;;; CHICKEN Transducers - Transducers for working with foldable data types.
;;;
;;; Copyright (c) 2022 Jeremy Steward
;;;
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;;; of this software and associated documentation files (the "Software"), to deal
;;; in the Software without restriction, including without limitation the rights
;;; to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
;;; copies of the Software, and to permit persons to whom the Software is
;;; furnished to do so, subject to the following conditions:
;;;
;;; The above copyright notice and this permission notice shall be included in all
;;; copies or substantial portions of the Software.
;;;
;;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
;;; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
;;; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
;;; AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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;;; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
;;; SOFTWARE.

(module (transducers ports) ()
  (import (scheme))
  (import (chicken base)
          (chicken module)
          (check-errors)
          (transducers base))

  (export reader-fold
          collect-writer
          chain-reader
          interleave-reader
          zip-reader)

  ;; A reducer that aggregates the transduction into a new list in the order of
  ;; the items reduced.
  (define (collect-writer writer)
    (case-lambda
      (() (void))
      ((result) result)
      ((result item) (writer item))))

  ;; Transducer-aware folding operation over ports given a reader procedure.
  ;;
  ;; This procedure takes in a reducer `f`, a `sentinel` value for that
  ;; reducer, and a reader `reader` to call `(f sentinel x)` on repeatedly,
  ;; taking the result of the reducer as the new sentinel value each time.
  (define (reader-fold f sentinel reader)
    (check-procedure 'reader-fold reader 'reader)
    (let loop ((collector sentinel)
               (x (reader)))
      (if (eof-object? x)
        collector
        (let ((result (f collector x)))
          (if (reduced? result)
            (unwrap result)
            (loop result (reader)))))))

  ;; A transducer that chains another reader onto the current transduction
  (define-chain-transducer chain-reader reader-fold)

  ;; A transducer that interleaves the contents of a list
  (define-interleave-transducer
    interleave-reader
    (lambda (reader) (reader))
    (lambda (_ s) (eof-object? s))
    (lambda (_ s) s)
    (lambda (reader _) (reader)))

  ;; A transducer that zips the contents of a list
  (define-zip-transducer
    zip-reader
    (lambda (reader) (reader))
    (lambda (_ s) (eof-object? s))
    (lambda (_ s) s)
    (lambda (reader _) (reader)))

  ;; End-of-module
  )