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Comment by shakna | Hacker News Reader

I'm not seeing the less friendly... Eval, and sandboxing eval, are builtin features of the Scheme standard.

The less-eval-friendly thing is R5RS only requires two possible "environment specifiers" (2nd arg to eval), returned by SCHEME-REPORT-ENVIRONMENT and NULL-ENVIRONMENT, with no special form or procedure to get the current lexical environment; and R4RS (I just checked) does not specify eval. There's pretty much no manipulation of environments in the standard; the best is something like

  ((eval `(lambda (foo) ,exp) (scheme-report-environment 5))
   foo)

(can't really think of applications for getting the current environment right now except for pseudo-macro stuff)

Scheme is hygeneic, yes. Which is why sandboxing works.

If you want to manipulate the current environment, and defeat half the point of the language, then you're allowed that through (user-initial-environment).

(define-syntax) will also let you rewrite the bindings however you require.

Edit: Here's an eval macro I use fairly often:

  (define-syntax constexpr
  (syntax-rules ()
    ((_ expr)
     (begin
       (syntax->datum
        (datum->syntax
         (quote-syntax here)
         (eval 'expr (interaction-environment))))))))
         
  (define (factorial n)
  (if (<= n 1)
      1
      (* n (factorial (- n 1)))))
         
  (define n
    (constexpr (factorial 10)))

By the current lexical environment I meant the lexical environment at some place in the code (not just the top-level one, which user-initial-environment gives, though it's optional). I'm not attempting to make a case for anything; but without that RnRS' eval doesn't grant more expressivity than just tacking on an interpreter to every program, which you could also just do yourself. (It also doesn't complicate the implementation; variable names don't need to be preserved after compiling.) I.e. it makes sense to say standard Scheme has been less "eval-friendly," even if there were good reasons or one doesn't care.

It has eval but it lacks reader macros (or at least it did thirty years ago) which is what makes eval useful on lisp2 systems.

I'm interested in reading more about how reader macros make `eval` better/useful.

R5RS supports reader macros? Which would be 1998, and I guess only 28 years ago.

That's why SRFI-49 and its descendants were a possibility. Wisp, JavaScript, brainfuck, etc. implementations all exist because the tools exist in the language.

It does not. https://conservatory.scheme.org/schemers/Documents/Standards... Particular implementations could.

That document, shows several forms:

https://conservatory.scheme.org/schemers/Documents/Standards...

Common Lisp reader macros hook into the reader itself as it's parsing S-exps and meets some character combination: http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec... Define-syntax is for making special forms (i.e. normal macros).

Not that I know why eval should be paired with reader macros.

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