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This section describes the simplest kinds of ports: input ports that read their input from given strings, and output ports that accumulate their output and return it as a string. It also describes “truncating” output ports, which can limit the length of the resulting string to a given value.
Returns a new string port that delivers characters from string.
The optional arguments start and end may be used to specify
that the string port delivers characters from a substring of
string; if not given, start defaults to 0
and
end defaults to (string-length string)
.
Thunk must be a procedure of no arguments.
with-input-from-string
creates a new input port that reads from
string, makes that port the current input port, and calls
thunk. When thunk returns, with-input-from-string
restores the previous current input port and returns the result yielded
by thunk.
(with-input-from-string "(a b c) (d e f)" read) ⇒ (a b c)
Note: this procedure is equivalent to:
(with-input-from-port (open-input-string string) thunk)
open-output-string
returns a new output port that accumulates in
a buffer everything that is written to it. The accumulated output can
subsequently be obtained by calling get-output-string
on the
port.
Procedure is called with one argument, an output port. The value
yielded by procedure is ignored. When procedure returns,
call-with-output-string
returns the port’s accumulated output as
a newly allocated string. This is equivalent to:
(define (call-with-output-string procedure) (let ((port (open-output-string))) (procedure port) (get-output-string port)))
Thunk must be a procedure of no arguments.
with-output-to-string
creates a new output port that accumulates
output, makes that port the default value returned by
current-output-port
, and calls thunk with no arguments.
When thunk returns, with-output-to-string
restores the
previous default and returns the accumulated output as a newly allocated
string.
(with-output-to-string (lambda () (write 'abc))) ⇒ "abc"
Note: this procedure is equivalent to:
(call-with-output-string (lambda (port) (with-output-to-port port thunk)))
Similar to with-output-to-string
, except that the output is
limited to k characters. If thunk attempts to write more
than k characters, it will be aborted by invoking an escape
procedure that returns from with-output-to-truncated-string
.
The value of this procedure is a pair; the car of the pair is #t
if thunk attempted to write more than k characters, and
#f
otherwise. The cdr of the pair is a newly allocated string
containing the accumulated output.
This procedure is helpful for displaying circular lists, as shown in this example:
(define inf (list 'inf)) (with-output-to-truncated-string 40 (lambda () (write inf))) ⇒ (#f . "(inf)") (set-cdr! inf inf) (with-output-to-truncated-string 40 (lambda () (write inf))) ⇒ (#t . "(inf inf inf inf inf inf inf inf inf inf")
Writes object to a string output port, and returns the resulting
newly allocated string. If k is supplied and not #f
, this
procedure is equivalent to
(with-output-to-truncated-string k (lambda () (write object)))
otherwise it is equivalent to
(with-output-to-string (lambda () (write object)))
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