Re: [racket-users] [ANN] Transducers library

[William G Hatch]

On Wed, Oct 23, 2019 at 12:39:43PM -0700, Jack Firth wrote:

and it uses *no macros at all*:


Gotcha!  You're using the `define` and `#%app` macros in this example,
aren't you!

More seriously, ever since I watched a Rich Hickey talk about
transducers I've wanted Racket to have them.  Thanks for making this;
I'm excited to use it.

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[racket-users] [ANN] Transducers library

[Jack Firth]

Rebellion  now provides
the rebellion/streaming/transducer
 library for
processing streams of data using transducers.

A *transducer* is a kind of sequence transformation that comes with
guarantees about memory usage and about how much of the sequence is
consumed in between each produced output. Transducers are first class
objects that chain together fluently, without any macros involved. For
example, this code defines a function that finds and reports all of the
lines in a string that are longer than 80 characters and it uses *no macros
at all*:


(require rebellion/streaming/reducer
 rebellion/streaming/transducer)

(define (report-all-long-lines str)
  (transduce str
 (batching into-line)
 enumerating
 (filtering long-line?)
 #:into (into-for-each report-long-line)))

(define (long-line? e)
  (define line (enumerated-element e))
  (> (string-length line) 80))

(define (report-long-line e)
  (define line (enumerated-element e))
  (define number (enumerated-position e))
  (printf "Line ~a is longer than 80 characters: ~a\n" number line))


You may find transducers more useful than for-style loops in the following
cases:

   - The for/xxx form you need doesn't exist
   - You want to sort
   

   elements or search for the N largest or N smallest elements
   - You're reading data from external sources and want control over when
   resources are acquired and released
   - You're processing a stream that's too large to fit into memory
   - You care a lot about processing your streams in only one pass
   - You want to combine elements into batches
   

   or group
   

   them by key
   - You want to remove duplicate
   

   elements
   - You prefer writing sequence transformations in a point-free style
   - You already like reducers
   
   - You've got a rebellious streak

For some real-world examples of transducers in action, I cannot thank Sam
Phillips enough for putting together this gist

containing scripts he wrote for his day job using transducers.

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