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lisp(3gv) lisp(3gv)
NAME
geomview lisp interpreter
NOTE
This document describes the geomview 1.3 lisp interpreter.
This version is incompatible with previous versions in
several ways. Since the previous one was used mostly just
by Geometry Center staff, I am not going to write a docu-
ment detailing the changes. The geomview lisp interpreter
is not very well documented in general because I am
strongly considering phasing it out and replacing it with
a real lisp interpreter in a future version of geomview.
If you have any questions about the current version or how
to convert programs from an older version please contact
me directly [ mbp@geomtech.com ].
SYNOPSIS
#include "lisp.h"
void LInit();
Lake * LakeDefine(FILE *streamin, FILE *streamout, void *river);
void LakeFree(Lake *lake);
LObject * LNew(LType *type, LCell *cell);
LObject * LRefIncr(LObject *obj);
void LRefDecr(LObject *obj);
void LWrite(FILE *fp, LObject *obj);
void LFree(LObject *obj);
LObject * LCopy(LObject *obj);
LObject * LSexpr(Lake *lake);
LObject * LEval(LObject *obj);
LObject * LEvalSexpr(Lake *lake);
LList * LListNew();
LList * LListAppend(LList *list, LObject *obj);
void LListFree(LList *list);
LList * LListCopy(LList *list);
LObject * LListEntry(LList *list, int n);
int LListLength(LList *list);
int LParseArgs(char *name, Lake *lake, LList *args, ...);
int LDefun(char *name, LObjectFunc func, char *help);
void LListWrite(FILE *fp, LList *list);
LInterest * LInterestList(char *funcname);
LObject * LEvalFunc(char *name, ...);
int LArgClassValid(LType *type);
void LHelpDef(char *key, char *message);
LDEFINE(name, ltype, doc)
LDECLARE((name, LBEGIN, ..., LEND));
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DESCRIPTION
Geomview contains a minimal lisp interpreter for parsing
and evaluating commands. This lisp interpreter is part of
the "-loogutil" library and thus any program which links
with this library may use the interpreter. This provides
a simple but powerful way to build up a command language.
This manual page assumes that you are familiar with the
syntax of lisp. The first part describes the basics of
using the interpreter. Some gory details that don't con-
cern most users then follow.
The main steps in using the lisp interpreter are
1. call Linit() to initialize the interpreter 2. make calls to LDefun(), one for each lisp function you want the interpreter to know about 3. define the "i/o lake" 4. parse input with calls to LSexpr() and evaluate the resulting lisp objects with LEval() (or use LEvalSexpr() to combine both steps).
For example the following code defines a single function
"f" and executes commands from standard input:
#include "lisp.h" Lake *lake; LObject *obj, *val;
LInit(); LDefun("f", f, NULL); lake = LakeDefine(stdin, stdout, NULL); while (!feof(stdin)) { obj = LSexpr(lake); val = LEval(obj); LFree(obj); LFree(val); }
The second argument to LDefun() is a function pointer;
LDefun() sets up a correspondence between the string "f"
and the function f, which is assumed to have been previ-
ously declared. The section FUNCTION DEFINITIONS below
gives the expected call syntax and behavior of such func-
tions. (The third argument to LDefun() is a pointer to a
string which documents the function and may be NULL if you
don't care about documentation.) LakeDefine() defines an
i/o lake; this is a generalization of the notion of an i/o
stream. Most programs don't need to use the generaliza-
tion, though, and can simply pass FILE pointers as LakeDe-
fine()'s first two arguments and NULL as the third one.
The section LAKES below gives the details for those who
are interested. LSexpr() [which stands for Lisp Symbolic
EXPRession] parses a single lisp expression from the lake,
returning a lisp object which represents that expression.
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The lisp object is returned as an LObject pointer, which
points to an opaque structure containing a representation
of the expression. LEval() then evaluates the object; it
is during the call to LEval() that the action of the
expression takes place. Note that the last two lines of
code in this example could have been replaced by the sin-
gle line LEval(LSexpr(lake)) or, more efficiently, by LEv-
alSexpr(lake).
FUNCTION DEFINITIONS
The functions defined by calls to LDefun() are expected to
have a certain call syntax; LEval() calls them when it
encounters a call to the lisp function named with the cor-
responding string. The macro LDEFINE is provided for
declaring them. For example:
LDEFINE(f, LSTRING, "(f a b) returns a string representing the0um of the integer a with the floating point number b.") { int a; float b; char buf[20], *s;
LDECLARE(("f", LBEGIN, LINT, &a, LFLOAT, &b, LEND)); sprintf(buf,"%f",a+b); s = strdup(buf); return LNew(LSTRING, &s); }
The important things about this function are:
1. It is declared with the LDEFINE macro, the general syntax of which is LDEFINE(name, type, helpstr). name should be a valid C identifer and will be used to construct the actual name of the C function by prepending an 'L' and the name of the help string by prepending an 'H'. type should be a lisp object type identifier (see below) and determines the type of object that the function returns. helpstr is a documentation string.
2. The use of the LDECLARE macro. More about this below.
3. It returns an LObject *. All lisp functions must actually return a value. If you don't care what value they return you can return one of the pre-defined values Lnil or Lt (and specify LVOID as the type in the LDEFINE header).
This particular example is a function which takes two
arguments, an int and a float, and returns a string object
representing their sum. A lisp call to this function
might look like "(f 1 3.4)".
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The LDECLARE macro, defined in lisp.h, sets up the corre-
spondence between variables in the C code and arguments in
the lisp call to the function. Note that the arguments to
LDECLARE are delimited by *two* pairs of parentheses (this
is because C does not allow macros with a variable number
of arguments; LDECLARE thus actually takes one argument
which is a parenthesized list of an arbitrary number of
items). The general usage of LDECLARE is
LDECLARE(( name, LBEGIN, <argspec>, ..., LEND ));
where name is the name of the function (as specified to
LDefun()). <argspec> is an argument specification, which
in general consists of a lisp type identifier followed by
an address. The identifier indicates the data type of the
argument. The builtin type identifiers are LINT (inte-
ger), LFLOAT (float), LSTRING (string), LLOBJECT (lisp
object), and LLIST (lisp list). Applications may define
additional types whose identifiers may also be used here;
see the section CUSTOM LISP TYPES below for details.
There may be any number of <argspec>'s; the last must be
followed by the special keyword LEND.
STOP HERE
Most users of the lisp interpreter can stop reading this
man page here. What follows is only used in advanced sit-
uations.
EVALUATION OF FUNCTION ARGUMENTS
Normally the lisp interpreter evaluates function arguments
before passing them to the function; to prevent this eval-
uation from happening you can insert the special token
LHOLD in an LDECLARE argument specification before the
type keyword. For example
LHOLD, LLIST, &list,
specifies an unevalutated list argument. This feature is
really useful only for LLIST, LLOBJECT, and array types
(see below) since the other types evalutate to themselves.
ARRAYS
In general an <argspec> in the LDECLARE call consists of a
keyword followed by the address of a scalar data type.
Since it is relatively common to use a lisp list to repre-
sent an array of values, however, the special <argspec>
keyword LARRAY is provided for dealing with them. It has
a different syntax: it should be followed by a lisp type
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identifier which specifies the type of the elements of the
array and then by two addresses --- the address of an
array and the address of an integer count. Upon entry to
LDECLARE the count specifies how many elements may be
written into the array. LDECLARE then modifies this num-
ber to indicate the number of entries actually parsed.
For example:
LDEFINE(myfunc, ...) { float f[2]; int fn = 2; LDECLARE(("myfunc", LEBGIN LHOLD, LARRAY, f, &fn, LEND)); /* at this point the value of fn has been modified to be the number of entries actually appearing in the list argument; and this number of values have been written into the array f. */ ... }
defines a function "myfunc" which takes a list of up to 2
floats as its only argument. Valid calls to "myfunc"
would be "(myfunc ())", "(myfunc (7))", and "(myfunc
(7
8))".
Note the use of LHOLD; this is necessary because otherwise
the lisp system would attempt to evaluate the list as a
function call before passing it off to myfunc.
OPTIONAL ARGUMENTS
Normally the lisp interpreter will generate (to stderr) a
reasonable error message if a function is called with
fewer arguments than were specified in LDECLARE. Some
functions, however, may have some arguments that are
optional. You can define a function which takes optional
arguments by putting the keyword LOPTIONAL after the last
required argument in the LDECLARE call. Any arguments
specified in the list after that are considered optional;
the interpreter doesn't complain if they are not supplied.
Note that all optional arguments must come after all
required arguments.
Normally excess arguments also elicit an error message.
The LREST keyword allows control over this situation. If
LREST is followed by a pointer to an LList * variable,
then trailing arguments are parsed, evaluated (unless
LHOLD was used), and the list of them is stored in the
given variable. (Note that the value is an LList, not an
LObject of type LLIST -- if there are no excess arguments,
the value is NULL, not an empty LLIST.) If LREST is fol-
lowed by a NULL pointer, excess arguments are silently
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ignored. LREST might be useful when a function's argument
types are not known. It's not necessary to specify LEND
after LREST.
LISP OBJECTS
The basic data type of the lisp interpreter is the lisp
object; it is represented by an LObject pointer, which
points to an opaque data structure. The functions for
manipulating lisp objects (i.e. the object's methods) are:
LNew(): creates a new lisp object of the given type with the given value. The "type" argument is one of the values LSTRING or LLIST, or a type pointer defining a custom object type (see CUSTOM OBJECT TYPES below). LRefIncr(): increments the reference count of a lisp object. The lisp interpreter uses the convention that when a procedure returns a lisp object, the caller owns the object and thus has responsibility for freeing it. LRefIncr() can be used to increment the reference count of an existing object about to be returned. New objects created by LNew() have their reference count initialized to 1 and hence do not need to be LRefIncr()'ed. LRefDecr(): decrements the reference count of a lisp object. This should probably not be called by application programs; it is used internally. LWrite(): writes a formatted string representation of a lisp object to a stream. LFree(): free the space assoicated with a lisp object LCopy(): construct a copy of a lisp object
CUSTOM OBJECT TYPES
In addition to the predefined lisp object types you may
define your own custom types. This is done by construct-
ing a structure containing various function pointers for
manipulating objects of your new type. The address of
this structure is then the type identifier for this type
and may be used in LDECLARE <argspec>'s and in LNew()
calls. (The type names LINT, LSTRING and the other
builtin types are actually pointers to predefined struc-
tures.) The structure is of type LType as defined in
lisp.h:
struct LType {
/* name of type */ char *name;
/* size of corresponding C type */ int size;
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/* extract cell value from obj */ int (*fromobj)(/* LObject *obj, void *x */);
/* create a new LObject of this type */ LObject *(*toobj)(/* void *x */);
/* free a cell of this type */ void (*free)(/* void *x */);
/* write a cell value to a stream */ void (*write)(/* FILE *fp, void *x */);
/* test equality of two cells of this type */ int (*match)(/* void *a, void *b */);
/* pull a cell value from a va_list */ void (*pull)(/* va_list *a_list, void *x */);
/* parse an object of this type */ LObject *(*parse)(/* Lake *lake */);
/* magic number; always set to LTypeMagic */ int magic;
};
The void * pointers in the above point to objects of the
type you are defining. For examples of how to define new
types see the code in lisp.c that defines the string and
list types. See also the file TYPES.DOC in the lisp
source code directory for further details.
LISTS
The LList pointer is used to refer to objects of type
LLIST, which implement a linked list. The operations on
these objects are LListNew(), LListLength(), LListEntry(),
LListAppend(), LListCopy(), and LListFree(). These are
mostly used internally by the lisp system but are avail-
able for outside use. Maybe I'll write more documentation
for them later if it seems necessary.
LAKES The Lake structure is a generalization of an input stream.
It contains three members: an input FILE pointer
("streamin"), an output FILE pointer ("streamout"), and
an
arbitrary pointer ("river"). The input FILE pointer is
required; the lisp interpreter assumes that every lake has
a valid input file pointer. The output FILE pointer is
required if you do any operations that result in the
intepreter producing any output. The third pointer may
point to whatever you want. The lisp interpreter itself
does not directly refer to this pointer. It may be used
by the parser that you supply when defining a new lisp
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object type.
The term "Lake" is supposed to connote something more gen-
eral than a stream; it also seemed particularly appropri-
ate since this interpreter was written in the City of
Lakes.
HIDDEN LAKE ARGUMENTS AND OTHER WET THINGS
This section is X rated. Don't read it unless you are
really serious.
The lisp interpreter works by first parsing (LSexpr()) an
expression then evaluating it (LEval()). The LDECLARE
macro is a mechanism which allows both the syntax (for
parsing) and the semantics (for evaluation) of an expres-
sion to be specified in the same convenient place --- at
the top of the C function which implements the function.
The call syntax of all such C functions is
LObject *func(Lake *lake, LList *args)
When parsing a call to the corresponding lisp function,
LSexpr() calls func with that lake pointer, and with args
pointing to the head of the list in the parse tree corre-
sponding to this function call. LDECLARE parses the argu-
ments in the call (by reading them from the lake) and
appends them to this list. (Note: the head of this list
is the function itself, so the first argument becomes
entry #2 in the list.)
When evaluating the function call, LEval() calls func with
lake=NULL and with args pointing to the call's argument
list. (In this case the first entry of the list is the
first argument.) LDECLARE then converts the arguments in
the list into the appropriate C data types, writing their
values into the addresses in the <argspec>s.
One side-effect of using lake=NULL as the signal to evalu-
ate rather than to parse is that the value of the lake
pointer is not available at evaluation time. Some func-
tions, however, may want to do something with the lake
they were parsed from. For example, the "write" function
in geomview writes data to the output stream associated
with its input stream. (In geomview these streams are all
stored in a general "Pool" structure which is retained as
the "river" member of the lake.) The special token LLAKE
may be used to cause the lake pointer to be saved in the
args list at parse time and written into a variable at
evaluation time. It is used exactly like the other
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(scalar) argument keywords:
LObject *func(Lake *lake, LList *args) Lake *mylake; LDECLARE(("myfunc", LBEGIN LARG_LAKE, &mylake, ... LARG_END));
At evaluation time LDECLARE will set mylake to have the
value that lake had at parse time. This looks just like a
specification for an argument to the lisp function but it
is not --- it is just a way to tell LDECLARE to remember
the lake pointer between parse- and evaluation-time.
BUGS
The documentation is incomplete.
AUTHOR
The lisp interpreter was written mostly by Mark Phillips
with lots of input and moral support from Stuart Levy and
Tamara Munzner.
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