Feature Request: read-handler parameter for custom reader extensibility
8 views
Skip to first unread message
Michał Pociecha-Łoś
Mar 10, 2026, 2:51:51 PM (4 days ago) Mar 10
to chez-scheme
Hi everyone,
I’ve found Chez Scheme to be an excellent foundation for implementing custom languages. Parameters like `library-extensions` and `current-expand` already provide great hooks for customising the load and expansion phases.
To complete this extensibility suite, I am proposing the addition of a `read-handler` parameter. This would allow users to provide a custom read function, bridging the gap between raw source files and the expander.
I’ve implemented a small patch in `s/read.ss` (around line 1670) to demonstrate the integration:
(define do-read
(lambda (who ip sfd a? fp)
(when (port-closed? ip)
($oops who "not permitted on closed port ~s" ip))
(let ([fp (or fp
(and ($port-flags-set? ip (constant port-flag-char-positions))
(port-has-port-position? ip)
(port-position ip)))])
((read-handler) ip sfd a? fp))))
(set-who! default-read-handler
(lambda (ip sfd a? fp)
(let ([rcb (make-rcb ip sfd (and a? sfd fp #t) who)] [tb ""] [bfp fp] [it #f])
(call-with-token rd-top-level))))
(set! read-handler
(make-parameter default-read-handler)))
Do you think this hook fits the design philosophy of Chez Scheme, and would you be open to a formal Pull Request for it?
Best regards,
Michał
I’ve found Chez Scheme to be an excellent foundation for implementing custom languages. Parameters like `library-extensions` and `current-expand` already provide great hooks for customising the load and expansion phases.
To complete this extensibility suite, I am proposing the addition of a `read-handler` parameter. This would allow users to provide a custom read function, bridging the gap between raw source files and the expander.
I’ve implemented a small patch in `s/read.ss` (around line 1670) to demonstrate the integration:
(define do-read
(lambda (who ip sfd a? fp)
(when (port-closed? ip)
($oops who "not permitted on closed port ~s" ip))
(let ([fp (or fp
(and ($port-flags-set? ip (constant port-flag-char-positions))
(port-has-port-position? ip)
(port-position ip)))])
((read-handler) ip sfd a? fp))))
(set-who! default-read-handler
(lambda (ip sfd a? fp)
(let ([rcb (make-rcb ip sfd (and a? sfd fp #t) who)] [tb ""] [bfp fp] [it #f])
(call-with-token rd-top-level))))
(set! read-handler
(make-parameter default-read-handler)))
Do you think this hook fits the design philosophy of Chez Scheme, and would you be open to a formal Pull Request for it?
Best regards,
Michał
Michał Pociecha-Łoś
Mar 10, 2026, 8:41:50 PM (4 days ago) Mar 10
to chez-scheme
Alternatively, to allow intercepting reading from source files only, Chez Scheme could provide `make-read-handler`.
(set-who! default-make-read-handler
(lambda (ip sfd fp)
(lambda ()
(let-values ([(x new-fp) (do-read who ip sfd #t fp)])
(set! fp new-fp)
x))))
(set! make-read-handler
(make-parameter default-make-read-handler))
(set! $make-read
(lambda (ip sfd fp)
(define who 'read)
(unless (and (input-port? ip) (textual-port? ip))
($oops who "~s is not a textual input port" ip))
(unless (or (not sfd) (source-file-descriptor? sfd))
($oops who "~s is not a source-file descriptor" sfd))
((make-read-handler) ip sfd fp)))
(lambda (ip sfd fp)
(lambda ()
(let-values ([(x new-fp) (do-read who ip sfd #t fp)])
(set! fp new-fp)
x))))
(set! make-read-handler
(make-parameter default-make-read-handler))
(set! $make-read
(lambda (ip sfd fp)
(define who 'read)
(unless (and (input-port? ip) (textual-port? ip))
($oops who "~s is not a textual input port" ip))
(unless (or (not sfd) (source-file-descriptor? sfd))
($oops who "~s is not a source-file descriptor" sfd))
((make-read-handler) ip sfd fp)))
Michał
Reply all
Reply to author
Forward
0 new messages