data structures <--> functions

[Jonathan A Rees <JAR@AI.AI.MIT.EDU>]

    Date: Sun, 6 Dec 87 01:51:10 est
    From: gjs at ZOHAR.AI.MIT.EDU (Gerald Jay Sussman)
    To:   JAR at AI.AI.MIT.EDU
    Re:   data structures <--> functions

    I really feel the need for EVAL (or ENCLOSE) quite seriously.

    In particular I find the need to execute code that my programs write.

    I wish this would be somehow solved.

How about if we put a meta-circular evaluator into the scheme library
that only knows about the standard R^3RS global variables?  Then if you
wanted to evaluate (+ (EXPT 2 23) 7), you could do

    (LOAD "scheme-library/eval")  ;or whatever

    (EVAL-IN-R^3RS-ENV '(+ (EXPT 2 23) 7))

Then, although particular implementations could choose to do "efficient"
implementations of this library routine, the beast is implementable in
R^3 Scheme, so no one could complain about its semantics or any
threat to change the language definition itself.

The only problem is, what if the expression E has free variables (e.g.
FOO) that are defined in the current environment, but not in the R^3RS
global environment?  Well, if FOO isn't going to be assigned, how about

    ((EVAL-IN-R^3RS-ENV '(LAMBDA (FOO ...) E))
     FOO
     ...)

or, if it is going to be assigned, something more complicated, like

    ((EVAL-IN-R^3RS-ENV '(LAMBDA (GET-FOO SET-FOO! ...) E'))
     (LAMBDA () FOO)
     (LAMBDA (VAL) (SET! FOO VAL))
     ...)

where E' = E with (GET-FOO) substituted for FOO  and SET-FOO! substituted
for SET! FOO.  Or, another entry point to the interpreter could be created
that would do substitutions of this kind automagically.

I think this would serve moderately well.

I'd be happy to submit such a package to the library (name and arguments
negotiable) if there's interest.  This could at least hold us until we
can come to agreement over something that would give real level-crossing
access to environments.