When is `eval` in Ruby justified?

Question

\"Is \'eval\' supposed to be nasty?\" inspired this one:

Mostly everybody agrees that eval is bad, and in most cases there is more elegant/safer replace

Answer 1

There is one very important use-case for eval() which cannot (AFAIK) be achieved using anything else, and that is to find the corresponding object reference for a binding.

Say you have been passed a block but (for some reason) you need access to object context of the binding, you would do the following:

obj = eval('self', block.binding)

It is also useful to define the following:

class Proc
    def __context__
        eval('self', self.binding)
    end
end

Answer 2

When is it justified? I'd say when there's no reasonable alternative. I was able to think of one use where I can't think of an alternative: irb, which, if you dig deep enough (to workspace.rb, around line 80 in my copy if you're interested) uses eval to execute your input:

def evaluate(context, statements, file = __FILE__, line = __LINE__)
  eval(statements, @binding, file, line)
end

That seems pretty reasonable to me - a situation where you specifically don't know what code you're going to have to execute until the very moment that you're asked to do so. Something dynamic and interactive seems to fit the bill.