Bug #16251: Evaluation in binding differs from ruby execution

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.6.2p47 (2019-03-13 revision 67232) [x86_64-darwin18]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN

[teslur (Tetsushi FUKABORI)]

In specific situation, I found that result of string evaluation in Binding returns different from ruby execution result.

In following sample code, ruby evaluates method_or_local_var as method call and returns "method".
However, binding.eval evaluates method_or_local_var as local variable and returns nil.

Here is sample code.

def method_or_local_var
  'method'
end

if true
  puts "execute method_or_local_var:"
  p method_or_local_var #=> "method"

  puts "execute method_or_local_var by bind.eval('method_or_local_var'):"
  p binding.eval('method_or_local_var') #=> nil
else
  method_or_local_var = 'local variable'
end

and here is results of execute sample code.

❯ ruby sample_code.rb
execute method_or_local_var:
"method"
execute method_or_local_var by bind.eval('method_or_local_var'):
nil

I expect evaluation result of method_or_local_var in binding to be method, and returns "method".
Is this the expected behavior?

[mame (Yusuke Endoh)]

I have no idea whether this is a spec or undefined behavior, but the current implementation is actually intentional.

def x
  "x"
end

def foo
  eval("x = 1") # foo has no variable named x, so 1 is assigned to a temporal variable
  p eval("x") #=> "x" # there is no variable named x in this scope, so it is regarded as a method call
end
foo

def bar
  eval("x = 1") # foo has a variable named x, so 1 is assigned to the variable
  p eval("x") #=> 1 # foo has a variable named x, so it is regarded as a variable reference
  x = nil
end
bar

IMO, you'd better not to depend upon the behavior in any way.

[Eregon (Benoit Daloze)]

Local variables are "hoisted" to the beginning of the method/block in Ruby (and start with value nil).
With that in mind, I think the behavior is logical. IMHO, it is spec.

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Closed

[kernigh (George Koehler)]

This is a simpler example of the behavior:

$ ruby -e 'p x; x = 6'
Traceback (most recent call last):
-e:1:in `<main>': undefined local variable or method `x' for main:Object (NameError)
$ ruby -e 'p eval("x"); x = 6'
nil
$ ruby -e 'p binding.eval("x"); x = 6'
nil

A plain x raises NameError, but an eval("x") or binding.eval("x") fetches nil from the local variable x. I expect this behavior, because Ruby parses a script before running it:

1. Ruby parses x = 6 and adds x to the binding.
2. Ruby runs eval("x") with x in the binding.
3. Ruby runs x = 6 and changes the value of x from nil to 6.

Binding#local_variables reveals the local variable named x:

$ ruby -e 'p binding.local_variables; x = 6'
[:x]

Because x exists, binding.eval("x") has the expected behavior, so there is no bug.