From 474fc238647205566e40ae7205950e5106338cb7 Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Fri, 18 Jan 2019 18:04:03 -0800
Subject: [PATCH] the break

---
 quad/qtest/markdown.rkt | 20 ++++++--------------
 quad/quad/atomize.rkt   | 17 +++++++++++------
 quad/quad/quad.rkt      |  3 ---
 3 files changed, 17 insertions(+), 23 deletions(-)

diff --git a/quad/qtest/markdown.rkt b/quad/qtest/markdown.rkt
index 052a9a31..89818815 100644
--- a/quad/qtest/markdown.rkt
+++ b/quad/qtest/markdown.rkt
@@ -145,19 +145,11 @@
                                                   (pt-y (size (car pcs)))))]))
     (values (cons new-run runs) rest)))
 
-
-(struct line-break quad () #:transparent)
-(define (line-break-copy x as es)
-  (struct-copy line-break x
-               [attrs #:parent quad as]
-               [elems #:parent quad es]))
-(define lbr (q #:type line-break
-               #:copier line-break-copy
-               #:printable #f))
-(define pbr (q #:type line-break
-               #:copier line-break-copy
-               #:printable #f
-               #:elems '("¶¶")))
+(struct line-break quad ())
+(define lbr (q #:type line-break #:printable #f))
+;; treat paragraph break as special kind of line break
+(struct para-break line-break ())
+(define pbr (q #:type para-break #:printable #f))
 
 (module+ test
   (check-true (line-break? (second (quad-elems (q "foo" pbr "bar")))))
@@ -188,7 +180,7 @@
                                                     ;; when `line-heights` is empty, this is just h
                                                     (pt w (apply max (cons h line-heights))))]
                                             [elems new-elems]))
-                         (if (and q (line-break? q) (equal? (quad-elems q) '("¶¶")))
+                         (if (para-break? q)
                              (list q:line-spacer)
                              null)))))
 
diff --git a/quad/quad/atomize.rkt b/quad/quad/atomize.rkt
index f9c03e1b..ce712c25 100644
--- a/quad/quad/atomize.rkt
+++ b/quad/quad/atomize.rkt
@@ -62,14 +62,19 @@
                     (values next-key next-attrs)]))
     (match (quad-elems x)
       [(? pair? elems)
+       ;; we don't use `struct-copy` here because it needs to have the structure id at compile time.
+       ;; whereas with this technique, we can extract a constructor for any structure type.
+       ;; notice that the technique depends on
+       ;; 1) we only need to update attrs and elems
+       ;; 2) we make them the first two fields, so we know to drop the first two fields of x-tail
+       (define x-maker (let-values ([(x-structure-type _) (struct-info x)])
+                         (struct-type-make-constructor x-structure-type)))
+       (define x-tail (drop (struct->list x) 2))
        (append* 
         (for/list ([elem (in-list (merge-adjacent-strings elems 'isolate-white))])
-                  (match elem
-                    [(? string?)
-                     (define-values (xtype _) (struct-info x))
-                     (define x-constructor (struct-type-make-constructor xtype))
-                     (list (apply x-constructor (list* next-attrs (list elem) (cddr (struct->list x)))))]
-                    [_ (loop elem next-attrs next-key)])))]
+                  (if (string? elem)
+                      (list (apply x-maker next-attrs (list elem) x-tail))
+                      (loop elem next-attrs next-key))))]
       [_ (list x)])))
 
 (module+ test
diff --git a/quad/quad/quad.rkt b/quad/quad/quad.rkt
index 213a9f4b..b2c21936 100644
--- a/quad/quad/quad.rkt
+++ b/quad/quad/quad.rkt
@@ -72,9 +72,6 @@
 ;; todo: convert immutable hashes to mutable on input?
 (define (make-quad
          #:type [type quad]
-         #:copier [copier (λ (x as es) (struct-copy quad x
-                                                    [attrs as]
-                                                    [elems es]))]
          #:attrs [attrs (make-hasheq)]
          #:elems [elems null]
          #:size [size '(0 0)]