`in-iterate` sequence generator

in-nest-sequence.rkt

#lang racket
(require (for-syntax unstable/syntax))
(provide (rename-out [*in-nest-sequence in-nest-sequence]))

(define in-nest-sequence
  (case-lambda
    [(func init)
     (make-do-sequence
      (thunk (values identity func init #f #f #f)))]
    [(func . inits)
     (make-do-sequence
      (thunk (values (curry apply values)
                     (lambda (args)
                       (call-with-values (thunk (apply func args)) list))
                     inits #f #f #f)))]))

(define-sequence-syntax *in-nest-sequence
  (lambda () #'in-nest-sequence)
  (lambda (stx)
    (syntax-case stx ()
      [[(x ...) (_ func init ...)]
       (unless (= (syntax-length #'(x ...)) (syntax-length #'(init ...)))
         (raise-syntax-error 'in-nest-sequence
                             (format "~a values required" (syntax-length #'(x ...)))
                             stx #'(init ...)))
       (with-syntax ([for-arity (syntax-length #'(init ...))]
                     [(value ...) (generate-temporaries #'(init ...))]
                     [(y ...) (generate-temporaries #'(init ...))])
         #'[(x ...) (:do-in ([(f) func])
                            (unless (procedure-arity-includes? f for-arity)
                              (raise-arity-error f (procedure-arity f) init ...))
                            ([value init] ...)
                            #t
                            ([(x ...) (values value ...)]
                             [(y ...) (f value ...)])
                            #t
                            #t
                            (y ...))])])))