correct ambiguity with certain escape sequences

brag-lib/brag/rules/lexer.rkt

@@ -38,39 +38,54 @@
 (require syntax-color/racket-lexer)
 
 (define (unescape-double-quoted-lexeme lexeme)
+  ;; use `read` so brag strings have all the notational semantics of Racket strings
   (list->string `(#\" ,@(string->list (read (open-input-string lexeme))) #\")))
 
 (define (convert-to-double-quoted lexeme)
+  ;; brag supports single-quoted strings, for some reason
+  ;; (Racket does not. A single quote denotes a datum)
+  ;; let's convert a single-quoted string into standard double-quoted style
+  ;; so we can use Racket's `read` function on it.
+  ;; and thereby support all the standard Racket string elements:
+  ;; https://docs.racket-lang.org/reference/reader.html#%28part._parse-string%29
   (define outside-quotes-removed (string-trim lexeme "'"))
   (define single-quotes-unescaped (string-replace outside-quotes-removed "\\'" "'"))
   (define double-quotes-escaped (string-replace single-quotes-unescaped "\"" "\\\""))
   (define double-quotes-on-ends (string-append "\"" double-quotes-escaped "\""))
   double-quotes-on-ends)
 
-(define-lex-abbrev escaped-single-quote "\\'")
+(define-lex-abbrev backslash "\\")
 (define-lex-abbrev single-quote "'")
-(define-lex-abbrev escaped-double-quote "\\\"")
+(define-lex-abbrev escaped-single-quote (:: backslash single-quote))
 (define-lex-abbrev double-quote "\"")
-(define-lex-abbrev escaped-backslash "\\\\")
+(define-lex-abbrev escaped-double-quote (:: backslash double-quote))
+(define-lex-abbrev escaped-backslash (:: backslash backslash))
 
 (define lex/1
   (lexer-src-pos
-   ;; handle whitespace & escape chars within quotes as literal tokens: "\n" "\t" '\n' '\t'
-   ;; match the escaped version, and then unescape them before they become token-LITs
+   [(:: double-quote ;; start with double quote
+        (intersection ;; two conditions need to be true inside the quotes:
+         ;; we can have anything except
+         ;; a plain double-quote (which would close the quote)
+         ;; plus we specially allow escaped double quotes and backslashes
+         (:* (:or escaped-double-quote escaped-backslash (:~ double-quote)))
+         ;; we must forbid one situation with the string \\"
+         ;; the problem is that it's ambiguous:
+         ;; it can be lexed as (:: escaped-backlash double-quote) = \\ + "
+         ;; or  (:: backlash escaped-double-quote) = \ + \"
+         ;; because escapes should be "left associative",
+         ;; we forbid the second possibility         
+         (complement (:: any-string backslash escaped-double-quote any-string)))
+        double-quote) ;; end with double quote
+    (token-LIT (unescape-double-quoted-lexeme lexeme))]
+   ;; single-quoted string follows the same pattern,
+   ;; but with escaped-single-quote instead of escaped-double-quote
    [(:: single-quote
-        (:or
-         (:+ escaped-backslash) ; aka '\\'
-         (intersection (:* (:or escaped-single-quote (:~ single-quote)))
-                       (complement (:: escaped-backslash any-string))))
+        (intersection
+         (:* (:or escaped-single-quote escaped-backslash (:~ single-quote)))
+         (complement (:: any-string backslash escaped-single-quote any-string)))
         single-quote)
     (token-LIT (unescape-double-quoted-lexeme (convert-to-double-quoted lexeme)))]
-   [(:: double-quote
-        (:or
-         (:+ escaped-backslash) ; aka "\\"
-         (intersection (:* (:or escaped-double-quote (:~ double-quote)))
-                       (complement (:: escaped-backslash any-string))))
-        double-quote)
-    (token-LIT (unescape-double-quoted-lexeme lexeme))]
    [(:or "()" "Ø" "∅") (token-EMPTY lexeme)]
    ["("
     (token-LPAREN lexeme)]