The variable *EMPTY* is initialized to a unique object (a list cell whose car is *EMPTY* -- this is so that no other object can be EQ to it, but it can be easily recognized when printed) which is used to initialize components of structures. (We will see later how such structures are defined.) We do not use, say, NIL to represent an empty component because NIL might be a meaningful value for that component. The predicate EMPTY is true of the unique object.
TRIVFN is a predicate which is true of "trivial" functions. A function is trivial if it is a MacLISP primitive (an EXPR, SUBR, or LSUBR), or has been declared to be primitive via a *EXPR or *LEXPR proclamation.
(INCREMENT FOO) expands into the code (ASET' F00 (+ F00 1)).
CATENATE is a utility macro which may be thought of as a function. Given any number of S-expressions it produces an atomic symbol whose print name is the concatenation of the print names of the S-expressions. Usually the S-expressions will be atomic symbols or numbers.
(CATENATE 'F00 '- 43) => F00-43
GENTEMP is used to generate a new unique symbol, given a specified prefix. The global variable *GENTEMPNUM* starts at zero and increases
monotonicially. Each call to GENTEMP catenates the prefix, a hyphen, and a new
value of *GENTEMPNUM*. Because the numeric suffixes of the generated symbols
increase with time, one can determine in which order symbols were generated. We
also will use different prefixes for different purposes, so that one can tell
which part of the compiler generated a given symbol. This information can be
invaluable for debugging purposes; from the names of the symbols appearing in a
data structure, one can determine how that structure was created and in what
order. (The generated symbols are themselves used primarily as simple markers,
or as simple structures (property lists). The use of the print names amounts to
tagging each marker or structure with a type and a creation timestamp. A LISP-like language encourages the inclusion of such information.)
(GENTEMP 'NODE) => NODE-2534
A list of all generated symbols is maintained in *GENTEMPLIST*. GENFLUSH can be called to excise all generated symbols from the MacLISP obarray; this is periodically necessary when compiling a large file so that unneeded symbols may be garbage-collected. The symbols are initially interned on the obarray in the first place for ease of debugging (one can refer to them by name from a debugging breakpoint). GEN-GLOBAL-NAME is used to generate a symbol to be used as a run-time name by the compiled code. The prefix for such names is initially "?" for testing purposes, but is initialized by the file transducer as a function of the name of the file being compiled. This allows separately compiled files to be loaded together without fear of naming conflicts.
