Fix make-rule-parser (#28)

When using `make-rule-parser` for any rule that is not the start
rule, and applying the resulting parser while giving it a source-path
it would just use the entire grammar from the start rule.

brag-lib/brag/codegen/expander.rkt

@@ -71,41 +71,38 @@
                           (cons eof token-EOF) 
                           (cons 'TOKEN-TYPE TOKEN-TYPE-CONSTRUCTOR) ...)))
              
-            (define-syntax (MAKE-RULE-PARSER rule-id-stx)
-              (syntax-case rule-id-stx ()
-                [(_ start-rule)
-                 (and (identifier? #'start-rule)
-                      (member (syntax-e #'start-rule) 'RULE-IDS))
+            (define-syntax (MAKE-RULE-PARSER stx)
+              (syntax-case stx ()
+                [(_ START-RULE-ID)
+                 (and (identifier? #'START-RULE-ID) (member (syntax-e #'START-RULE-ID) 'RULE-IDS))
                  ;; The cfg-parser depends on the start-rule provided in (start ...) to have the same
                  ;; context as the rest of this body. Hence RECOLORED-START-RULE
-                 (with-syntax ([RECOLORED-START-RULE (datum->syntax #'RULES-STX (syntax-e #'start-rule))])
-                   #'(let ([THE-GRAMMAR (cfg-parser (tokens enumerated-tokens)
+                 (with-syntax ([RECOLORED-START-RULE (datum->syntax #'RULES-STX (syntax-e #'START-RULE-ID))])
+                   #'(let ()
+                       (define (rule-parser tokenizer)
+                         (define rule-grammar (cfg-parser (tokens enumerated-tokens)
                                                           (src-pos)
                                                           (start RECOLORED-START-RULE)
                                                           (end EOF)
-                                                    (error THE-ERROR-HANDLER)
-                                                    (grammar . GENERATED-RULE-CODES))])
-                       (procedure-rename
-                        (case-lambda [(tokenizer)
-                                      (define next-token
-                                        (make-permissive-tokenizer tokenizer all-tokens-hash/mutable))
-                                      ;; little post-processor to support cuts on top rule name
-                                      (define parse-tree-stx (THE-GRAMMAR next-token))
-                                      (define top-rule-name-stx (syntax-case parse-tree-stx ()
-                                                                  [(TRN . REST) #'TRN]
-                                                                  [_ (error 'malformed-parse-tree)]))
-                                      (if (eq? (syntax-property top-rule-name-stx 'hide-or-splice?) 'hide)
-                                          ;; use `remove-rule-name` so we get the same housekeeping
-                                          (remove-rule-name parse-tree-stx) 
+                                                          (error the-error-handler)
+                                                          (grammar . GENERATED-RULE-CODES)))
+                         (define next-token (make-permissive-tokenizer tokenizer all-tokens-hash/mutable))
+                         ;; here's how we support grammar "cuts" on top rule name
+                         (define parse-tree-stx (rule-grammar next-token))                         
+                         (syntax-case parse-tree-stx ()
+                           [(TOP-RULE-NAME . _)
+                            (if (eq? (syntax-property #'TOP-RULE-NAME 'hide-or-splice?) 'hide)
+                                (remove-rule-name parse-tree-stx) ; use `remove-rule-name` so we get the same housekeeping
                                 parse-tree-stx)]
+                           [_ (error 'malformed-parse-tree)]))
+                       (case-lambda [(tokenizer) (rule-parser tokenizer)]
                                     [(source tokenizer)
                                      (parameterize ([current-source source])
-                                        (PARSE tokenizer))])
-                        (string->symbol (format "~a-rule-parser" 'start-rule)))))]
+                                       (rule-parser tokenizer))])))]
                 [(_ not-a-rule-id)
                  (raise-syntax-error #f
                                      (format "Rule ~a is not defined in the grammar" (syntax-e #'not-a-rule-id))
-                                     rule-id-stx)]))
+                                     stx)]))
 
             ;; start-id has to be a value, not an expr, because make-rule-parser is a macro
             (define PARSE (procedure-rename (MAKE-RULE-PARSER START-ID) 'PARSE))

brag-lib/brag/codegen/runtime.rkt

@@ -7,7 +7,7 @@
          brag/private/internal-support)
 
 
-(provide THE-ERROR-HANDLER 
+(provide the-error-handler 
          make-permissive-tokenizer
          atomic-datum->syntax
          positions->srcloc
@@ -19,7 +19,7 @@
 ;; The level of indirection here is necessary since the yacc grammar wants a
 ;; function value for the error handler up front.  We want to delay that decision
 ;; till parse time.
-(define (THE-ERROR-HANDLER tok-ok? tok-name tok-value start-pos end-pos)
+(define (the-error-handler tok-ok? tok-name tok-value start-pos end-pos)
   (match (positions->srcloc start-pos end-pos)
     [(list src line col offset span)
      ((current-parser-error-handler) tok-name 

brag-lib/brag/examples/subrule.rkt

@@ -0,0 +1,4 @@
+#lang brag
+
+start: next
+next: "0"

brag-lib/brag/test/test-make-rule-parser.rkt

@@ -0,0 +1,17 @@
+#lang racket/base
+(require rackunit
+         brag/support
+         brag/examples/subrule)
+
+(define parse-next (make-rule-parser next))
+(define parse-start (make-rule-parser start))
+
+(check-equal? (syntax->datum (parse #f "0")) '(start (next "0")))
+(check-equal? (syntax->datum (parse #f "0")) (syntax->datum (parse "0")))
+
+(check-equal? (syntax->datum (parse-start #f "0")) '(start (next "0")))
+(check-equal? (syntax->datum (parse-start #f "0")) (syntax->datum (parse-start "0")))
+
+(check-equal? (syntax->datum (parse-next #f "0")) '(next "0"))
+(check-equal? (syntax->datum (parse-next #f "0")) (syntax->datum (parse-next "0")))
+