Editing Scheme (programming language) (section)

===Input/output===
Scheme's input and output is based on the ''port'' datatype. (R5RS sec 6.6)<ref name="r5rs"/>  R5RS defines two default ports, accessible with the procedures <code>current-input-port</code> and <code>current-output-port</code>, which correspond to the Unix notions of [[Standard streams|standard input and standard output]]. Most implementations also provide <code>current-error-port</code>. [[Redirection (computing)|Redirection]] of input and standard output is supported in the standard, by standard procedures such as <code>with-input-from-file</code> and <code>with-output-to-file</code>. Most implementations provide string ports with similar redirection capabilities, enabling many normal input-output operations to be performed on string buffers instead of files, using procedures described in SRFI 6.<ref name="srfi-6">{{Cite web |last=William D Clinger |date=1999-07-01 |title=SRFI 6: Basic String Ports |url=http://srfi.schemers.org/srfi-6/srfi-6.html |access-date=2012-08-09 |publisher=The SRFI Editors, schemers.org}}</ref> The R6RS standard specifies much more sophisticated and capable port procedures and many new types of port.

The following examples are written in strict R5RS Scheme.

Example 1: With output defaulting to (current-output-port):
<syntaxhighlight lang="Scheme">
(let ((hello0 (lambda() (display "Hello world") (newline))))
  (hello0))
</syntaxhighlight>

Example 2: As 1, but using optional port argument to output procedures
<syntaxhighlight lang="Scheme">
(let ((hello1 (lambda (p) (display "Hello world" p) (newline p))))
  (hello1 (current-output-port)))
</syntaxhighlight>

Example 3: As 1, but output is redirected to a newly created file
<syntaxhighlight lang="Scheme">
;; NB: with-output-to-file is an optional procedure in R5RS
(let ((hello0 (lambda () (display "Hello world") (newline))))
  (with-output-to-file "helloworldoutputfile" hello0))
</syntaxhighlight>

Example 4: As 2, but with explicit file open and port close to send output to file
<syntaxhighlight lang="Scheme">
(let ((hello1 (lambda (p) (display "Hello world" p) (newline p)))
      (output-port (open-output-file "helloworldoutputfile")))
  (hello1 output-port)
  (close-output-port output-port))
</syntaxhighlight>

Example 5: As 2, but with using call-with-output-file to send output to a file.
<syntaxhighlight lang="Scheme">
(let ((hello1 (lambda (p) (display "Hello world" p) (newline p))))
  (call-with-output-file "helloworldoutputfile" hello1))
</syntaxhighlight>

Similar procedures are provided for input. R5RS Scheme provides the predicates <code>input-port?</code> and <code>output-port?</code>. For character input and output, <code>write-char</code>, <code>read-char</code>, <code>peek-char</code> and <code>char-ready?</code> are provided. For writing and reading Scheme expressions, Scheme provides <code>read</code> and <code>write</code>. On a read operation, the result returned is the end-of-file object if the input port has reached the end of the file, and this can be tested using the predicate <code>eof-object?</code>.

With the standard, SRFI 28 also defines a basic formatting procedure resembling Common Lisp's <code>format</code> function, after which it is named.<ref name="srfi-28">{{Cite web |last=Scott G. Miller |date=2002-06-25 |title=SRFI 28: Basic Format Strings |url=http://srfi.schemers.org/srfi-28/srfi-28.html |publisher=The SRFI Editors, schemers.org |access-date=2012-08-09}}</ref>