Re: [Scheme-reports] Proposal to add fexprs

[John Cowan <cowan@x>]

Kalinni Gorzkis scripsit:

> Macros are like functions, but arguments are passed to them with their
> original syntax, instead of evaluated. 

They are not.  Macros are (conceptually, and often actually) expanded
*before* run time, whereas functions are called at run time.

> You can already emulate [fexprs] with functions that are called like
> (macro ‘(anything you want) ‘(just everything she wants))

That is also not the case, because Scheme does not have an `eval` function
with access to lexical bindings.  Suppose you have a procedure `choose`
that works like `if` but accepts three arguments:

(define (choose p t e)
  (if p (eval t) (eval e)))

Now invoke it like this:

(let ((a 1) (b 2))
  (choose #t 'a 'b))

The call to `eval` within `choose` will look in the global environment
for the definition of `a`, and will not find the value 1 bound in the
lexical environment.

(In fact, this does not work, because you must explicitly specify which
global environment you want `eval` to use.)

> Scheme is intended to be a language which is designed to be small but
> powerful and flexible.

Up to a point.

> There have been recent interests in reintroducing fexprs to Lisp. Many
> newer Lisp dialects, for example newLISP and Kernel include fexpr.

Newlisp is dynamically scoped, like Elisp; unlike Elisp, it has namespaces.
The only way to re-create the safe lexical scoping of Scheme within
Newlisp is to have one namespace per function.  That may be a reasonable
approach to Lisp, but it is not Scheme.

As for Kernel, it is closer to Scheme in some ways, but further away
in others.  It too is a reasonable approach to Lisp.

> So I propose to reintroduce fexprs, or first-class macros, into Scheme.
> What do you think?

I think it would be a serious mistake.

-- 
John Cowan  cowan@x  http://ccil.org/~cowan
If I have not seen as far as others, it is because giants were standing
on my shoulders.
        --Hal Abelson

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