In Ruby
2.1.2 :068 > a=1
=> 1
2.1.2 :069 > eval("a=4")
=> 4
2.1.2 :070 > a
=> 4
2.1.2 :071 > eval("b=4")
=> 4
2.1.2 :072 > b
NameError: undefined local variable or method `b' for main:Object
So,the question is why the variable 'b' will be 'undefined local variable or method' but the variable 'a' equals 4 ?
When you call the eval lambda you create a new scope for the code that you execute.
It would be like to create and execute a new lambda. If you declare a before and then use a variable called a in your lambda, you shall use the same. But if you don't declare b before the lambda, eval will create its own variable, and will delete it at the end.
b is local to the eval'd code. puts "outside binding #{binding.__id__}"
a = 1
puts "outside a #{a.__id__}"
eval 'b="b"; puts "inside binding #{binding.__id__}"; puts "inside a #{binding.local_variable_get(:a).__id__}"; a="c"; puts "inside after a #{a.__id__}"'
puts "outside after a #{a.__id__}"
run the code above, you can see that the outside binding and the inside binding is using different ids. but share the same a. when we run eval, ruby will copy the outside binding, so it will use the same a; but b is defined in the inside binding, when eval is done, inside binding is lost and so is b. Actually block scope is same as this.
