Skip to content
Search

DEV Community 👩‍💻👨‍💻

Vincent Cantin
Vincent Cantin

Posted on • Updated on

Transducer Exercises

#clojure #intermediate

Contains exercises on implementing custom transducers in Clojure.

UPDATE: All the solutions are now posted on Github. This present article is no longer updated, please use the github version.

Fair notice

Implementing transducers is hard and you may feel confused.

Cry Kid Meme

That's normal and to be expected, it happens also to me when I read my own code. Just don't give up.

No Operation transducer

Implement a transducer that does nothing but passing the data from his input to his output. Functionally speaking, it is an identity transducer.

(into [] nop (range 3))
; => [0 1 2]
Enter fullscreen mode Exit fullscreen mode

Use it as your transducer template for the following.

Prepare for battle

  • Implement the (debug in out) transducer that helps to debug. 
; We use this function instead of `into` for debugging.
; The reason is that this avoids using transient
; structures which do not `print` nicely.
(defn slow-into [to xf from]
  (transduce xf conj to from))

(slow-into [] (debug "in" "out") (range 3))
;; Outputs:
; in 0 
; out [0] 
; in 1 
; out [0 1] 
; in 2 
; out [0 1 2]
Enter fullscreen mode Exit fullscreen mode
  • Add the (debug), (debug indent) and (debug indent in out) variants for convenience. They are all calling the (debug in out) transducer under the hood. 
(slow-into []
           (comp (debug)
                 (debug 2)
                 (debug 4 ">" "<"))
                 (debug "      >" "      <")) ; 6-spaces prefix
           (range 3))
;; Outputs:
; > 0 
;   > 0 
;     > 0 
;       > 0 
;       < [0] 
;     < [0] 
;   < [0] 
; < [0] 
; > 1 
;   > 1 
;     > 1 
;       > 1 
;       < [0 1] 
;     < [0 1] 
;   < [0 1] 
; < [0 1] 
; > 2 
;   > 2 
;     > 2 
;       > 2 
;       < [0 1 2] 
;     < [0 1 2] 
;   < [0 1 2] 
; < [0 1 2]
Enter fullscreen mode Exit fullscreen mode

Strange patterns, it reminds me of ...

The PHP Hadouken !!!

PHP Hadouken

We are now ready to face real transducer implementations and confront an army of problems.

May I beg your pardon?

You heard me well, I want you to implement the following transducer.

(def beg-data (list :may :i :beg :your :pardon :?))

(into [] (beg 2) beg-data)
; => [:may :may :i :i :beg :beg :your :your :pardon :pardon :? :?]
Enter fullscreen mode Exit fullscreen mode

Make sure that you are handling the early termination as well.

(into []
      (comp (take 3)
            (beg 2))
      beg-data)
; => [:may :may :i :i :beg :beg]
Enter fullscreen mode Exit fullscreen mode

Test for both sides.

(into []
      (comp (beg 2)
            (take 3))
      beg-data)
; => [:may :may :i]
Enter fullscreen mode Exit fullscreen mode

Test with the debug transducer (expect problems and losing some hair).

(slow-into []
           (comp (debug 0)
                 (beg 2)
                 (debug 2)
                 (take 3)
                 (debug 4))
           beg-data)
; Output:
; > :may 
;   > :may 
;     > :may 
;     < [:may] 
;   < [:may] 
;   > :may 
;     > :may 
;     < [:may :may] 
;   < [:may :may] 
; < [:may :may] 
; > :i 
;   > :i 
;     > :i 
;     < [:may :may :i] 
;   < #reduced[{:status :ready, :val [:may :may :i]} 0x8cdcdd1] 
; < #reduced[{:status :ready, :val [:may :may :i]} 0x8cdcdd1] 

; Result:
; => [:may :may :i]
Enter fullscreen mode Exit fullscreen mode

The beg transducer should not continue sending data downstream after it receives a reduced result. Fix your implementation if needed.

All your data are belong to me

Catducer

Implement the my-cat transducer. For each step, you will need to adapt your implementation to the new requirements described by the test samples.

  • Step 1, shapeless cat

It functions similarly to clojure.core/cat. Don't handle early termination at the moment.

(def cat-data [[1 2 :fish 3] [:heat 4] [5 :sleep 6] [7]])

(into [] my-cat cat-data)
; => [1 2 :fish 3 :heat 4 5 :sleep 6 7]