From 8dc6346c58cfdf5ea1f6cf4d36b23c73a4dfa231 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Mon, 8 Oct 2018 20:53:53 +0100
Subject: [PATCH] Special cases for label[...] and label{...}

---
 implementations/racket/preserves/main.rkt | 18 ++++++++++--------
 preserves.md                              |  6 ++++++
 2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/implementations/racket/preserves/main.rkt b/implementations/racket/preserves/main.rkt
index 5bd01b4..bdaa1c2 100644
--- a/implementations/racket/preserves/main.rkt
+++ b/implementations/racket/preserves/main.rkt
@@ -283,8 +283,8 @@
   (define (read-sequence terminator)
     (sequence-fold '() (lambda (acc) (cons (read-value) acc)) reverse terminator))
 
-  (define (read-dictionary-or-set)
-    (sequence-fold #f
+  (define (read-dictionary-or-set seed)
+    (sequence-fold seed
                    (lambda (acc)
                      (define k (read-value))
                      (skip-whitespace)
@@ -447,18 +447,20 @@
 
   (define (collect-fields head)
     (match (peek-char i)
-      [#\(
-       (read-char i)
-       (collect-fields (build-record head (read-sequence #\))))]
-      [_
-       head]))
+      [#\( (read-char i)
+           (collect-fields (build-record head (read-sequence #\))))]
+      [#\[ (read-char i)
+           (collect-fields (build-record head (list (read-sequence #\]))))]
+      [#\{ (read-char i)
+           (collect-fields (build-record head (list (read-dictionary-or-set (hash)))))]
+      [_   head]))
 
   (define (read-value)
     (skip-whitespace)
     (collect-fields
      (match (peek-char i)
        [(? eof-object? o) o]
-       [#\{ (read-char i) (read-dictionary-or-set)]
+       [#\{ (read-char i) (read-dictionary-or-set #f)]
        [#\[ (read-char i) (read-sequence #\])]
        [(or #\- #\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9) (read-number)]
        [#\" (read-char i) (read-string #\")]
diff --git a/preserves.md b/preserves.md
index f2431ed..d4fe242 100644
--- a/preserves.md
+++ b/preserves.md
@@ -317,6 +317,12 @@ tokens `#set{` and `}`.[^printing-collections]
     commas separating, and commas terminating elements or key/value
     pairs within a collection.
 
+The special cases of records with a single field, which is in turn a
+sequence or dictionary, may be written omitting the parentheses.
+
+           Record =/ Value Sequence
+           Record =/ Value Dictionary
+
 `Boolean`s are the simple literal strings `#true` and `#false`.
 
            Boolean = %s"#true" / %s"#false"