From 37355f10373c7537751491cdd028606159364109 Mon Sep 17 00:00:00 2001
From: Peter McGoron <code@mcgoron.com>
Date: Sun, 22 Sep 2024 09:46:58 -0400
Subject: [PATCH] read: factor out ADD-ALL-AS-SKIP to UPDATE-LIST

---
 read.scm | 62 +++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 50 insertions(+), 12 deletions(-)

diff --git a/read.scm b/read.scm
index a8b8c50..b3c9567 100644
--- a/read.scm
+++ b/read.scm
@@ -13,6 +13,34 @@
 
 ;;; 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.
+;;; 
+;;; Printing circular structures and shared structures:
+;;; 
+;;; The only way to do so is to find a way to sort lists in a way that
+;;; respects EQ?. This is impossible in standard Scheme and also in
+;;; many implementations because of moving collectors.
+;;; 
+;;; A list could be maintained of all previous values, and each print
+;;; could check the list using EQ? to find a match, but for R7RS WRITE
+;;; and WRITE-SHARED this would be an O(N^2) operation.
 
 (load "chez-compat.scm")
 (load "util.scm")
@@ -35,6 +63,9 @@
 
 ;;; 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
@@ -163,6 +194,14 @@
                    (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
@@ -283,15 +322,6 @@
 ;;; Comments and whitespace reader
 ;;; ;;;;;;;;;;;;;;;;;;;;
 
-;;; Add all chars in TO-SKIP to TABLE as skipped characters. Used for
-;;; whitespace.
-(define readtable:add-all-as-skip
-  (lambda (table to-skip)
-    (fold (lambda (char table)
-            (readtable:update table char readtable:skip))
-          table
-          to-skip)))
-
 ;;; Readtable for a line comment.
 (define readtable:read-to-newline
   (readtable:process
@@ -321,7 +351,9 @@
       (let ((table (readtable:process
                     (readtable:empty/default (readtable:error
                                               "improper list has 1 cdr"))
-                    (list readtable:add-all-as-skip readtable:ASCII-whitespace)
+                    (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)))))
@@ -438,6 +470,10 @@
 
 ;;; Reads the next toplevel datum, discards it, and then continues at the
 ;;; toplevel.
+;;; 
+;;; TODO: The R7RS reader can cause side-effects due to #!FOLD-CASE. This
+;;; must be supressed in datum comments. A method could be added to PORT
+;;; that saves and restores mutable state (besides stream position).
 (define readtable:datum-comment
   (lambda (_ __ toplevel port)
     (readtable:next toplevel #f port)
@@ -459,7 +495,9 @@
   (lambda ()
     (readtable:process
      (readtable:empty/default readtable:read-ident)
-     (list readtable:add-all-as-skip readtable:ASCII-whitespace)
+     (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 "unbalanced list"))
@@ -506,4 +544,4 @@
 (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 "(this has a #;(call with (current continuation)) datum comment)")