Kernel#eval

There's an interesting post in ruby mailing list today. The discussion started by a question asked by Jonathan Wills:

a = 1
eval('a=2')
puts a

This will print out '2', as I want. However if I remove the first line,

eval('a=2')
puts a

r.rb:2:in `

': undefined local variable or method `a' for
main:Object (NameError)

Then Mike Gold gives an excellent explaination:

The thing to remember is that local variables are always determined at parse time.

eval("a = 3", binding)
p local_variables #=> ["a"]
p a #=> undefined local variable or method `a'

We see that the local exists, but because the parser has not seen the "a = ..." syntax, we can't access it.

Locals are not handled in the same way. The local assignment syntax is subject to special examination by the parser. This does not happen for instance variables and globals. Again, it is the parser that determines locals. If all assignments were the same, this would print '3':

eval("a = 3", binding)
puts a

Pickaxe gives this example:

def a
print "Function 'a' called\n"
99
end

for i in 1..2
if i == 2
print "a=", a, "\n"
else
a = 1
print "a=", a, "\n"
end
end

produces:

a=1
Function 'a' called
a=99

But constants and instance variables are different. Which means

eval("@a=2")
@a # => 2

Another interested thing Mike metioned is, when you see this error:

undefined local variable or method `a' for main:Object (NameError)

It in fact means 'undefined method a':

... that error message comes from rb_method_missing(), i.e., it was determined to be a method call. Presumably it is worded that way for the user's benefit, in case there was a misspelling or some such. Locals are determined by the parser; method lookup is done when no such local exists.