From: Gavin Sinclair <gsinclair@...>
Date: 2010-07-19T23:38:22+09:00
Subject: Re: Instance eval in 1.8 and 1.9

>>
>> It seems like this could be a regression.
>
> Seconded. The program works OK for me with ruby 1.9.2dev (2009-07-18
> trunk 24186) [i686-linux]

Data so far:
 * works on 1.9.1-p376 [Caleb, Gavin]
 * works on 1.9.2dev (2009-07-18) [Brian]
 * doesn't work on 1.9.2dev (2010-07-02) [Gavin]

That agrees with Nobu's comment on ruby-core (which I should have
relayed here earlier, sorry):

    No, it not the point. Now instance_eval yields the self.

This code (to boil the problematic case down) fails...

    code = lambda { 42 }
    instance_eval &code

...because instance_eval passes 'self' as a parameter to the block.
Lambdas are fussy about parameters being accounted for, hence the
ArgumentError.  Procs are not fussy about parameters, so replace
'lambda' with 'proc' in the above code and it works.

For that matter, _this_ works too:

    code = lambda { |s| 42 }
    instance_eval &code

From what I gather:
 * in 1.8.x, instance_eval passed self but it was an undocumented feature
   and lambdas weren't fussy so it didn't matter
 * in 1.9.1 instance_eval stopped passing self (don't know whether that change
   was intentional)
 * heading towards 1.9.2 it started passing self again, from about December 2009

For a (still?)-undocumented feature, the message "ArgumentError: 0 for
1" is not very helpful.

Gavin