[racket] redefining #%top (and #%app) with conditions

From: Matthew Butterick (mb at mbtype.com)
Date: Fri Feb 7 00:37:58 EST 2014

Reflecting on it as an issue of detecting bound identifiers, I've come up with a possible approach — could it be this simple, or am I overlooking some complication?


#lang racket

(provide bound/c (rename-out (top~ #%top)))

(define-syntax-rule (top~ . id)
    (λ x `(id , at x)))

(define-syntax (bound/c stx)
    (syntax-case stx ()
      [(_ x)
       (if (identifier-binding #'x )
           #'(#%top . x))]))


#lang racket
(require "bound.rkt")
(define foo displayln) ; foo is now bound
(foo "hello")
((bound/c foo) "hello")
(bar "hello") ; bar is unbound
;((bound/c bar) "hello")

This does the right thing:

'(bar "hello")

And then when the last line is uncommented

((bound/c bar) "hello")

It triggers the usual error on compile:

bar: unbound identifier in module in: bar

On Feb 6, 2014, at 8:38 PM, Matthew Butterick <mb at mbtype.com> wrote:

> I'm trying to figure out how to make #%top change behavior based on different kinds of function names, though my experiments keep leading to infinite loops that blow up DrRacket. Oops.
> In my #lang project based on Scribble, I've been using this simple redefinition of #%top for convenience:
> (define-syntax-rule (#%top . id)
>    (λ x `(id , at x)))
> IOW, if the thing in the function position is undefined, it's treated as the opening tag of an x-expression. This makes it easy to mix undefined and defined names.
> But it can make debugging difficult. Because if you expect a name to be defined as a function and it isn't, then you don't get the syntax error you ordinarily would. The function call silently gets converted an x-expression. Spooky side effects follow.
> I'd like to improve this by making a syntactic prefix that suppresses this behavior and that can be attached to any function name. For instance, def-*. So if I write (def-foobar ..) in the code, this will mean:
> 1) try to invoke a function called (foobar ...); 
> 2) if the function (foobar ...) isn't defined, raise the usual exception + syntax error.
> In other words, the standard #%top behavior.
> Whereas if I just write (foobar ...) without the def-* prefix, this will mean:
> 1) try to invoke the function (foobar ...); 
> 2) if the function (foobar ...) isn't defined, turn it into the x-expression `(foobar ,@ ...). 
> In other words, my special #%top behavior shown above.
> Having explained the logic, I think the flaw in my experiments so far is that this actually requires manipulation of #%app too, because once you hit #%top, you've missed your chance to apply functions. (Right?) But even supposing one creates two evaluation paths under #%app, it's not clear to me how to preserve those paths on the way up to #%top.

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