chickadee » transducers » inspect

A transducer that "inspects" a value by applying a procedure f to the value and then forwarding the original value on. This is primarily useful for debugging but can also be used for side-effects.

 
(import transducers)

(transduce list-fold
           (compose
             (map add1)
             (inspect print)
             (map (lambda (x) (* 3 x)))
             (inspect print))
           (collect-list)
           (list 2 4 1 3 5 9))

; 3
; 9
; 5
; 15
; 2
; 6
; 4
; 8
; 6
; 18
; 10
; 30
; => (9 15 6 8 18 30)