8.6 I/O Patterns

If you want to process individual lines of a file, then you can use for with in-lines:

> (define (upcase-all in)       (for ([l (in-lines in)])         (display (string-upcase l))         (newline)))

> (upcase-all (open-input-string                  (string-append                   "Hello, World!\n"                   "Can you hear me, now?")))

HELLO, WORLD!   CAN YOU HEAR ME, NOW?

If you want to determine whether “hello” appears in a file, then you could search separate lines, but it’s even easier to simply apply a regular expression (see Regular Expressions) to the stream:

> (define (has-hello? in)       (regexp-match? #rx"hello" in))

> (has-hello? (open-input-string "hello"))

#t

> (has-hello? (open-input-string "goodbye"))

#f

If you want to copy one port into another, use copy-port from racket/port, which efficiently transfers large blocks when lots of data is available, but also transfers small blocks immediately if that’s all that is available:

> (define o (open-output-string))

> (copy-port (open-input-string "broom") o)

> (get-output-string o)

"broom"