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

A tool like eval is about evaluating code at runtime vs. "compile" time. Do you know what the code is when you launch Ruby? Then you probably don't need eval. Is your code generating code during runtime? then you probably need to eval it.

For example, the methods/functions needed in a recursive decent parser depend on the language being parsed. If your application builds such a parser on-the-fly, then it might make sense to use eval. You could write a generalized parser, but it might not be as elegant a solution.

"Programatically filling in a letrec in Scheme. Macros or eval?" is a question I posted about eval in Scheme, where its use is mostly unavoidable.

Answer 2

In general eval is a useful language feature when you want to run arbitrary code. This should be a rare thing but maybe you are making your own REPL or you want to expose the ruby run-time to the end user for some reason. It could happen and that is why the feature exists. If you are using it to work around some part of the language (e.g. global variables) then either the language is flawed or your understanding of the language is flawed. The solution is typically not to use eval but to either better understand the language or pick a different language.

It's worth noting that in ruby particulary instance_eval and class_eval have other uses.

Answer 3

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.

Answer 4

IMO mostly for Domain Specific Languages.

"Evaluation Options in Ruby" is an article by Jay Fields about it on InfoQ.

Answer 5

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.

Answer 6

eval is a tool, it is neither inherently good nor evil. It is justified whenever you are certain it is the right tool for what you are trying to accomplish.