Transducer Exercises

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 This present article is no longer updated, please use the github version.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.

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]

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]

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]

Strange patterns, it reminds me of ...

The 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 :? :?]

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]

Test for both sides.

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

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]

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

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]

Step 2, hungry cat

As you can see, the transducer is keeping for itself all the fishes and the heat.

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

Step 3, sleepy cat

That version of the transducer falls asleep an the :sleep keyword and do not process any subsequent data.

(into [] my-cat cat-data)
; => [1 2 3 4 5]

Step 4, correct cat

At last, we want our transducer to respect the normal early termination (with reduced? tested on the downstream result) in a correct manner.

(into [] (comp (take 2)
               my-cat)
      cat-data)
; => [1 2 3 4]

(into [] (comp my-cat
               (take 2))
      cat-data)
; => [1 2]

(slow-into [] (comp (debug 0)
                    my-cat    ; try replacing it with `cat` and compare
                    (debug 2)
                    (take 2)
                    (debug 4))
           cat-data)
;; Outputs:
; > [1 2 :fish 3] 
;   > 1 
;     > 1 
;     < [1] 
;   < [1] 
;   > 2 
;     > 2 
;     < [1 2] 
;   < #reduced[{:status :ready, :val [1 2]} 0x517a7e8e] 
; < #reduced[{:status :ready, :val [1 2]} 0x517a7e8e]

Provide an idiomatic equivalent to my-cat.

A.D.D. transducer

Implement a transducer that daydream during a number of elements. While in the daydream state, it buffers its input. When it stops daydreaming, it processes all of its buffer as a batch, then daydreams again.

The a-d-d transducer never loses data.

(into [] (a-d-d 3) (range 10))
; => [0 1 2 3 4 5 6 7 8 9]

; Try:
(slow-into [] (comp (debug 0)
                    (a-d-d 3)
                    (debug 2))
           (range 10))

Change a-d-d so that it works similarly to clojure.core/partition-all.

(into [] (a-d-d 3) (range 10))
; => [[0 1 2] [3 4 5] [6 7 8] [9]]

Verify that it works well with early termination.

(slow-into [] (comp (debug 0)
                    (a-d-d 3)
                    (debug 2)
                    (take 2))
           (range 10))
; => [[0 1 2] [3 4 5]]

A reducer inside a transducer

Implement serieduce which provides a transducer which reduces incoming elements and emits all intermediary elements.

(into [] (serieduce conj [1 2]) (range 3 6))
; => [[1 2 3] [1 2 3 4] [1 2 3 4 5]]

(into [] (serieduce +) (range 5))
; => [0 1 3 6 10]

Congratulations if you made it that far!

You are one of a few.

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Vincent Cantin