Andy Wingo <[email protected]> writes:
> What are the meanings of these expressions:
I found it amusing to see what my definitions using
with-current-continuation will produce here.
> ;; Toplevel
> (local-eval '(define foo 42) (the-environment))
guile> (my-eval '(define foo 42) (my-env))
guile> foo
42
> ;; Lexical, tail context
> (local-eval '(define foo 42) (let ((x 100)) (the-environment)))
guile> (my-eval '(define foo 42) (let ((x 100)) (my-env)))
Backtrace:
In standard input:
1: 0* [my-eval (define foo 42) ...
1: 1* (let* ((x 100)) (#<continuation 1401 @ 9253970> (define foo 42)))
1: 2 (begin #<continuation 1581 @ 9252000>)
1: 3 [#<continuation 1401 @ 9253970> ...
1: 4* (define foo 42)
standard input:1:11: In procedure memoization in expression (define foo 42):
standard input:1:11: In file "standard input", line 0: Bad define placement
(define foo 42).
ABORT: (syntax-error)
guile>
> ;; Lexical, tail context -- but with a definition
> (local-eval '(begin (define foo 42) foo) (let ((x 100)) (the-environment)))
guile> (my-eval '(begin (define foo 42) foo) (let ((x 100)) (my-env)))
Backtrace:
In standard input:
6: 0* [my-eval (begin (define foo 42) foo) ...
6: 1* (let* ((x 100)) (#<continuation 1401 @ 9219b38> (begin # foo)))
6: 2 (begin #<continuation 1581 @ 9259a80>)
6: 3 [#<continuation 1401 @ 9219b38> ...
6: 4* (begin (define foo 42) foo)
6: 5* (define foo 42)
standard input:6:18: In procedure memoization in expression (define foo 42):
standard input:6:18: In file "standard input", line 5: Bad define placement
(define foo 42).
ABORT: (syntax-error)
guile>
> ;; Lexical, tail context -- but with a definition, and nested reference
> (local-eval '(begin (define foo 42) (bar))
> (let ((x 100)) (define (bar) foo) (the-environment)))
guile> (my-eval '(begin (define foo 42) (bar)) (let ((x 100)) (define (bar)
foo) (my-env)))
Backtrace:
In standard input:
13: 0* [my-eval (begin (define foo 42) (bar)) ...
13: 1* (let* ((x 100)) (letrec (#) (# #)))
In unknown file:
?: 2 (letrec ((bar #)) (#<continuation 1401 @ 925e7c0> (begin # #)))
In standard input:
...
13: 3 [#<continuation 1401 @ 925e7c0> ...
13: 4* (begin (define foo 42) (bar))
13: 5* (define foo 42)
standard input:13:18: In procedure memoization in expression (define foo 42):
standard input:13:18: In file "standard input", line 12: Bad define placement
(define foo 42).
ABORT: (syntax-error)
> ;; Lexical, not a definition context
> (local-eval '(define foo 42) (let ((x 100)) not-a-definition
> (the-environment)))
guile> (my-eval '(define foo 42) (let ((x 100)) "hello" (my-env)))
Backtrace:
In standard input:
12: 0* [my-eval (define foo 42) ...
12: 1* (let* ((x 100)) "hello" (#<continuation 1401 @ 9253970> (define foo
42)))
12: 2 (begin #<continuation 1581 @ 9252000>)
12: 3 [#<continuation 1401 @ 9253970> ...
12: 4* (define foo 42)
standard input:12:11: In procedure memoization in expression (define foo 42):
standard input:12:11: In file "standard input", line 11: Bad define placement
(define foo 42).
ABORT: (syntax-error)
guile>
> What about this one:
>
> ;; Keeping in mind that `or' expands to (let ((t ...)) (if t t ...)),
> ;; hygienically
> (local-eval 't '(let ((t 42)) (or #f (the-environment))))
Assuming that the second quote mark is a typo.
guile> (my-eval 't (let ((t 42)) (or #f (my-env))))
42
guile>
Now of course, the continuation based approach that just hijacks the
expander and jumps in and out of it is not really a measure of how
things should work. But it makes clear that (the-environment) is a bit
of a chimera: it captures content at a level conceptually relevant for
(define), but returns a value and has to be placed accordingly, like in
a function call or at the providing side of a binding construct.
If those different syntactic aspects prove to be too hard to conciliate,
it might help to look at the kind of interface that some other chimeras
like call-with-values or call-with-current-continuation have taken.
--
David Kastrup
