From: harryo@... (Harry Ohlsen)
Date: 2002-04-04T17:11:11+09:00
Subject: Re: Is eval a code/design smell?

> I never use eval.  However, I use module_eval all the time.  That is,
> although I never use eval at "runtime", it's extremely useful at, err,
> "compile" time, to generate code.

Chris and I have been having a discussion about the use of eval because
we've been corresponding about some changes to his XMLSerialization
module that would make it more usable for me.

In this particular case, we're not doing a general eval, either,
but a couple of instance_eval calls, to allow the
serialise/deserialise to work with classes that may wish to restrict
access to some of their attributes (for example, if the programmer
wants some attribute to be read-only because it's a derived value,
or for some other reason).

So, the use of instance_eval is very specific.  Ie, all it's ever
doing is setting or getting the value of an attribute, to avoid
dictating to the class designer how they should write their class
in order to be able to use the module.

I think it was you who pointed to this being something that perhaps
the language should be providing support for.  That would be even
nicer, but isn't there right now, so far as I know.

I've also sent Chris a patch that uses class_eval to avoid calling
initialize, based on some code someone provided in answer to my
query about that issue.  Again, this is doing something very
specific, based on a design requirement, rather than doing a general
eval on arbitrary data provided as input to the module.

I'd be keen to know whether these kinds of uses of eval match with what
you would consider "clean", or whether you'd still  have reservations.