When is `eval` in Ruby justified?

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.

The reason eval is there is because when you need it, when you really need it, there are no substitutes. There's only so much you can do with creative method dispatching, after all, and at some point you need to execute arbitrary code.

Just because a language has a feature that might be dangerous doesn't mean it's inherently a bad thing. When a language presumes to know more than its user, that's when there's trouble.

I'd argue that when you find a programming language devoid of danger, you've found one that's not very useful.

When is eval justified? In pragmatic terms, when you say it is. If it's your program and you're the programmer, you set the parameters.

The only case I know of (other than "I have this string and I want to execute it") is dynamically dealing with local and global variables. Ruby has methods to get the names of local and global variables, but it lacks methods to get or set their values based on these names. The only way to do AFAIK is with eval.

Any other use is almost certainly wrong. I'm no guru and can't state categorically that there are no others, but every other use case I've ever seen where somebody said "You need eval for this," I've found a solution that didn't.

Note that I'm talking about string eval here, by the way. Ruby also has instance_eval, which can take either a string or a block to execute in the context of the receiver. The block form of this method is fast, safe and very useful.

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