> On Feb 20, 2019, at 8:47 AM, Kees-Jochem Wehrmeijer <henc...@gmail.com> wrote:
>
> While this scheme works, it silently breaks if a user accidentally calls
> (yield) instead of ((yield)). It might not be a big deal, but in an ideal
> world I'd only allow the 'correct' way.
You can make a new `define` form where `yield` means something different:
#lang racket
(require (except-in racket/generator yield)
(prefix-in gen: (only-in racket/generator yield))
racket/stxparam)
(define-syntax-parameter yield (λ (stx) #'"error: `yield` not parameterized"))
(define-syntax (yield* stx)
(syntax-case stx ()
[(_ . vals)
#'(call-with-values (thunk (gen:yield . vals))
(case-lambda
[() (void)]
[(v) (if (exn? v)
(raise v)
v)]
[vs vs]))]))
(define-syntax (define/ideal-world stx)
(syntax-case stx ()
[(_ ID . BODY)
#'(define ID
(syntax-parameterize ([yield (make-rename-transformer #'yield*)])
. BODY))]))
(define/ideal-world mygenerator (generator ()
(with-handlers ([exn? (lambda (e)
(yield 42))])
(yield 1)
(yield 2)
(yield 3))))
(mygenerator)
(mygenerator (exn:fail "Welp" (current-continuation-marks)))
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