7dc01d44885e76321bcbd135e03d3e8f84cf8c30 — Alex Karle 1 year, 5 months ago 95960e0
parser: Add synchronization at declaration level

Before the parser stopped entirely at the first parse error:

$ cat examples/sync.lox
print 1 + for;
print 2 + 2;
print 1 + this;

$ ./fisl.scm examples/sync.lox
examples/sync.lox:1:Error at for. Unknown token

Now it synchronizes by going to the next statement and finds all parse
errors (but does NOT execute them):

$ ./fisl.scm examples/sync.lox
examples/sync.lox:1:Error at for. Unknown token
examples/sync.lox:3:Error at this. Unknown token
3 files changed, 38 insertions(+), 25 deletions(-)

A examples/sync.lox
M fisl.scm
M parser.scm
A examples/sync.lox => examples/sync.lox +3 -0
@@ 0,0 1,3 @@
print 1 + for;
print 2 + 2;
print 1 + this;

M fisl.scm => fisl.scm +3 -2
@@ 18,8 18,9 @@
  (let ((tokens (scan code)))
    (if tokens
	(let ((stmts (parse tokens)))
	  (if stmts
	      (interpret stmts))))))
          (unless had-err
            (if stmts
              (interpret stmts)))))))

(define (prompt)
  ;; HACK: srfi-18 blocks for IO, so having run-prompt

M parser.scm => parser.scm +32 -23
@@ 1,7 1,7 @@
;; parser.scm -- parser routines
(import (chicken format))

(define parser-abort #f)
(define parser-sync #f)


@@ 62,7 62,6 @@

(define (parse-declaration tokens)
  (if (top-type? tokens '(VAR))
      ;; TODO: sync on failure
      (parse-var-decl (cdr tokens))
      (parse-statement tokens)))

@@ 74,8 73,8 @@
                      (values '() (cdr tokens)))))
        (if (top-type? toks '(SEMICOLON))
            (values (make-var-stmt (car tokens) init) (cdr toks))
            (parse-err! (car toks) "Expected ';' after variable declaration")))
      (parse-err! (car tokens) "expected variable name")))
            (parse-err! toks "Expected ';' after variable declaration")))
      (parse-err! tokens "expected variable name")))

(define (parse-statement tokens)
  (if (top-type? tokens '(PRINT))

@@ 89,7 88,7 @@
      (values (maker expr) (cdr toks))
      (if in-repl
        (values (maker expr) toks)
        (parse-err! (car toks) "expected ;")))))
        (parse-err! toks "expected ;")))))

(define (parse-print-statement tokens)
  (parse-generic-stmt tokens make-print-stmt))

@@ 154,25 153,35 @@
      (let-values (((e2 t2) (parse-expression expr rest)))
        (if (top-type? t2 '(RIGHT_PAREN))
            (values (make-grouping e2) (cdr t2))
            (parse-err! (car t2) "Expected ')'"))))
     (else (parse-err! (car toks) "Unknown token")))))
            (parse-err! t2 "Expected ')'"))))
     (else (parse-err! toks "Unknown token")))))

(define (parse-err! tok msg)
  (if (eq? (token-type tok) 'EOF)
      (fname-err! (format "~A:~A ~A" (token-line tok) "Error at end." msg))
(define (parse-err! toks msg)
  (let ((top (car toks)))
    (if (top-type? toks '(EOF))
      (fname-err! (format "~A:~A ~A" (token-line top) "Error at end." msg))
      (fname-err! (format "~A:~A ~A. ~A"
                    (token-line tok)
                    "Error at"
                    (token-lexeme tok)
  ;; TODO: synchronize instead of abort
  (parser-abort #f))
                          (token-line top)
                          "Error at"
                          (token-lexeme top)
    (let ((t2 (synchronize (cdr toks))))
      (parser-sync t2))))

;; Given a list of tokens, returns the next statement (best guess based
;; on keyword being a statement keyword OR seeing a semicolon)
(define (synchronize tokens)
    ((null? tokens) '())
    ((top-type? tokens '(SEMICOLON)) (cdr tokens))
    ((top-type? tokens '(CLASS FUN VAR FOR IF WHILE PRINT RETURN)) tokens)
    (else (synchronize (cdr tokens)))))

(define (parse tokens)
  (call/cc (lambda (cc)
	     (set! parser-abort cc)
	     (let loop ((toks tokens))
	       (if (not (top-type? toks '(EOF)))
		   (let-values (((expr rest) (parse-declaration toks)))
		     (cons expr (loop rest)))
  ;; Loop through declarations, starting with tokens BUT using call/cc
  ;; to bookmark the loop so we can synchronize on parse-err!
  (let loop ((toks (call/cc (lambda (cc) (set! parser-sync cc) tokens))))
    (if (and toks (not (top-type? toks '(EOF))))
      (let-values (((expr rest) (parse-declaration toks)))
        (cons expr (loop rest)))