6.18.4 String ports

String ports are the ports that you can read from or write to memory.

Function: open-input-string string

[SRFI-6] Creates an input string port that has the content string. This is a more efficient way to access a string in order rather than using string-ref with incremental index.

(define p (open-input-string "foo x")) (read p) => foo (read-char p) => #\space (read-char p) => #\x (read-char p) => #<eof> (read-char p) => #<eof>

Function: open-output-string

[SRFI-6] Creates an output string port. Anything written to the port is accumulated in the buffer, and can be obtained as a string by get-output-string. This is a far more efficient way to construct a string sequentially than pre-allocate a string and fill it with string-set!.

Function: get-output-string port

[SRFI-6] Takes an output string port port and returns a string that has been accumulated to port so far. If a byte data has been written to the port, this function re-scans the buffer to see if it can consist a complete string; if not, an incomplete string is returned.

Function: call-with-input-string string proc

Function: call-with-output-string proc

Function: with-input-from-string string thunk

Function: with-output-to-string thunk

These utility functions are trivially defined as follows. The interface is parallel to the file port version.

(define (call-with-output-string proc) (let ((out (open-output-string))) (proc out) (get-output-string out))) (define (call-with-input-string str proc) (let ((in (open-input-string str))) (proc in))) (define (with-output-to-string thunk) (let ((out (open-output-string))) (with-output-to-port out thunk) (get-output-string out))) (define (with-input-from-string str thunk) (with-input-from-port (open-input-string str) thunk))

Function: call-with-string-io str proc

Function: with-string-io str thunk

(define (call-with-string-io str proc) (let ((out (open-output-string)) (in (open-input-string str))) (proc in out) (get-output-string out))) (define (with-string-io str thunk) (with-output-to-string (lambda () (with-input-from-string str thunk))))

Function: write-to-string obj &optional writer

Function: read-from-string string &optional start end

These convenience functions cover commin idioms using string ports.

(write-to-string obj writer) == (with-output-to-string (lambda () (writer obj))) (read-from-string string) == (with-input-from-string string read)

The default value of writer is the procedure write. The default values of start and end is 0 and the length of string.

Portability note: Common Lisp has these functions, with different optional arguments. STk has read-from-string without optional argument.