way to reference
> the most recent if-expression.  I recall how a lone underscore in irb
> will contain the value of whatever was last evaluated.  So perhaps
> something like this would be nice:
> 
>  if response_status_and_flash.delete(:notice)
>    flash[:notice] = _
>  end
> 
> instead of what we see above:
> 
>  if notice = response_status_and_flash.delete(:notice)
>    flash[:notice] = notice
>  end
Interesting, but I think using _ is even more surprising isn''t it? 
What happens when you have this...

if something-that-returns-a-value-and-passes-the-if-test
  if something-that-returns-a-value-but-fails-the-if-test
    # never happens
  else
    # what does _ reference?  The very top level if?  Or the second one?  
    # Both were evaluated, but one failed so does it count towards _ or not?
    # How do I access the first _ if Ruby decides _ is the second one?  
  end
end

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Kai Middleton

2011-Feb-15 01:50 UTC

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Re: An idea for new syntactic sugar in Ruby

Ok, I found this feature request:

http://redmine.ruby-lang.org/issues/show/1141

It was rejected.  I guess one could do:

notice = response_status_and_flash.delete(:notice)
flash[:notice] if notice

It''s one line shorter, and perhaps less confusing.  The only drawback
would be that the notice variable stays in scope, which is not so
nice.


On Feb 14, 5:18 pm, Kai Middleton
<kai.middle...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> I was looking at this bit of code within Rails:
>
> # File actionpack/lib/action_controller/metal/flash.rb, line 12
> def redirect_to(options = {}, response_status_and_flash = {}) #:doc:
>   if alert = response_status_and_flash.delete(:alert)
>     flash[:alert] = alert
>   end
>
>   if notice = response_status_and_flash.delete(:notice)
>     flash[:notice] = notice
>   end
>
>   if other_flashes = response_status_and_flash.delete(:flash)
>     flash.update(other_flashes)
>   end
>
>   super(options, response_status_and_flash)
> end
>
> And I thought: ouch, aren''t those supposed to be double-equals?
 Well,
> no, it''s an assignment within an if statement.  It saves a line of
> code per usage, thus three lines of code in that method.
>
> But I thought: wow, that sure violates the principle of least
> surprise.  Perhaps it would be nice if there were a way to reference
> the most recent if-expression.  I recall how a lone underscore in irb
> will contain the value of whatever was last evaluated.  So perhaps
> something like this would be nice:
>
>   if response_status_and_flash.delete(:notice)
>     flash[:notice] = _
>   end
>
> instead of what we see above:
>
>   if notice = response_status_and_flash.delete(:notice)
>     flash[:notice] = notice
>   end
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Kai Middleton

2011-Feb-15 01:53 UTC

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Re: An idea for new syntactic sugar in Ruby

I also found this on stackoverflow:

http://stackoverflow.com/questions/3835170/ruby-assignment-and-comparison-in-one-line

On Feb 14, 5:18 pm, Kai Middleton
<kai.middle...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> I was looking at this bit of code within Rails:
>
> # File actionpack/lib/action_controller/metal/flash.rb, line 12
> def redirect_to(options = {}, response_status_and_flash = {}) #:doc:
>   if alert = response_status_and_flash.delete(:alert)
>     flash[:alert] = alert
>   end
>
>   if notice = response_status_and_flash.delete(:notice)
>     flash[:notice] = notice
>   end
>
>   if other_flashes = response_status_and_flash.delete(:flash)
>     flash.update(other_flashes)
>   end
>
>   super(options, response_status_and_flash)
> end
>
> And I thought: ouch, aren''t those supposed to be double-equals?
 Well,
> no, it''s an assignment within an if statement.  It saves a line of
> code per usage, thus three lines of code in that method.
>
> But I thought: wow, that sure violates the principle of least
> surprise.  Perhaps it would be nice if there were a way to reference
> the most recent if-expression.  I recall how a lone underscore in irb
> will contain the value of whatever was last evaluated.  So perhaps
> something like this would be nice:
>
>   if response_status_and_flash.delete(:notice)
>     flash[:notice] = _
>   end
>
> instead of what we see above:
>
>   if notice = response_status_and_flash.delete(:notice)
>     flash[:notice] = notice
>   end
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Kai Middleton

2011-Feb-15 01:57 UTC

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Re: An idea for new syntactic sugar in Ruby

Yeah, great point.  How to know how deep to go.  I was looking at some
of the Ruby global vars:

http://www.rubyist.net/~slagell/ruby/globalvars.html

This one is similar:
$n	 the nth subexpression in the last match (same as $~[n])

But it seems somehow not so clean.

We have this nifty idiom:

x ||= "hello"

But I can''t think of a neat syntax for what I''m wanting.

On Feb 14, 5:45 pm, Philip Hallstrom <phi...-LSG90OXdqQE@public.gmane.org>
wrote:> > I was looking at this bit of code within Rails:
>
> > # File actionpack/lib/action_controller/metal/flash.rb, line 12
> > def redirect_to(options = {}, response_status_and_flash = {}) #:doc:
> >  if alert = response_status_and_flash.delete(:alert)
> >    flash[:alert] = alert
> >  end
>
> >  if notice = response_status_and_flash.delete(:notice)
> >    flash[:notice] = notice
> >  end
>
> >  if other_flashes = response_status_and_flash.delete(:flash)
> >    flash.update(other_flashes)
> >  end
>
> >  super(options, response_status_and_flash)
> > end
>
> > And I thought: ouch, aren''t those supposed to be
double-equals?  Well,
> > no, it''s an assignment within an if statement.  It saves a
line of
> > code per usage, thus three lines of code in that method.
>
> > But I thought: wow, that sure violates the principle of least
> > surprise.  Perhaps it would be nice if there were a way to reference
> > the most recent if-expression.  I recall how a lone underscore in irb
> > will contain the value of whatever was last evaluated.  So perhaps
> > something like this would be nice:
>
> >  if response_status_and_flash.delete(:notice)
> >    flash[:notice] = _
> >  end
>
> > instead of what we see above:
>
> >  if notice = response_status_and_flash.delete(:notice)
> >    flash[:notice] = notice
> >  end
>
> Interesting, but I think using _ is even more surprising isn''t it?
 What happens when you have this...
>
> if something-that-returns-a-value-and-passes-the-if-test
>   if something-that-returns-a-value-but-fails-the-if-test
>     # never happens
>   else
>     # what does _ reference?  The very top level if?  Or the second one?  
>     # Both were evaluated, but one failed so does it count towards _ or
not?
>     # How do I access the first _ if Ruby decides _ is the second one?  
>   end
> end
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Colin Law

2011-Feb-15 09:22 UTC

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Re: An idea for new syntactic sugar in Ruby

On 15 February 2011 01:18, Kai Middleton
<kai.middleton-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> I was looking at this bit of code within Rails:
>
> # File actionpack/lib/action_controller/metal/flash.rb, line 12
> def redirect_to(options = {}, response_status_and_flash = {}) #:doc:
>  if alert = response_status_and_flash.delete(:alert)
>    flash[:alert] = alert
>  end
>
>  if notice = response_status_and_flash.delete(:notice)
>    flash[:notice] = notice
>  end
>
>  if other_flashes = response_status_and_flash.delete(:flash)
>    flash.update(other_flashes)
>  end
>
>  super(options, response_status_and_flash)
> end
>
> And I thought: ouch, aren''t those supposed to be double-equals?
 Well,
> no, it''s an assignment within an if statement.  It saves a line of
> code per usage, thus three lines of code in that method.
>
> But I thought: wow, that sure violates the principle of least
> surprise.
In that situation I would code it as
 if ( other_flashes = response_status_and_flash.delete(:flash) )
which arguably makes the intention clearer with only a few extra
characters an no run time penalty.

Colin

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Curtis j Schofield

2011-May-20 21:05 UTC

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Re: An idea for new syntactic sugar in Ruby

>
>  if notice = response_status_and_flash.delete(:notice)
>    flash[:notice] = notice
>  end
>
>This is a common C idiom.

The best solution is to always write your == with the constant on the LEFT
instead
of the right.

:D



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Rob Biedenharn

2011-May-20 21:23 UTC

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Re: An idea for new syntactic sugar in Ruby

On May 20, 2011, at 5:05 PM, Curtis j Schofield wrote:>  if notice = response_status_and_flash.delete(:notice)
>    flash[:notice] = notice
>  end
>
>
> This is a common C idiom.
>
> The best solution is to always write your == with the constant on  
> the LEFT instead
> of the right.
>
> :D
> -- 
> make haste slowly \
> festina lente  \
> -
> mobile  +1_415_632_6001
> curtis.schofield-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
> http://robotarmyma.de
>
assuming that the intent is to call  
response_status_and_flash.delete(:notice) just once, test the result,  
and then use a truthy result in the flash[:notice], this is almost  
exactly what I typically type.

However, I''d really put ( ) around the expression.

if (notice = response_status_and_flash.delete(:notice))
   flash[:notice] = notice
end

This is a trick from JSLint which will complain about assignments in  
the conditional expression of an if, but an extra pair of parentheses  
will signal your intent and silence the warning.

-Rob


Rob Biedenharn		
Rob-xa9cJyRlE0mWcWVYNo9pwxS2lgjeYSpx@public.gmane.org
http://AgileConsultingLLC.com/
rab-/VpnD74mH8+00s0LW7PaslaTQe2KTcn/@public.gmane.org	
http://GaslightSoftware.com/

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David Kahn

2011-May-20 21:48 UTC

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Re: An idea for new syntactic sugar in Ruby

On Fri, May 20, 2011 at 4:23 PM, Rob Biedenharn
<Rob-GBZH0y1GwQfnZcttdmLDtcI/UQi/AW5J@public.gmane.org>wrote:
> On May 20, 2011, at 5:05 PM, Curtis j Schofield wrote:
>
>  if notice = response_status_and_flash.delete(:notice)
>>    flash[:notice] = notice
>>  end
>>
>>
> This is a common C idiom.
>
> The best solution is to always write your == with the constant on the LEFT
> instead
> of the right.
>
> :D
> --
> make haste slowly \
> festina lente  \
> -
> mobile  +1_415_632_6001
> curtis.schofield-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
<curtis-DDU1nqEjlGkHaT8GDLgCUg@public.gmane.org>
> http://robotarmyma.de
>
>
> assuming that the intent is to call
> response_status_and_flash.delete(:notice) just once, test the result, and
> then use a truthy result in the flash[:notice], this is almost exactly what
> I typically type.
>
> However, I''d really put ( ) around the expression.
>
> if (notice = response_status_and_flash.delete(:notice))
>   flash[:notice] = notice
> end
>
This is definitely understandable without surprise, while I for sure did a
double take before understanding Kai''s initial example.

>
> This is a trick from JSLint which will complain about assignments in the
> conditional expression of an if, but an extra pair of parentheses will
> signal your intent and silence the warning.
>
> -Rob
>
>
> Rob Biedenharn
> Rob-xa9cJyRlE0mWcWVYNo9pwxS2lgjeYSpx@public.gmane.org
http://AgileConsultingLLC.com/
> rab-/VpnD74mH8+00s0LW7PaslaTQe2KTcn/@public.gmane.org
<Rob-/VpnD74mH8+00s0LW7PaslaTQe2KTcn/@public.gmane.org>
> http://GaslightSoftware.com/
>
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Rails - Feb 2011 - An idea for new syntactic sugar in Ruby

An idea for new syntactic sugar in Ruby

Re: An idea for new syntactic sugar in Ruby

Re: An idea for new syntactic sugar in Ruby

Re: An idea for new syntactic sugar in Ruby

Re: An idea for new syntactic sugar in Ruby

Re: An idea for new syntactic sugar in Ruby

Re: An idea for new syntactic sugar in Ruby

Re: An idea for new syntactic sugar in Ruby

Re: An idea for new syntactic sugar in Ruby