I'm not sure I understand what you want, but here are some ideas about
evaluating a text that would a module if only a "#lang" line were
added.
To start, here's a function to `require` an input port that contains a
module's source. It uses the current namespace, and it gensyms a name
for the module if you don't provide one. The part that I think is least
obvious is using `current-module-declare-name` to set the name of the
module to that it can be found by `dynamic-require`:
(require syntax/modread)
(define (require-input-port p [name (gensym)])
(define module-name (make-resolved-module-path name))
(parameterize ([current-module-declare-name module-name])
(eval-syntax (check-module-form ; ensures that `module` is bound
(with-module-reading-parameterization
(lambda ()
(read-syntax (object-name p) p)))
'ignored
#f)))
(dynamic-require module-name #f))
The `require-input-port` function assumes that the source starts with
"#lang". You could use `input-port-append`, as others have suggested,
to add a "#lang" line:
(define p (input-port-append #t
(open-input-string "#lang racket/base\n")
(open-input-file "body.rktd")))
(port-count-lines! p)
(require-input-port p)
A problem with `input-port-append` is that line numbers are off by one
for error reporting, and positions are off by the length of the first
line. That's an annoyingly difficult problem to fix, but
`prefix-input-port` below is my attempt (and maybe `input-port-append`
should just work better along similar lines).
(define p (prefix-input-port #"#lang racket/base\n"
(open-input-file "body.rktd")))
(port-count-lines! p)
(require-input-port p)
----------------------------------------
;; prefix-input-port : bytes input-port -> input-port
;; Directs position requests to the given port after the
;; prefix is read.
;; Closes the given input port when the result port is closed.
(define (prefix-input-port prefix base-p)
(define-values (prefix-i prefix-o) (make-pipe))
(write-bytes prefix prefix-o)
(close-output-port prefix-o)
(define (prefix-done?)
(zero? (pipe-content-length prefix-i)))
(make-input-port
(object-name base-p)
;; read
(lambda (bstr)
(define n (read-bytes-avail!* bstr
(if (prefix-done?)
base-p
prefix-i)))
(if (equal? n 0)
(wrap-evt base-p (lambda (v) 0))
n))
;; peek
(lambda (bstr offset evt)
(define pre-n (pipe-content-length prefix-i))
(define n (if (offset . >= . pre-n)
(peek-bytes-avail!* bstr
(- offset pre-n)
#f
base-p)
(peek-bytes-avail!* bstr
offset
#f
prefix-i)))
(if (equal? n 0)
(wrap-evt base-p (lambda (v) 0))
n))
;; close
(lambda ()
(close-input-port base-p))
;; get-progress-evt
;; Difficult (impossible?) to support at the
;; prefix--base boundary.
#f
;; commit
#f
;; get-location
(lambda ()
(if (prefix-done?)
(port-next-location base-p)
(port-next-location prefix-i)))
;; count-lines!
(lambda ()
(port-count-lines! prefix-i)
(port-count-lines! base-p))))
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