From 2e43f36471d0484c92354f971f3b44266190b7a5 Mon Sep 17 00:00:00 2001
From: Peter McGoron <code@mcgoron.com>
Date: Sun, 22 Sep 2024 00:46:41 -0400
Subject: [PATCH] read: block comments

---
 read.scm | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 76 insertions(+), 2 deletions(-)

diff --git a/read.scm b/read.scm
index d9ea7f7..8d68093 100644
--- a/read.scm
+++ b/read.scm
@@ -120,6 +120,18 @@
 
 ;;; ;;;;;;;;;;;;;;;;;;;;;;
 ;;; 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)
@@ -167,7 +179,7 @@
           functions)))
 
 ;;; ;;;;;;;;;;;;;;;;;;
-;;; Default readtables
+;;; Default actions
 ;;; ;;;;;;;;;;;;;;;;;;
 
 ;;; Return an error.
@@ -208,6 +220,12 @@
     (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
 ;;; ;;;;;;;;;;;;;;;;;
@@ -359,16 +377,69 @@
                               port)))
 
 ;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;;; Reader for datum that start with "#"
+;;; Reader for stuff that start with "#"
 ;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (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 readtable:hash
   (readtable:process
    (readtable:empty/default (readtable:error "unimplemented"))
+   (list readtable:update #\| (readtable:jump/next readtable:block-comment))
    (list readtable:update %bol readtable:vector)))
 
 ;;; ;;;;;;;;;;;;;;;;
@@ -424,3 +495,6 @@
 (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))")