Can eval be retrofitted to an existing Scheme implementation?
Firstly, eval should only be used when its absolutely necessary – “eval is evil” is a common saying among Schemers. Beginners often make the mistake of employing eval when a combination of closures, quasiquotation and macros would achieve the same effect. There are however some cases where eval is the only option of accomplishing a particular task – see for instance here for examples. eval only became a mandatory part of the Scheme standard with the R5RS revision. Hence some Schemes do not support it. Petrofsky posted an implementation of eval in Scheme itself – see http://groups.google.com/groups?selm=876617n453.fsf%40radish.petrofsky.org – and it should be possible to retrofit this to existing Schemes that do not have eval.
