From: ts <decoux@...>
Date: 2002-06-01T18:14:20+09:00
Subject: Re: possible bug: stack dump with <<-String, #{...} and large loops

>>>>> "C" == Christoph  <chr_news@gmx.net> writes:

C> It does not really retrieve it - it is just telling Ruby to interpret  ``a''
C> as
C> an initialized local variable.

 There is different way to see it :-) 

C> Without the `eval part' you would
C> get a NameError, this difference is really surprising (to me at
C> least;-).

pigeon% ruby
def aa(x)
   a = 1
end
^D
pigeon% 

C> It is at least interesting that running the free form (no method) of the
C> same Ruby snippet
C> ---
C> if x
C>    eval "p a" # no premonition
C> end
C>  a  = 1
C> ---
C> also raises a NameError.

 it raise a NameError for `x'

pigeon% cat b.rb
#!/usr/bin/ruby
if x
   eval "p a"
end

a = 1
pigeon%
 
pigeon% b.rb
./b.rb:2: undefined local variable or method `x' for #<Object:0x401c6ce0> (NameError)
pigeon% 

 Now this work

pigeon% cat b.rb
#!/usr/bin/ruby
def x
   p "ruby has called me"
   true
end

if x
   eval "p a"
end

a = 1
pigeon% 

pigeon% b.rb
"ruby has called me"
nil
pigeon% 



C> so basically it uses the `insight'  (that `b' is a local variable and not
C> dynamic
C> variable )  gained at the end of the method body and to reinterpret the
C> previous lines in that new light?

 No : ruby first compile the def, find the local variable `b' and store
 the name of this variable in a node of this def. When it has found the
 line `eval "p a"' it was interpreted as `eval("p a")' and ruby has created
 a node to call the method #eval

 When ruby call the def, it retrieve the node where are stored the name of
 the local variables and create a local variable `b'. When #eval is called,
 it compile the string, at this time ruby know that it exist a local
 variable `b' and it interpret "b = 1" like an access to this local
 variable.

 Without the line "b = 2", the local variable is not created when the
 method is called and eval "b = 1" will also be interpreted as an access to
 a *new* local variable 


Guy Decoux