in-value exaple

in-value.rkt

#lang racket/base

(define (expensive-or-complicated v)
  v)  ;not really, but for example

(for*/list ([x (in-list '(1 2 3 4))]
            [y (in-value (expensive-or-complicated x))]
            #:unless (odd? y))
  y)
;; '(2 4)