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21.6.5 Function Slots and Property Lists

In traditional Lisp dialects, symbols are often understood as having three kinds of value at once:

• a variable value, which is used when the symbol appears in code in a variable reference context
• a function value, which is used when the symbol appears in code in a function name position (i.e. as the first element in an unquoted list)
• a property list value, which is used when the symbol is given as the first argument to Lisp's put or get functions.

Although Scheme (as one of its simplifications with respect to Lisp) does away with the distinction between variable and function namespaces, Guile currently retains some elements of the traditional structure in case they turn out to be useful when implementing translators for other languages, in particular Emacs Lisp.

Specifically, Guile symbols have two extra slots. for a symbol's property list, and for its "function value." The following procedures are provided to access these slots.

symbol-fref symbol	Scheme Procedure
scm_symbol_fref (symbol)	C Function

Return the contents of symbol's function slot.

symbol-fset! symbol value	Scheme Procedure
scm_symbol_fset_x (symbol, value)	C Function

Set the contents of symbol's function slot to value.

symbol-pref symbol	Scheme Procedure
scm_symbol_pref (symbol)	C Function

Return the property list currently associated with symbol.

symbol-pset! symbol value	Scheme Procedure
scm_symbol_pset_x (symbol, value)	C Function

Set symbol's property list to value.

symbol-property sym prop

Scheme Procedure

From sym's property list, return the value for property prop. The assumption is that sym's property list is an association list whose keys are distinguished from each other using equal?; prop should be one of the keys in that list. If the property list has no entry for prop, symbol-property returns #f.

set-symbol-property! sym prop val

Scheme Procedure

In sym's property list, set the value for property prop to val, or add a new entry for prop, with value val, if none already exists. For the structure of the property list, see symbol-property.

symbol-property-remove! sym prop

Scheme Procedure

From sym's property list, remove the entry for property prop, if there is one. For the structure of the property list, see symbol-property.

Support for these extra slots may be removed in a future release, and it is probably better to avoid using them. (In release 1.6, Guile itself uses the property list slot sparingly, and the function slot not at all.) For a more modern and Schemely approach to properties, see Object Properties.