THE EVALUATOR

Strings, characters, numbers of any type, objects, arrays, structures, streams, subrs, fsubrs and closures evaluate to themselves.

Symbols act as variables and are evaluated by retrieving the value associated with their current binding.

Lists are evaluated by examining the first element of the list and then taking one of the following actions:

• If it is a symbol, the functional binding of the symbol is retrieved.
• If it is a lambda expression, a closure is constructed for the function described by the lambda expression.
• If it is a subr, fsubr or closure, it stands for itself.
• Any other value is an error.

• If it is a subr or closure, the remaining list elements are evaluated and the subr or closure is applied to these evaluated expressions.
• If it is an fsubr, the fsubr is called with the remaining list elements as arguments (unevaluated).
• If it is a macro, the macro is expanded with the remaining list elements as arguments (unevaluated). The macro expansion is then evaluated in place of the original macro call. If the symbol *displace-macros* is not NIL, then the expanded macro will (destructively) replace the original macro expression. This means that the macro will only be expanded once, but the original code will be lost. The displacement will not take place unless the macro expands into a list. The standard XLISP practice is the macro will be expanded each time the expression is evaluated, which negates some of the advantages of using macros.