Side-effect free evaluation?

[Snyder]

Is there any Scheme (or other Lisp) implementation that allows
for side-effect free evaluation of expressions? Like, when I
receive an expression from an untrusted source and I want to
compute just its value and be safe from any side-effects (such as
mutation, IO, etc.).

[Jussi Piitulainen]

Snyder writes:

You could wrap the untrusted expression in a let expression that binds
every dangerous identifier in the report environment to something safe
and then evaluate the wrapped expression in the report environment:

(eval `(let ((set! '(no can do))
(vector-set! '(no can do))
(set-car! '(no can do))
(display '(no can do))
... many more ...
(eval '(no can do)))
,untrusted-expression)
(scheme-report-environment 5))

Something like that. This will raise an error if any of those
identifiers are used in untrusted-expression in a way that would have
had an effect during the evaluation.

Or can we now create environments with just the bindings that we want
to allow? That might be nicer.

[Grant Rettke]

Here is one approach in PLT Scheme:

http://www.cs.brown.edu/courses/csci1730/2008/Manual/reference/Sandboxed_Evaluation.html

[Aaron W. Hsu]

Jussi Piitulainen <jpii...@ling.helsinki.fi> writes:

>Snyder writes:

>> Is there any Scheme (or other Lisp) implementation that allows for
>> side-effect free evaluation of expressions? Like, when I receive an
>> expression from an untrusted source and I want to compute just its
>> value and be safe from any side-effects (such as mutation, IO,
>> etc.).

[...]

>Or can we now create environments with just the bindings that we want
>to allow? That might be nicer.

Yes, in R6RS you can do this. The solution is to define a library with
only the bindings you want to use. You can then evaluate your code however
you like using:

(eval code (environment '(my safe environment)))

Chez Scheme also allows you to copy an environment, selectively taking
the bindings you want from it. Other Schemes give you similar features.

Aaron W. Hsu

[Jussi Piitulainen]

Aaron W. Hsu writes:

> Jussi Piitulainen writes:
> >Snyder writes:
>
> >> Is there any Scheme (or other Lisp) implementation that allows
> >> for side-effect free evaluation of expressions? Like, when I

[...]

> [...]
>
> > Or can we now create environments with just the bindings that we
> > want to allow? That might be nicer.
>
> Yes, in R6RS you can do this. The solution is to define a library
> with only the bindings you want to use. You can then evaluate your
> code however you like using:
>
> (eval code (environment '(my safe environment)))

Thanks. R6RS was what I meant by "now".