Use module-lexer to parse comments #750

Copilot · 2025-11-10T17:07:09Z

The custom comment parser was incomplete (missing expression comments like #;(foo)), language-specific to #lang racket, and difficult to maintain.

Changes

Replaced custom lexer with syntax-color/module-lexer: Removed br-parser-tools/lex implementation in favor of Racket's standard lexer
Added expression comment support: Now properly handles #; comments via skip-one-sexp helper that tracks parenthesis depth
Fixed port compatibility: Thread lexer mode state through iterations instead of using file-position, which fails on reencoded ports
Added test cases: Created comment-preservation-test.rkt with 6 #lang resyntax/test test cases demonstrating expression (#;) and block (#|) comment preservation during refactorings

Example

; Line comment
#| Block comment |#
#;(commented-expression arg1 arg2)  ; Now detected
#;unused-atom                        ; Also detected

Impact

Fixes Comment parser failures #149 (expression comments not preserved)
Enables automatic support for comments in other #lang variants (Refactoring custom languages #270)
Added 5 new test cases for comment parsing in comment-reader.rkt, all 1043 tests pass
Added 6 new test cases demonstrating comment preservation in refactorings
Fixes Use module-lexer to parse comments #299

Original prompt

This section details on the original issue you should resolve

<issue_title>Use module-lexer to parse comments</issue_title>
<issue_description>The current comment parsing code in Resyntax is specific to #lang racket, incomplete, and irritating to maintain. Resyntax should use module-lexer to find comment locations in the file. This will properly handle expression comments like #;(foo). It should also work automatically for comments in other languages, hopefully making #270 easier to implement.</issue_description>

Comments on the Issue (you are @copilot in this section)
@jackfirth Note that this would also fix #149.

Fixes Use module-lexer to parse comments #299

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

- Use syntax-color/module-lexer instead of br-parser-tools/lex - Support expression comments (#;) - Add tests for expression comments - All existing tests pass Co-authored-by: jackfirth <8175575+jackfirth@users.noreply.github.com>

- Track lexer mode state through iterations instead of rewinding port - Update skip-one-sexp to return final mode - Handle reencoded ports that don't support file-position - All 1037 tests pass Co-authored-by: jackfirth <8175575+jackfirth@users.noreply.github.com>

github-actions

Resyntax analyzed 1 file in this pull request and has added suggestions.

github-actions · 2025-11-10T20:43:54Z

private/comment-reader.rkt

+       (if is-line-comment?
+           ;; Peek at the next token to see if it's a newline
+           (let ()
+             (define-values (next-lexeme next-type next-paren next-start next-end next-backup next-mode)
+               (module-lexer in 0 mode-out))
+             (cond
+               [(and (equal? next-type 'white-space) (equal? next-lexeme "\n"))
+                ;; Include the newline in the comment range and continue with the mode after the newline
+                (loop (cons (closed-open-range comment-start (sub1 next-end) #:comparator natural<=>) ranges)
+                      next-mode)]
+               [(eof-object? next-lexeme)
+                ;; EOF after comment
+                (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)
+                      next-mode)]
+               [else
+                ;; Non-newline token after comment; we need to "un-consume" it
+                ;; by processing it in the next iteration. But we can't easily do that
+                ;; with module-lexer. Let's use a different approach.
+                (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)
+                      next-mode)]))
+           ;; Block comment - don't peek ahead
+           (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)
+                 mode-out))]


if-let-to-cond: cond with internal definitions is preferred over if with let, to reduce nesting

Suggested change

(if is-line-comment?

;; Peek at the next token to see if it's a newline

(let ()

(define-values (next-lexeme next-type next-paren next-start next-end next-backup next-mode)

(module-lexer in 0 mode-out))

(cond

[(and (equal? next-type 'white-space) (equal? next-lexeme "\n"))

;; Include the newline in the comment range and continue with the mode after the newline

(loop (cons (closed-open-range comment-start (sub1 next-end) #:comparator natural<=>) ranges)

next-mode)]

[(eof-object? next-lexeme)

;; EOF after comment

(loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)

next-mode)]

[else

;; Non-newline token after comment; we need to "un-consume" it

;; by processing it in the next iteration. But we can't easily do that

;; with module-lexer. Let's use a different approach.

(loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)

next-mode)]))

;; Block comment - don't peek ahead

(loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)

mode-out))]

(cond

[is-line-comment?

;; Peek at the next token to see if it's a newline

(define-values (next-lexeme next-type next-paren next-start next-end next-backup next-mode)

(module-lexer in 0 mode-out))

(cond

[(and (equal? next-type 'white-space) (equal? next-lexeme "\n"))

;; Include the newline in the comment range and continue with the mode after the newline

(loop (cons (closed-open-range comment-start (sub1 next-end) #:comparator natural<=>)

ranges)

next-mode)]

[(eof-object? next-lexeme)

;; EOF after comment

(loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>)

ranges)

next-mode)]

[else

;; Non-newline token after comment; we need to "un-consume" it

;; by processing it in the next iteration. But we can't easily do that

;; with module-lexer. Let's use a different approach.

(loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>)

ranges)

next-mode)])]

;; Block comment - don't peek ahead

[else

(loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>)

ranges)

mode-out)])]

Debugging details

Textual replacement

(line-replacement #:new-lines '#(" (cond" " [is-line-comment?" " ;; Peek at the next token to see if it's a newline" " (define-values (next-lexeme next-type next-paren next-start next-end next-backup next-mode)" " (module-lexer in 0 mode-out))" " (cond" " [(and (equal? next-type 'white-space) (equal? next-lexeme \"\\n\"))" " ;; Include the newline in the comment range and continue with the mode after the newline" " (loop (cons (closed-open-range comment-start (sub1 next-end) #:comparator natural<=>)" " ranges)" " next-mode)]" " [(eof-object? next-lexeme)" " ;; EOF after comment" " (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>)" " ranges)" " next-mode)]" " [else" " ;; Non-newline token after comment; we need to \"un-consume\" it" " ;; by processing it in the next iteration. But we can't easily do that" " ;; with module-lexer. Let's use a different approach." " (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>)" " ranges)" " next-mode)])]" " ;; Block comment - don't peek ahead" " [else" " (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>)" " ranges)" " mode-out)])]") #:original-lines '#(" (if is-line-comment?" " ;; Peek at the next token to see if it's a newline" " (let ()" " (define-values (next-lexeme next-type next-paren next-start next-end next-backup next-mode)" " (module-lexer in 0 mode-out))" " (cond" " [(and (equal? next-type 'white-space) (equal? next-lexeme \"\\n\"))" " ;; Include the newline in the comment range and continue with the mode after the newline" " (loop (cons (closed-open-range comment-start (sub1 next-end) #:comparator natural<=>) ranges)" " next-mode)]" " [(eof-object? next-lexeme)" " ;; EOF after comment" " (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)" " next-mode)]" " [else" " ;; Non-newline token after comment; we need to \"un-consume\" it" " ;; by processing it in the next iteration. But we can't easily do that" " ;; with module-lexer. Let's use a different approach." " (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)" " next-mode)]))" " ;; Block comment - don't peek ahead" " (loop (cons (closed-open-range comment-start comment-end-base #:comparator natural<=>) ranges)" " mode-out))]") #:start-line 44)

Syntactic replacement

(syntax-replacement #:introduction-scope #<procedure:...and/syntax-local.rkt:148:2> #:new-syntax #<syntax:default-recommendations/let-replacement/cond-let-replacement.rkt:47:2 (cond (is-line-comment? (define-values (next-lexeme next-type next-paren next-start next-end next-backup next-mode) (module-lexer in 0 mode-out)) (cond ((and (equal? next-type (quote white-space)) (equal? next-lexeme "\n")) (loop (cons (closed-open-rang...> #:original-syntax #<syntax:private/comment-reader.rkt:44:7 (if is-line-comment? (let () (define-values (next-lexeme next-type next-paren next-start next-end next-backup next-mode) (module-lexer in 0 mode-out)) (cond ((and (equal? next-type (quote white-space)) (equal? next-lexeme "\n")) (loop (cons (closed-open...> #:source (file-source #<path:/home/runner/work/resyntax/resyntax/private/comment-reader.rkt>) #:uses-universal-tagged-syntax? #f)

coveralls · 2025-11-10T20:51:44Z

coverage: 93.655% (+0.03%) from 93.63%
when pulling 284c128 on copilot/use-module-lexer-for-comments
into 186fd5e on master.

jackfirth · 2025-11-11T02:22:03Z

@copilot Add some #lang resyntax/test test cases that demonstrate that Resyntax can preserve expression (#;) comments and block (#| |#) comments.

- Added comment-preservation-test.rkt with 6 test cases - Tests show expression comments (#;) are preserved - Tests show block comments (#|) are preserved - All 1043 tests pass Co-authored-by: jackfirth <8175575+jackfirth@users.noreply.github.com>

Copilot · 2025-11-11T02:35:08Z

@copilot Add some #lang resyntax/test test cases that demonstrate that Resyntax can preserve expression (#;) comments and block (#| |#) comments.

Added in 5d4ee83. Created comment-preservation-test.rkt with 6 test cases demonstrating that both expression comments (#;) and block comments (#|) are preserved during let-to-define refactorings.

github-actions

Resyntax analyzed 3 files in this pull request and found no issues.

github-actions

Resyntax analyzed 4 files in this pull request and found no issues.

Initial plan

08e1e5b

Copilot AI assigned Copilot and jackfirth Nov 10, 2025

Copilot started work on behalf of jackfirth November 10, 2025 17:07 View session

Copilot AI and others added 2 commits November 10, 2025 17:29

Replace custom comment lexer with module-lexer

20e8b57

- Use syntax-color/module-lexer instead of br-parser-tools/lex - Support expression comments (#;) - Add tests for expression comments - All existing tests pass Co-authored-by: jackfirth <8175575+jackfirth@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Use module-lexer to parse comments in Resyntax~~ Use module-lexer to parse comments Nov 10, 2025

Copilot finished work on behalf of jackfirth November 10, 2025 17:46

Copilot AI requested a review from jackfirth November 10, 2025 17:46

jackfirth marked this pull request as ready for review November 10, 2025 20:38

github-actions bot reviewed Nov 10, 2025

View reviewed changes

Copilot started work on behalf of jackfirth November 11, 2025 02:22 View session

Copilot finished work on behalf of jackfirth November 11, 2025 02:36

Redo copilot implementation

284c128

jackfirth approved these changes Nov 12, 2025

View reviewed changes

github-actions bot approved these changes Nov 12, 2025

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jackfirth enabled auto-merge (squash) November 12, 2025 01:57

Increase default analyzer timeout in tests

46bedf9

github-actions bot approved these changes Nov 12, 2025

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jackfirth merged commit 1c757cf into master Nov 12, 2025
5 checks passed

jackfirth deleted the copilot/use-module-lexer-for-comments branch November 12, 2025 02:13

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