This WG2 proposal extends Scheme symbols in a variety of traditional Lisp ways: it provides uninterned symbols, symbol values, 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 values and property lists.
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 symbols 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.
(symbol-ref symbol)
Returns an object which is the value of symbol. The value may or may not have anything to do with the global binding of the identifier corresponding to symbol.
(symbol-set! symbol obj)
Changes the value of symbol to obj. This may or may not affect the global binding of the identifier corresponding to symbol. An unspecified value is returned.
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-append symbol ... )
Returns a standard symbol whose name is the concatenation (as if by string-append) of the names of the symbols. This may be used to construct pseudo-structured symbol names; for example, (symbol-append prefix ":" name).