@page
@node Deprecated
@chapter Deprecated

@menu
* Shared And Read Only Strings::
@end menu


@node Shared And Read Only Strings
@section Shared And Read Only Strings

The procedures described in this section are deprecated because explicit
shared substrings are planned to disappear from Guile.

Instead, all strings will be implemented using sharing internally,
combined with a copy-on-write strategy.  Once internal string sharing
and copy-on-write have been implemented, it will be unnecessary to
preserve the concept of read only strings.

@menu
* Shared Substrings::           Strings which share memory with each other.
* Read Only Strings::           Treating certain non-strings as strings.
@end menu


@node Shared Substrings
@subsection Shared Substrings

Whenever you extract a substring using @code{substring}, the Scheme
interpreter allocates a new string and copies data from the old string.
This is expensive, but @code{substring} is so convenient for
manipulating text that programmers use it often.

Guile Scheme provides the concept of the @dfn{shared substring} to
improve performance of many substring-related operations.  A shared
substring is an object that mostly behaves just like an ordinary
substring, except that it actually shares storage space with its parent
string.

@deffn {Scheme Procedure} make-shared-substring str [start [end]]
Return a shared substring of @var{str}.  The arguments are the
same as for the @code{substring} function: the shared substring
returned includes all of the text from @var{str} between
indexes @var{start} (inclusive) and @var{end} (exclusive).  If
@var{end} is omitted, it defaults to the end of @var{str}.  The
shared substring returned by @code{make-shared-substring}
occupies the same storage space as @var{str}.
@end deffn

Example:

@example
(define foo "the quick brown fox")
(define bar (make-shared-substring some-string 4 9))

foo => "t h e   q u i c k   b r o w n   f o x"
bar =========> |---------|
@end example

The shared substring @var{bar} is not given its own storage space.
Instead, the Guile interpreter notes internally that @var{bar} points to
a portion of the memory allocated to @var{foo}.  However, @var{bar}
behaves like an ordinary string in most respects: it may be used with
string primitives like @code{string-length}, @code{string-ref},
@code{string=?}.  Guile makes the necessary translation between indices
of @var{bar} and indices of @var{foo} automatically.

@example
(string-length? bar) @result{} 5	; bar only extends from indices 4 to 9
(string-ref bar 3)  @result{} #\c	; same as (string-ref foo 7)
(make-shared-substring bar 2)
  @result{} "ick"			; can even make a shared substring!
@end example

Because creating a shared substring does not require allocating new
storage from the heap, it is a very fast operation.  However, because it
shares memory with its parent string, a change to the contents of the
parent string will implicitly change the contents of its shared
substrings.

@example
(string-set! foo 7 #\r)
bar @result{} "quirk"
@end example

Guile considers shared substrings to be immutable.  This is because
programmers might not always be aware that a given string is really a
shared substring, and might innocently try to mutate it without
realizing that the change would affect its parent string.  (We are
currently considering a "copy-on-write" strategy that would permit
modifying shared substrings without affecting the parent string.)

In general, shared substrings are useful in circumstances where it is
important to divide a string into smaller portions, but you do not
expect to change the contents of any of the strings involved.


@node Read Only Strings
@subsection Read Only Strings

In previous versions of Guile, there was the idea that some string-based
primitives such as @code{string-append} could equally accept symbols as
arguments.  For example, one could write

@lisp
(string-append '/home/ 'vigilia)
@end lisp

@noindent
and get @code{"/home/vigilia"} as the result.  The term @dfn{read only
string} was adopted to describe the argument type expected by such
primitives.

This idea has now been removed.  The predicate @code{read-only-string?}
still exists, but deprecated, and is equivalent to

@lisp
(lambda (x) (or (string? x) (symbol? x)))
@end lisp

@noindent
But no Guile primitives now use @code{read-only-string?} to validate
their arguments.

String-based primitives such as @code{string-append}
now require strings:

@lisp
(string-append '/home/ 'vigilia)
@result{}
ERROR: Wrong type argument (expecting STRINGP): /home/
@end lisp

@deffn {Scheme Procedure} read-only-string? obj
Return @code{#t} if @var{obj} is either a string or a symbol,
otherwise return @code{#f}.
@end deffn