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comp.lang.ruby

accessor_writer methods and instance_eval

Thomas Gantner

8/23/2007 11:36:00 PM

Hi

I stumbled upon a problem with 'instance_eval' (using ruby 1.8.5 on
linux-x86_64). Simplified code example:

irb:01> S = Struct.new(:a)
=> S
irb:02> s = S.new(1)
=> #<struct S a=1>
irb:03> s.a
=> 1
irb:04> s.a = 2
=> 2
irb:05> s
=> #<struct S a=2>
irb:06> s.instance_eval { puts a }
2
=> nil
irb:07> s.instance_eval { a = 3 }
=> 3
irb:08> s
=> #<struct S a=2>
irb:09> s.instance_eval { self.a = 3 }
=> 3
irb:10> s
=> #<struct S a=3>

My problem is in line 07: it seems that the 'a' in the block is treated as a
local variable, therefore all the writer-accessor methods ('a=') are hidden
within the block. I get the same result when instance_eval-ing the
string "a = 3" instead of the block variant.

Is there a way to avoid this behaviour without using 'self.a' or passing the
struct as a parameter into the block (which quite obsoletes the meaning of
instance_eval)?

My real world use of this pattern is using a block with 'initialize()':

class C
def initialize(&blk)
@format = Struct.new( ... many, many elements with default values ...)
@format.instance_eval(&blk)
end
end

To explicitly override some default format-values I want to use

c = C.new { elem8 = 8; elem17 = 'test'; ... }

This won't work as explained above. I know I can use

c = C.new {|format| format.elem8 = 8; format.elem17 = 'test'; ... }

or

c = C.new { self.elem8 = 8; self.elem17 = 'test'; ... }

but both seem somewhat redundant and need too much typing (yeah I know I'm
lazy ;-).

Thanks for any suggestions.

-Thomas

--
<sig. under construction>
3 Answers

Ezra Zygmuntowicz

8/24/2007 12:46:00 AM

0

Hi~

On Aug 23, 2007, at 4:40 PM, Thomas Gantner wrote:

> Hi
>
> I stumbled upon a problem with 'instance_eval' (using ruby 1.8.5 on
> linux-x86_64). Simplified code example:
>
> irb:01> S = Struct.new(:a)
> => S
> irb:02> s = S.new(1)
> => #<struct S a=1>
> irb:03> s.a
> => 1
> irb:04> s.a = 2
> => 2
> irb:05> s
> => #<struct S a=2>
> irb:06> s.instance_eval { puts a }
> 2
> => nil
> irb:07> s.instance_eval { a = 3 }
> => 3
> irb:08> s
> => #<struct S a=2>
> irb:09> s.instance_eval { self.a = 3 }
> => 3
> irb:10> s
> => #<struct S a=3>
>
> My problem is in line 07: it seems that the 'a' in the block is
> treated as a
> local variable, therefore all the writer-accessor methods ('a=')
> are hidden
> within the block. I get the same result when instance_eval-ing the
> string "a = 3" instead of the block variant.
>
> Is there a way to avoid this behaviour without using 'self.a' or
> passing the
> struct as a parameter into the block (which quite obsoletes the
> meaning of
> instance_eval)?
>
> My real world use of this pattern is using a block with 'initialize
> ()':
>
> class C
> def initialize(&blk)
> @format = Struct.new( ... many, many elements with default
> values ...)
> @format.instance_eval(&blk)
> end
> end
>
> To explicitly override some default format-values I want to use
>
> c = C.new { elem8 = 8; elem17 = 'test'; ... }
>
> This won't work as explained above. I know I can use
>
> c = C.new {|format| format.elem8 = 8; format.elem17 = 'test'; ... }
>
> or
>
> c = C.new { self.elem8 = 8; self.elem17 = 'test'; ... }
>
> but both seem somewhat redundant and need too much typing (yeah I
> know I'm
> lazy ;-).
>
> Thanks for any suggestions.
>
> -Thomas
>
> --
> <sig. under construction>


No unfortunately you are stuck with the behavior you observed.
Whenever ruby sees 'a = b' it assumes it is a local variable
assignment. This happens at parse time so there is no way to affect
it at runtime. It's a tradeoff in order to allow local variables to
look like method calls and visca versca. So you have to prefix an
assignment with self. or another object, otherwise it is a local
variable assignment and you cannot work around it since it happens at
parse time.

Cheers-
-- Ezra Zygmuntowicz-- Founder & Ruby Hacker
-- ez@engineyard.com
-- Engine Yard, Serious Rails Hosting
-- (866) 518-YARD (9273)

Nobuyoshi Nakada

8/24/2007 1:30:00 AM

0

Hi,

At Fri, 24 Aug 2007 08:40:08 +0900,
Thomas Gantner wrote in [ruby-talk:266021]:
> Is there a way to avoid this behaviour without using 'self.a' or passing the
> struct as a parameter into the block (which quite obsoletes the meaning of
> instance_eval)?

Nothing. It's one of reasons why using instance_eval for such
purpose is not a good idea.

--
Nobu Nakada

Thomas Gantner

8/25/2007 8:38:00 PM

0

on Fri 24. August 2007 03.30, Nobuyoshi Nakada wrote:

> Hi,
>
> At Fri, 24 Aug 2007 08:40:08 +0900,
> Thomas Gantner wrote in [ruby-talk:266021]:
>> Is there a way to avoid this behaviour without using 'self.a' or passing
>> the struct as a parameter into the block (which quite obsoletes the
>> meaning of instance_eval)?
>
> Nothing. It's one of reasons why using instance_eval for such
> purpose is not a good idea.
>

Thanks for your quick answer (also to Ezra).

I found a solution by defining setter-methods for every struct member:

class C
def initialize(&blk)
@format = Struct.new( ... many, many elements ...) do
members.each do |key|
self.send(:define_method, "set_#{key}".to_sym) do |new_value|
self[key] = new_value
end
end
end
@format.instance_eval(&blk) if block_given?
end
end

Then I can use:

c = C.new { set_elem8(8); set_elem17('test'); ... }

Quite readable, but not many keystrokes saved. Is there a better
alternative? I understand you suggest something without instance_eval()?

-Thomas

--
<sig. under construction>