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Source for wiki SymbolsCowan version 3

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cowan

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127.10.177.1

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SymbolsCowan

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This WG2 proposal extends Scheme symbols in a variety of traditional Lisp ways: it provides uninterned symbols and property lists.  It can be implemented portably on top of standard Scheme symbols, provided that the `symbol?`, `symbol=?`, and `symbol->string` procedures are redefined to deal with uninterned symbols as well as standard symbols.  For a high-quality implementation, a record facility is needed for uninterned symbols, and a hash table or similar lookup table to associate standard symbols with their property lists.

== Uninterned symbols ==

Uninterned symbols are like standard symbols, except that they have two names, a unique name and a human-readable name (which need not be unique).  Implementations need not guarantee that the unique name is truly unique, but should make a best effort to do so, such as by using a large pseudo-random string of characters as the name.

`(symbol? `''obj''`)`

Returns `#t` if ''obj'' is either a standard symbol or an uninterned symbol, and `#f` otherwise.   

`(standard-symbol? `''obj''`)`

Returns `#t` if ''obj'' is a standard symbol, and `#f` otherwise.

`(uninterned-symbol? `''obj''`)`

Returns `#t` if ''obj'' is an uninterned symbol, and `#f` otherwise.

`(symbol=? `''symbol1 symbol2'' ... `)`

Returns `#t` if all the ''symbol''s have the same unique name in the sense of `string=?`, and `#f` otherwise.

`(symbol->string `''symbol''`)`

Returns the name of ''symbol'' as a string if it is a standard symbol, or its human-readable name if it is an uninterned symbol.  It is an error to mutate this string.

`(string->symbol `''string''`)`

Returns the standard symbol whose name is ''string''.  (Same as the standard version of this procedure.)

`(string->uninterned-symbol `''string''`)`

Returns a newly allocated uninterned symbol whose human-readable name is ''string'', and whose unique name is chosen by the implementation.

`(symbol-unique-name `''symbol''`)`

Return the name of ''symbol'' if it is a standard symbol, or its unique name if it is an uninterned symbol.

`(gensym ` [ ''string'' ] `)`

Returns a newly allocated uninterned symbol whose human-readable name is distinct from that of any symbol returned by a previous call to `gensym`.  If ''string'' is present, it is used as a prefix of the human-readable name.

== Property lists ==

A property list (plist) is a list that can be bound to a symbol.  The original value of a symbol's plist is implementation-dependent.  The structure of a plist is a sequence of symbols known as ''indicators'', some of which may be followed by an arbitrary object known as the indicator's ''value''.  Note that plists are a list of objects, unlike alists which are a list of pairs.

`(get-property `''symbol indicator'' [ ''obj'' ] `)`

If a symbol that is `symbol=?` to ''indicator'' is found on the list, the next object on the plist of ''symbol'' is returned; it is an error if there is no next object.  If there is not, ''obj'' is returned if it is present, and if absent, `#f` is returned.

`(get-property-list `''symbol indicator''`)`

If a symbol that is `symbol=?` to ''indicator'' is found on the plist of ''symbol'', the rest of the plist is returned.  If there is not, `#f` is returned.

`(set-property! `''symbol indicator obj''`)`

If a symbol that is `symbol=?` to ''indicator'' is found on the plist of ''symbol'', the next object on the plist is destructively replaced with ''obj''; it is an error if there is no next object.  If there is not, ''indicator'' and ''obj'' are consed on to the beginning of the plist.   An unspecified value is returned.

`(remove-property! `''symbol indicator''`)`

If a symbol that is `symbol=?` to ''indicator'' is found on the plist of ''symbol'', the indicator and the next object on the list are destructively removed from the plist and `#t` is returned; it is an error if there is no next object.  If there is not, nothing is done, and `#f` is returned.

`(symbol-property-list `''symbol''`)`

Returns the plist associated with ''symbol'' without copying it.  The caller is free to mutate the returned list.

`(symbol-property-list-set! `''symbol list''`)`

The plist associated with ''symbol'' is replaced with ''list'', which is not copied.  An unspecified value is returned.

`(symbol-add-indicators! `''symbol-list indicator''`)`

''Symbol-list'' is a list of symbols.  If a symbol that is `symbol=?` to ''indicator'' is found on the plist of ''symbol'', nothing is done.  If not, ''indicator'' is consed on to the beginning of the plist.  An unspecified value is returned.

`(symbol-remove-indicators! `''symbol-list indicator''`)`

''Symbol-list'' is a list of symbols.  If a symbol that is `symbol=?` to ''indicator'' is found on the plist of ''symbol'', the indicator is destructively removed from the plist.  If not, ''indicator'' is consed on to the beginning of the plist.  An unspecified value is returned.

== Symbol utilities ==

`(symbol-append ` ''symbol'' ... `)`

Returns a standard symbol whose name is the concatenation (as if by `string-append`) of the names of the ''symbol''s.  This may be used to construct pseudo-structured symbol names; for example, `(symbol-append prefix ":" name)`.

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2013-11-06 06:27:21