UNSLISP/read.scm

;;; Copyright (C) Peter McGoron 2024
;;; This program is free software: you can redistribute it and/or modify
;;; it under the terms of the GNU General Public License as published by
;;; the Free Software Foundation, version 3 of the License.
;;; 
;;; This program is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;; 
;;; You should have received a copy of the GNU General Public License
;;; along with this program.  If not, see <https://www.gnu.org/licenses/>.

;;; R7RS reader. This is the lexer-parser end, so it returns tokens and
;;; not concrete objects.
;;; 
;;; Notes:
;;; 
;;; Port stores datum labels. Datum labels are stored for the entirety of
;;; a READ: this is to emulate MIT Scheme, which allows for datum labels
;;; outside of the datum that the label appears in.
;;; 
;;; The reader does not return Scheme data: it returns annotated data
;;; containing the source location, datum label number, resolved datum
;;; label pointer. This is for advanced syntax systems.
;;; 
;;; How datum labels could work:
;;; 
;;; When encountering #[number]=, allocate a datum label and assign it
;;; nothing. Then call READ after "=", and destructively update the
;;; datum label with the resulting datum. A pass over the new read
;;; structure to convert it to regular Scheme data will resolve the
;;; indirection.
;;; 
;;; All tokens are procedure-encapsulated objects, since the reader should
;;; never return a literal procedure. Each procedure has a TYPE message.

(load "chez-compat.scm")
(load "util.scm")
(load "set.scm")
(load "linked-list.scm")

;;; My text editor cannot parse Scheme's character syntax.

(define %bol #\()
(define %eol #\))

;;; ;;;;;;;;;;;;;;;;;;;;;;;;
;;; Port reader wrapper
;;; ;;;;;;;;;;;;;;;;;;;;;;;;

(define port->read-function
  (lambda (port)
    (lambda ()
      (read-char port))))

;;; READ:
;;; 
;;; Stream readers contain mutable state. This is the case-folding mode
;;; and the current list of datum labels.
;;; 
;;; (POS): Return (LIST FILENAME LINE-NUMBER OFFSET).
;;; (READ): Read the next character in the stream. Returns #F on EOF.
;;; (PUSH CHAR): Push CHAR such that it will be the next character read
;;; when (READ) is called.
;;; (PEEK): Read character, push it back, and return it.
;;; (FOLD-CASE?): Returns a boolean if case folding is enabled.
;;; (FOLD-CASE! BOOL): Sets the case folding to BOOL.
(define port->read
  (lambda (read-function filename)
    (let ((line-number 1)
          (offset 0)
          (pushback-buffer '())
          (datum-labels '())
          (fold-case? #f))
      (letrec ((update-position!
                (lambda (ch)
                  (cond
                    ((eqv? ch #\newline)
                     (set! line-number (+ 1 line-number)) (set! offset 0))
                    ;; OFFSET is sometimes set to #F to denote an unknown
                    ;; offset.
                    (offset (set! offset (+ 1 offset))))))
               (location
                (lambda () (list filename line-number offset)))
               (set-datum-label!
                (lambda (label value)
                  (set! datum-labels
                        (car (smap:insert datum-labels label value)))))
               (get-datum-label
                (lambda (label)
                  (smap:search datum-labels label)))
               (clear-datum-labels!
                (lambda ()
                  (set! datum-labels '())))
               (dump-mutable
                (lambda ()
                  (list datum-labels fold-case?)))
               (restore-mutable!
                (lambda (state)
                  (set! datum-labels (car state))
                  (set! fold-case? (cadr state))))
               (process
                (lambda (ch)
                  (update-position! ch)
                  (cond
                    ((or (eof-object? ch) (not ch)) ch)
                    (fold-case? (char-downcase ch))
                    (else ch))))
               (port
                (lambda (op . args)
                  ;; TODO: turn into string map?
                  (cond
                    ((eq? op 'location) (location))
                    ((eq? op 'read)
                     (process
                      (if (null? pushback-buffer)
                          (read-function)
                          (let ((ch (car pushback-buffer)))
                            (set! pushback-buffer (cdr pushback-buffer))
                            ch))))
                    ((eq? op 'peek)
                     (let ((ch (port 'read)))
                       (port 'push ch)
                       ch))
                    ((eq? op 'push)
                     (let ((ch (car args)))
                       (if (eqv? ch #\newline)
                           (begin
                             (set! line-number (- line-number 1))
                             (set! offset #f))
                           (set! offset (- offset 1)))
                       (set! pushback-buffer (cons ch pushback-buffer))))
                    ((eq? op 'fold-case?) fold-case?)
                    ((eq? op 'fold-case!) (set! fold-case? (car args)))
                    ((eq? op 'set-datum-label!) (apply set-datum-label! args))
                    ((eq? op 'get-datum-label) (apply get-datum-label args))
                    ((eq? op 'clear-datum-labels!) (apply clear-datum-labels! args))
                    ((eq? op 'dump-mutable) (apply dump-mutable args))
                    ((eq? op 'restore-mutable!) (apply restore-mutable! args))
                    (else (error 'read->port 'invalid (cons op args)))))))
        port))))

;;; ;;;;;;;;;;;;;;
;;; Character maps
;;; ;;;;;;;;;;;;;;

(define integer<=>
  (lambda (x y)
    (cond
      ((< x y) '<)
      ((= x y) '=)
      (else '>))))

;;; Comparison on characters extended to #F, which is less than all
;;; characters.
(define char*<=>
  (lambda (x y)
    (cond
      ((and (not x) y) '<)
      ((and x (not y)) '>)
      ((and (not x) (not y) '=))
      (else (integer<=> (char->integer x)
                        (char->integer y))))))

(define %charmap:<=> (set:<=>-to-map char*<=>))
(define %charmap:update (set:update %charmap:<=>))

(define charmap:update (map:update %charmap:update))
(define charmap:insert (map:insert %charmap:update))
(define charmap:search (map:search %charmap:<=>))

;;; ;;;;;;;;;;;;;;;;;;;;;;
;;; Readtable constructors
;;; 
;;; Readtables are composed of a CHARMAP, which is a map from characters
;;; to actions, and a DEFAULT-ACTION, which is taken when there is no
;;; match in CHARMAP.
;;; 
;;; An "action" is a procedure that takes four arguments:
;;; 
;;; TABLE: The current table.
;;; CHAR: The character that was matched against the CHARMAP in TABLE.
;;; ACC: An arbitrary "accumulator" value that is different depending
;;; on the readtable in question.
;;; PORT: A port reader object.
;;; ;;;;;;;;;;;;;;;;;;;;;;

;;; (READTABLE:NEW DEFAULT-ACTION CHARMAP)
(define readtable:new cons)

(define %readtable:default-action car)
(define %readtable:charmap cdr)

;;; Run the action in TABLE assigned to CHAR, or the default action of
;;; TABLE if there is no entry for CHAR.
(define readtable:act
  (lambda (table char acc port)
    (let ((node (charmap:search (%readtable:charmap table)
                                char)))
      (let ((action (if (null? node)
                        (%readtable:default-action table)
                        (map:val node))))
        (action table char acc port)))))

;;; Run the action in TABLE with the next character from PORT.
(define readtable:next
  (lambda (table acc port)
    (readtable:act table (port 'read) acc port)))

;;; Return a new readtable where CHAR is bound to ACTION.
(define readtable:update
  (lambda (table char action)
    (readtable:new (%readtable:default-action table)
                   (car (charmap:insert
                         (%readtable:charmap table) char action)))))

;;; Update TABLE to act on all characters in LST with ACTION.
(define readtable:update-list
  (lambda (table lst action)
    (fold (lambda (char table)
            (readtable:update table char action))
          table
          lst)))

;;; Construct new readtable with no characters in its map and
;;; DEFAULT-ACTION as the default action.
(define readtable:empty/default
  (lambda (default-action)
    (readtable:new default-action '())))

;;; Each value in FUNCTIONS is a list (PROCEDURE ARGS...) which is called
;;; like (PROCEDURE TABLE ARGS...) and returns a table.
(define readtable:process
  (lambda (table . functions)
    (fold (lambda (function table)
            (apply (car function) table (cdr function)))
          table
          functions)))

;;; ;;;;;;;;;;;;;;;;;;
;;; Default actions
;;; ;;;;;;;;;;;;;;;;;;

;;; Return an error.
(define readtable:error
  (lambda emsg
    (lambda (table char acc port)
      (error emsg char acc table port))))

;;; Discard the current character and continue reading the readtable.
(define readtable:skip
  (lambda (table char acc port)
    (readtable:act table (port 'read) acc port)))

;;; Discard char and return constant.
(define readtable:return
  (lambda (return)
    (lambda (table char acc port)
      return)))

;;; Jump to a new readtable, discard it's return, and continue reading
;;; in the table.
(define readtable:jump-discard
  (lambda (newtable)
    (lambda (oldtable char acc port)
      (readtable:act newtable (port 'read) '() port)
      (readtable:act oldtable (port 'read) acc port))))

;;; Jump to a new readtable with the same characters.
(define readtable:jump
  (lambda (newtable)
    (lambda (oldtable char acc port)
      (readtable:act newtable char acc port))))

;;; Jump to a new readtable, reading the new character, with the old
;;; readtable as ACC.
(define readtable:next/old-as-acc
  (lambda (newtable)
    (lambda (oldtable __ _ port)
      (readtable:next newtable oldtable port))))

;;; Jump to a new readtable, reading the new character.
(define readtable:jump/next
  (lambda (newtable)
    (lambda (oldtable _ acc port)
      (readtable:next newtable acc port))))

;;; ;;;;;;;;;;;;;;;;;
;;; Identifier reader
;;; ;;;;;;;;;;;;;;;;;

(define read:ident
  (lambda (name location)
    (lambda (op . args)
      (cond
        ((eq? op 'type) 'ident)
        ((eq? op 'value) name)
        (else (error 'read:ident "invalid operation" op args))))))

(define read:ident-builder
  (lambda (location . start-char)
    (let ((char-list (linked-list:new)))
      (if (not (null? start-char))
          (char-list 'push (car start-char)))
      (lambda (op . args)
        (cond
          ((eq? op 'finalize->ident)
           (read:ident (list->string (char-list 'to-list)) location))
          (else (apply char-list op args)))))))

;;; Push back CHAR and return ACC.
(define readtable:return-acc-as-ident
  (lambda (table char acc port)
    (port 'push char)
    (acc 'finalize->ident)))

;;; Push CHAR to ACC and continue reading from TABLE.
(define readtable:push-char
  (lambda (table char acc port)
    (acc 'push-tail char)
    (readtable:act table (port 'read) acc port)))

;;; Define a readtable that constructs an identifier by accepting all
;;; characters that are not listed.
(define readtable:exclude-from-identifiers
  (lambda (table excluded)
    (fold (lambda (char table)
            (readtable:update table char readtable:return-acc-as-ident))
          table
          excluded)))

;;; ASCII whitespace.
(define readtable:ASCII-whitespace
  (list #\newline
        #\space
        (integer->char #x09)
        (integer->char #x0B)
        (integer->char #x0C)
        (integer->char #x0D)))

;;; Readtable for identifiers.
(define readtable:identifier
  (readtable:process
   (readtable:empty/default readtable:push-char)
   (list readtable:exclude-from-identifiers
         readtable:ASCII-whitespace)
   (list readtable:exclude-from-identifiers
         (list #\| %bol %eol #\' #\; #f))))

;;; Read an identifier starting with CHAR.
(define readtable:read-ident
  (lambda (table char acc port)
    (readtable:act readtable:identifier
                   (port 'read)
                   (read:ident-builder (port 'location)
                                       char)
                   port)))

;;; ;;;;;;;;;;;;;;;;;;;;
;;; Comments and whitespace reader
;;; ;;;;;;;;;;;;;;;;;;;;

;;; Readtable for a line comment.
(define readtable:read-to-newline
  (readtable:process
   (readtable:empty/default readtable:skip)
   (list readtable:update #\newline (readtable:return #f))))

;;; ;;;;;;;;;;;
;;; List reader
;;; 
;;; The reader updates the previous readtable to handle ). This means
;;; that this read table does not have to handle end-of-line, whitespace,
;;; etc.
;;; ;;;;;;;;;;;

;;; Read the end of an improper list.
(define readtable:read-improper-cdr
  (lambda (table acc port)
    (let ((val
           (readtable:act (readtable:update table
                                            %eol
                                            (readtable:error
                                             'read-improper-cdr
                                             "proper list must have cdr"))
                          (port 'read)
                          #f
                          port)))
      (acc 'set-cdr! val)
      (let ((table (readtable:process
                    (readtable:empty/default (readtable:error
                                              'read-improper-cdr
                                              "improper list has 1 cdr"))
                    (list readtable:update-list
                          readtable:ASCII-whitespace
                          readtable:skip)
                    (list readtable:update %eol
                          (lambda dummy 'end-of-list)))))
        (readtable:act table (port 'read) acc port)))))

;;; Generic reader loop for a list. It takes as input the table that has
;;; already been updated with end of list and improper list handlers.
(define readtable:read-list-loop
  (lambda (table port)
    (let ((acc (linked-list:new)))
      (letrec ((loop
                (lambda ()
                  (let ((value (readtable:act table
                                              (port 'read)
                                              acc
                                              port)))
                    (cond
                      ((eqv? value 'end-of-list) (acc 'to-list))
                      (else (acc 'push-tail value)
                            (loop)))))))
        (loop)))))

;;; Readtable for a list, generic to proper and improper list
;;; readers.
(define readtable:table-for-list
  (lambda (oldtable on-dot)
    (readtable:process
     oldtable
     (list readtable:update %eol (readtable:return 'end-of-list))
     (list readtable:update #\.
           (lambda (table char acc port)
             (let ((entire-identifier (readtable:read-ident
                                       table
                                       char
                                       #f
                                       port)))
               (if (equal? entire-identifier ".")
                   (on-dot table acc port)
                   entire-identifier)))))))

;;; Read a proper or improper list.
(define readtable:read-list
  (lambda (oldtable _ __ port)
    (readtable:read-list-loop (readtable:table-for-list
                               oldtable
                               readtable:read-improper-cdr)
                              port)))

;;; Read strictly a proper list. This assumes that BOL has been read.
(define readtable:read-proper-list
  (lambda (table port)
    (readtable:read-list-loop (readtable:table-for-list
                               table
                               (readtable:error
                                'read-proper-list
                                "expected proper list"))
                              port)))

(define readtable:expect-proper-list
  (lambda (table port)
    (if (not (eqv? (port 'read) %bol))
        (error 'expect-port-list 'no-list-found)
        (readtable:read-proper-list table port))))

;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Reader for stuff that start with "#"
;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define readtable:digits
  (list #\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9))

(define readtable:vector
  (lambda (_ __ toplevel port)
    (list 'vector (readtable:read-proper-list toplevel port))))

;;; Block comment reader.
;;; 
;;; The outermost block comment reader is passed the toplevel reader as
;;; ACC. When the outermost block is finished, it will tail-call ACC.
;;; (It is basically the continuation of the reader.)
;;; 
;;; When a nested block comment is found, it is passed #F as ACC, which
;;; it will not call. It will return an unspecified value.
;;; 
;;; Since the read tables are not procedures, references to other tables
;;; in the same LETREC declaration must be protected with explicit LAMBDAs.
;;; Macros could make this much easier to read.
(define readtable:block-comment
  (letrec ((potential-end
            (readtable:process
             (readtable:empty/default
              (lambda (this char acc port) (readtable:act
                                            loop
                                            char
                                            acc
                                            port)))
             (list readtable:update #\#
                   (lambda (this char acc port)
                     (if acc
                         (readtable:next acc #f port))))))
           (potential-start
            (readtable:process
             (readtable:empty/default
              (lambda (this char acc port) (readtable:act
                                            loop
                                            char
                                            acc
                                            port)))
             (list readtable:update #\|
                   (lambda (this char acc port)
                     (readtable:next loop #f port)
                     (readtable:next loop acc port)))))
           (loop
            (readtable:process
             (readtable:empty/default readtable:skip)
             (list readtable:update #\#
                   (lambda (this char acc port)
                     (readtable:next potential-start
                                     acc
                                     port)))
             (list readtable:update #\|
                   (lambda (this char acc port)
                     (readtable:next potential-end
                                     acc
                                     port))))))
    loop))

(define read:datum-label
  (lambda (location toplevel)
    (let ((finalized? #f)
          (value '())
          (name (read:ident-builder location)))
      (lambda (op . args)
        (cond
          ((eq? op 'finalize-value)
           (if finalized?
               (error 'datum-label "cannot finalize twice"))
           (set! finalized? #t)
           (set! value (car args))
           (set! toplevel #f))
          ((eq? op 'finalize->ident)
           (set! name (name 'finalize->ident)))
          ((eq? op 'value)
           (if (not finalized?)
               (error 'datum-label "value called before finalize"))
           value)
          ((eq? op 'toplevel) toplevel)
          ((eq? op 'type) 'datum-label)
          ((eq? op 'as-string) (name 'value))
          (else (apply name op args)))))))

;;; Readtable for the number part of a datum label / reference. A label
;;; looks like "#[NUMBER]=" and a reference looks like "#[NUMBER]#".
;;; 
;;; Datum label assignments #XXX=[DATUM] return DATUM.
;;; Datum label references #XXX# return a DATUM-LABEL object, because
;;; circular references must be resolved later. (With some trickery it
;;; cold be resolved as soon as possible, but that requires knowledge of
;;; what is complex data and what is an atom. This requires knowing all
;;; possible values that the toplevel could read, which is not possible
;;; because new items can be added to the toplevel.)
(define readtable:datum-label-next
  (readtable:process
   (readtable:empty/default (readtable:error 'datum-label-next
                                             "invalid datum label/ref"))
   (list readtable:update-list readtable:digits readtable:push-char)
   (list readtable:update #\=
         (lambda (_ __ acc port)
           (acc 'finalize->ident)
           (port 'set-datum-label! (acc 'as-string) acc)
           (let ((next-value (readtable:next (acc 'toplevel)
                                             #f
                                             port)))
             (if (eqv? acc next-value)
                 (error 'datum-label-next "datum label cannot be itself"))
             (acc 'finalize-value next-value)
             (acc 'value))))
   (list readtable:update #\#
         (lambda (_ __ acc port)
           (acc 'finalize->ident)
           (let ((datum-label-container (port 'get-datum-label
                                              (acc 'as-string))))
             (if (null? datum-label-container)
                 (error 'datum-label-next
                        "unknown reference to datum label" label)
                 (map:val datum-label-container)))))))

;;; Reads the next toplevel datum, discards it, and then continues at the
;;; toplevel.
;;; 
;;; The R7RS reader can cause side-effects due to #!FOLD-CASE and datum
;;; labels. This must be supressed in datum comments.
(define readtable:datum-comment
  (lambda (_ __ toplevel port)
    (let ((mutable (port 'dump-mutable)))
      (readtable:next toplevel #f port)
      (port 'restore-mutable! mutable))
    (readtable:next toplevel #f port)))

;;; ;;;;;;;;;;;;;;;;;;;;;;;;
;;; Handling hash messages
;;; 
;;; A "hash message" is any identifier that immediately follows a "#".
;;; This can be a constant, a part of a constant (like for numbers), or
;;; a directive (like #!FOLD-CASE).

;;; String map from constants to procedures with formal arguments
;;; (TOPLEVEL PORT)
;;; with TOPLEVEL being the current toplevel table and PORT being the
;;; current port being read from.
;;; 
;;; Each one must return something. Directives that return nothing must
;;; call the toplevel again.
(define read:hash-messages
  (smap:insert-many
   '()
   (cons "true" (lambda unused #t))
   (cons "false" (lambda unused #f))
   (cons "t" (lambda unused #t))
   (cons "f" (lambda unused #f))
   (cons "u8" (lambda (toplevel port)
                (list 'bytevector
                      (readtable:expect-proper-list toplevel port))))
   (cons "!fold-case"
         (lambda (toplevel port)
           (port 'fold-case! #t)
           (readtable:act toplevel (port 'read) #f port)))
   (cons "!no-fold-case"
         (lambda (toplevel port)
           (port 'fold-case! #f)
           (readtable:act toplevel (port 'read) #f port)))))

(define read:read-hash-messages
  (lambda (_ char acc port)
    (let ((ident (readtable:read-ident #f char #f port)))
      (let ((container (smap:search read:hash-messages
                                    (ident 'value))))
        ;; TODO: check if the hash message starts a number.
        (if (null? container)
            (error 'read-hash-messages 'unknown-constant
                   (ident 'value))
            ((map:val container) acc port))))))

;;; ;;;;;;;;;;;;;;;;;;;;;;
;;; Characters
;;; 
;;; For completeness, this adds a few more newline characters that are C
;;; escape sequences but are not standard Scheme.

(define read:named-characters
  (smap:insert-many
   '()
   (cons "null" (integer->char 0))
   (cons "alarm" (integer->char #x7))
   (cons "backspace" (integer->char #x8))
   (cons "tab" (integer->char #x9))
   (cons "newline" (integer->char #xA))
   (cons "vertical-tab" (integer->char #xB))
   (cons "form-feed" (integer->char #xC))
   (cons "return" (integer->char #xD))
   (cons "escape" (integer->char #x1B))
   (cons "space" (integer->char #x20))
   (cons "delete" (integer->char #x7F))))

(define readtable:character
  (lambda (_ char __ port)
    (let ((char (port 'read)))
      (let ((ident (readtable:read-ident #f char #f port)))
        (let ((container (smap:search read:named-characters
                                    (ident 'value))))
          (display (ident 'value))
          (newline)
          (cond
            ((and (null? container)
                (= (string-length (ident 'value)) 1))
             (string-ref (ident 'value) 0))
            ((null? container) (error 'character 'unknown-character ident))
            (else (map:val container))))))))

(define readtable:hash
  (readtable:process
   (readtable:empty/default read:read-hash-messages)
   (list readtable:update-list readtable:ASCII-whitespace
         (readtable:error 'hash 'whitespace-after-hash))
   (list readtable:update #\| (readtable:jump/next readtable:block-comment))
   (list readtable:update #\; readtable:datum-comment)
   (list readtable:update #\\ readtable:character)
   (list readtable:update-list readtable:digits ; Datum labels
         (lambda (_ char toplevel port)
           (readtable:act readtable:datum-label-next
                          char
                          (read:datum-label (port 'location) toplevel)
                          port)))
   (list readtable:update %bol readtable:vector)))

;;; ;;;;;;;;;;;;;;;;
;;; Toplevel reader.
;;; ;;;;;;;;;;;;;;;;
;;; This is defined as a function so that it dynamically loads each
;;; sub-readtable.
(define readtable:top
  (lambda ()
    (readtable:process
     (readtable:empty/default readtable:read-ident)
     (list readtable:update-list readtable:ASCII-whitespace readtable:skip)
     (list readtable:update #f (readtable:return 'eof))
     (list readtable:update %bol readtable:read-list)
     (list readtable:update %eol (readtable:error 'top "unbalanced list"))
     (list readtable:update #\# (readtable:next/old-as-acc
                                 readtable:hash))
     (list readtable:update #\;
           (readtable:jump-discard readtable:read-to-newline)))))

(define read/toplevel
  (lambda (port)
    ;; Does each invocation of READ keep datum labels from previous reads?
    ;; (port 'clear-datum-labels!)
    (readtable:next (readtable:top)
                    #f
                    port)))

(define intset:insert (set:insert (set:update integer<=>)))
(define intset:in (set:in integer<=>))

(define uncycle
  (lambda (value)
    (let ((cntr 0)
          (used-counters '())
          (pointers '()))
      (letrec ((uncycle
                (lambda (value)
                  (cond
                    ((pair? value)
                     (let ((pair (assq value pointers)))
                       (if (pair? pair)
                           (begin
                             (set! used-counters
                                   (car (intset:insert used-counters (cdr pair))))
                             (list 'ref (cdr pair)))
                           (begin
                             (set! pointers (cons (cons value cntr)
                                                  pointers))
                             (let ((cur-cntr cntr))
                               (set! cntr (+ 1 cntr))
                               (let ((returned (cons (uncycle (car value))
                                                     (uncycle (cdr value)))))
                                 (if (not (null? (intset:in used-counters cur-cntr)))
                                     (list 'def cur-cntr '= returned)
                                     returned)))))))
                    ((procedure? value)
                     (let ((type (value 'type)))
                       (cond
                         ((eq? type 'ident) (value 'value))
                         ((eq? type 'datum-label) (uncycle (value 'value)))
                         (else (vector 'unrepresentable type)))))
                    (else value)))))
        (uncycle value)))))

;;; ;;;;;;;;;;;
;;; Test reader
;;; ;;;;;;;;;;;
(define %list->read
  (lambda (seq)
    (port->read
     (lambda ()
       (if (null? seq)
           #f
           (let ((ch (car seq)))
             (set! seq (cdr seq))
             ch)))
     "test")))

(define read-all
  (lambda (str)
    (let ((reader (%list->read (string->list str))))
      (letrec ((loop
                (lambda ()
                  (if (not (reader 'peek))
                      #t
                      (let ((value (read/toplevel reader)))
                        (display (list "return" (uncycle value)))
                        (newline)
                        (loop))))))
        (loop)))))

(read-all "x yy zz ; this is a comment\nx call/cc ")
(read-all "(a   b    c     def (ghi j)  k    )")
(read-all "(      a . b   )")
(read-all "(   a .b . c)")
(read-all "#( a b y)")
(read-all "(x y #| this is a block\n comment\n |# z w)")
(read-all "#( a b #| this is a #| nested block |# comment|# z w)")
(read-all "#(a b #(c #|close#|comment|#|#y))")
(read-all "(this has a #;(call with (current continuation)) datum comment)")
(read-all "#0=(#0# not unix)")
(read-all "#!no-fold-case #!fold-case #!NO-FOLD-CASE #false")
(read-all "#!fold-case #TRUE")
(read-all "#u8(x y z w)")
(read-all "#\\newline")
(read-all "#\\a")