From: Robert Klemme <shortcutter@...>
Date: 2007-06-27T19:40:01+09:00
Subject: Re: Fun about inspect

On 26.06.2007 23:42, Victor "Zverok" Shepelev wrote:
> Hi all.
> 
> By some complex reasons, I've made an interesting conclusion: for
> "serializable" types it's always good to have eval(obj.inspect) == obj
> 
> For me it was a good thought, because previously, I've always doubt what
> #inspect should do, and typically have ended with #inspect as alias for
> #to_s
> 
> Here is a dumb test for some core classes:
> 
> # dumb testing function
> # tests, if eval(obj.inspect) == obj, and if any 
> # parsing errors are thrown by evaluating
> #
> def tst(obj)
>   begin
>     print "testing %-10.10s: " % obj.class.name 
>     res = eval(obj.inspect)
>     if res == obj
>       puts 'OK'
>     else
>       print 'WRONG : '
>       puts "\t%-7s => %-7s" % [obj.inspect, res.inspect]
>     end
>   rescue Exception
>     puts "ERROR"
>   end
> end
> 
> #the tests itself
> 
> tst(5)         #=> OK
> tst("str")     #=> OK
> tst(RuntimeError.new)     #=> WRONG : #<RuntimeError: RuntimeError> => nil
> 
> tst([1,2,3])   #=> OK
> tst(:a => 1)   #=> OK
> tst(Time.new)  #=> ERROR
> 
> There are several interesting things to note:
> * basic types (Numeric, String, Array, Hash) are all behave good here.

No, they don't.  See below.

> * Exception descendants are evaluated to nil, which can be considered OK, as
> we typically not plan to serialize and store exceptions. Even in this case,
> results of #inspect are evaluated without any error!
> * But Time is a bad guy! It's #inspect result is something like "Wed Jun 27
> 00:36:12 +0300 2007" and is equal to it's #to_s result and can't be
> evaluated in any manner. I think, it's not the most reasonable variant.
> 
> What do you think?

I think nobody should rely on #inspect creating something that will be 
able to resurrect the original.  Note, that it does not even work 
properly for Hashes:

irb(main):001:0> h1 = Hash.new 666
=> {}
irb(main):002:0> h1[:foo]
=> 666
irb(main):003:0> s1 = h1.inspect
=> "{}"
irb(main):004:0> h2 = eval s1
=> {}
irb(main):005:0> h2[:foo]
=> nil

The sole purpose of #inspect is to return something that will reveal 
human readable information about an object's state which can be used for 
logging and debugging.  By no means #inspect is meant for serialization, 
that's what Marshal, YAML and the likes are for.

Kind regards

	robert