> Date: Thu, 30 Jul 2026 16:39:28 -0700
> From: "Arthur A. Gleckler" <[email protected]>
>
> I also replaced the unless with an if so that assert-flonum! would
> always return a value, although that isn't necessary.
I think you'll find it is rather necessary if you want to actually
_use_ the replacement!
> That still captures a continuation every time around, even when there is no
> error. This duplicates a tiny bit of code, but would be faster:
>
> (define (assert-flonum! who x)
> (if (flonum? x)
> x
> (call-with-current-continuation
> (lambda (escape)
> (with-restart
> 'retry-flonum
> "Prompt for a new flonum value."
> (lambda (x*) (escape (assert-flonum! who x*)))
> (lambda () (prompt-for-evaluated-expression "New value"))
> (lambda ()
> (if (flonum? x)
> x
> (error:wrong-type-argument x "flonum" who))))))))
Here's a further likely improvement in performance (caveat: untested):
(define-integrable (assert-flonum! who x)
(let loop ((x x))
(if (flonum? x)
x
(loop (restartable-error:not-flonum who x)))))
(define (restartable-error:not-flonum who x)
(call-with-current-continuation
(lambda (escape)
(with-restart 'retry-flonum "Prompt for a new flonum value."
escape
(lambda () (prompt-for-evaluated-expression "New value"))
(lambda ()
(error:wrong-type-argument x "flonum" who))))))
This way, the type check can be done in-line in callers, while the
error path is out-of-line.
> But take all of this with a grain of salt because I'm a novice at using the
> restart system.
In practice, I haven't found restarts to be particularly useful -- I
don't think I have ever actually used them except while playing around
to see how they work. Generally, I find there are two better
approaches:
1. making a computation cheap to reproduce from all its inputs so you
can start it over from the beginning with one input tweaked, and/or
2. making a costly computation checkpoint its state so you can easily
roll back to a checkpoint and pick up where it left off -- either
because of a bug, or because you got bored and hit ^G, or because
of a power outage, or because you wanted to move to a faster
machine without losing your progress.
