MTS Volume 8 - LISP and SLIP Page 88 (88 of 160)

MTS 8: LISP and SLIP in MTS
Page Revised January 1983 June 1976
88 LISP

MTS 8: LISP and SLIP in MTS
June 1976
THE LISP EDITOR _______________
INTRODUCTION ____________
The LISP editor is a LISP program designed to examine and modify LISP
data structures, especially function definitions, as they exist within
the LISP interpreter. It does not edit MTS files, although there are
LISP editor commands which will read and write files on request. In the
following section, the term "expression" refers to any LISP data
structure (which is also known as an S-expression, denoting its external
representation).
The LISP editor has been checkpointed into the public file *LISPLIB.
It is automatically restored and invoked, using a command of the form
(EDIT fn)
The value of EDIT is NIL. The argument "fn" determines the expression
to edited as follows:
(1) If "fn" is an atom with an EXPR on its property list, then the
value of the EXPR property (normally a LAMBDA expression) is the
expression to be edited. This is the normal method of editing
function definitions.
(2) Otherwise, "fn" itself is the expression to be edited. Since
EDIT is an FSUBR, "fn" is not evaluated. Thus, the above form
is not normally useful for editing arbitrary structures. APPLY
or APPLY1 should be used instead, e.g.,
(APPLY1 ’EDIT list)
where the value of "list" is the expression to be edited.
The editor has its own command language; one or more commands may be
entered on each line, separated by blanks. There are several variable
length commands which must be separated from any subsequent commands on
the same line by a colon (:). These are INSERT, DELETE, EXTRACT, ML,
MR, BI, and BO, and are described in the subsection "Commands that
Modify the Current Expression." Commands are read from *SOURCE* using
the prefix character period (.), and output is written on *SINK*, using
the prefix colon.
All editor commands operate on a subexpression of the original
argument to EDIT; this subexpression is called the current expression. _______ __________
There are several commands available for specifying the current expres-
sion. They are described in later in this section.
The LISP Editor 89

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MTS 8: LISP and SLIP in MTS
Page Revised January 1983 June 1976
88 LISP

MTS 8: LISP and SLIP in MTS
June 1976
THE LISP EDITOR _______________
INTRODUCTION ____________
The LISP editor is a LISP program designed to examine and modify LISP
data structures, especially function definitions, as they exist within
the LISP interpreter. It does not edit MTS files, although there are
LISP editor commands which will read and write files on request. In the
following section, the term "expression" refers to any LISP data
structure (which is also known as an S-expression, denoting its external
representation).
The LISP editor has been checkpointed into the public file *LISPLIB.
It is automatically restored and invoked, using a command of the form
(EDIT fn)
The value of EDIT is NIL. The argument "fn" determines the expression
to edited as follows:
(1) If "fn" is an atom with an EXPR on its property list, then the
value of the EXPR property (normally a LAMBDA expression) is the
expression to be edited. This is the normal method of editing
function definitions.
(2) Otherwise, "fn" itself is the expression to be edited. Since
EDIT is an FSUBR, "fn" is not evaluated. Thus, the above form
is not normally useful for editing arbitrary structures. APPLY
or APPLY1 should be used instead, e.g.,
(APPLY1 ’EDIT list)
where the value of "list" is the expression to be edited.
The editor has its own command language; one or more commands may be
entered on each line, separated by blanks. There are several variable
length commands which must be separated from any subsequent commands on
the same line by a colon (:). These are INSERT, DELETE, EXTRACT, ML,
MR, BI, and BO, and are described in the subsection "Commands that
Modify the Current Expression." Commands are read from *SOURCE* using
the prefix character period (.), and output is written on *SINK*, using
the prefix colon.
All editor commands operate on a subexpression of the original
argument to EDIT; this subexpression is called the current expression. _______ __________
There are several commands available for specifying the current expres-
sion. They are described in later in this section.
The LISP Editor 89

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MTS 8: LISP and SLIP in MTS
June 1976
Ignoring dotted pairs for the moment, any proper subexpression of a
LISP expression (hence any current expression) must be one of the
following:
(1) An atom.
(2) A list which is an element of some higher level list.
(3) A proper sublist, which will be referred to as the tail of a ____
list.
For example, given the expression (A (B C) D), the atoms are A, B, C,
and D; the list (B C) is an element of the top-level list; the tails of
(A (B C) D) are ((B C) D) and (D); and the tail of (B C) is (C).
Dotted pairs (expressions whose CDR are atoms other than NIL) have
not been adequately described, and indeed, the behavior of the editor is
erratic at best when it encounters a dotted pair.
For the purpose of this description, editor commands are divided into
five major groups: printing commands, commands specifying the current
expression, commands modifying the current expression, commands undoing
previous modifications, and miscellaneous commands.
COMMANDS THAT PRINT THE CURRENT EXPRESSION __________________________________________
Command: P [n]
The P command prints the current expression up to level "n". The
optional argument n defaults to 2. To avoid excessive output,
lists at level n are printed as the character ampersand (&). A
current expression which is a tail of some list is indicated by
ellipsis marks (...) preceding the expression.
A P command is assumed at the end of every line unless a P, ?, or
PP command is the last command on the line.
Command: ?
The ? command is equivalent to a P 1000 command; it effectively
prints the entire current expression. This command is used in most
of the examples in this section.
Command: PP
The PP command also prints the entire current expression, but in an
indented format which makes the structure more clearly visible.
The PP command does not print ellipsis marks for tails of lists.
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MTS 8: LISP and SLIP in MTS
June 1976 Page Revised February 1979
Examples:
.?
:(LAMBDA (X) (COND ((NULL X) NIL) (T (CONS X X))))
.P
:(LAMBDA (X) (COND & &))
.P 3
:LAMBDA (X) (COND (& NIL) (T &)))
.PP
:(LAMBDA (X)
: (COND ((NULL X) NIL)
: (T (CONS X X))))
COMMANDS THAT SPECIFY THE CURRENT EXPRESSION ____________________________________________
Command: [±]n
A positive integer "n" or "+n" selects the nth element of the
current expression, counting from the left, and makes it the
current expression. A negative integer "-n" operates in the same
manner except that it counts from the right. Users should note
that zero is a separate command, described later in this section.
Examples:
.?
:(LAMBDA (X) (COND ((NULL X) NIL) (T (CONS X X))))
.3 ?
:(COND ((NULL X) NIL) (T (CONS X X)))
.2 1 ?
:(NULL X)
Command: UP
If the current expression is an element of a higher-level list, the
UP command specifies the tail of the higher-level list beginning
with the current expression, as the new current expression.
Otherwise, the UP command has no effect. Note that if the current
expression is the first element of a higher-level list, the UP
command produces the entire list, rather than just a tail of the
list.
Examples:
.?
:(LAMBDA (X) (COND ((NULL X) NIL) (T (CONS X X ))))
.3 ?
| :(COND ((NULL X) NIL) (T (CONS X X)))
The LISP Editor 91

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MTS 8: LISP and SLIP in MTS
Page Revised January 1983 June 1976
(DECLARE F =CHECK)
then when it is compiled, the generated code will include a
test to check that the correct number of arguments was passed
to the function, and various other error checks (for valid
atomic, list, or numeric arguments, etc.) will be made.
Normally, little or no error checking is performed by compiled
programs.
If the variable =CHECK is set to T, all functions will be
compiled as if they had been declared =CHECK.
Functions Known to the Compiler
The following functions have fixed definitions within compiled
code:
ABS ADD ADD1 AND APPLY APPLY1 ARG ASSOC ATOM C...R COND CONS
IDIVIDE EQ EQUAL EVAL EVEN GET GO GRAFT GREATER LAND LESS
LENGTH LIST LOR LXOR MAP MAPC MAPCAN MAPCAR MAPCON MAPLIST MAX
MEMBER MIN MINUS NOT NTH NUMBER OR PROG PROGN PUT QUOTE REMAIN
REPEAT RETURN REVERSE SELECT SET SETA SETQ SHIFT SUB SUB1
TIMES UNCONS ZERO
The following functions are declared to be fixed-link functions:
| CHECKPOINT, COPY, DECLARE, DEFUN, DISPLAY, LABEL, MODIFY,
| NEWWORLD, OBLIST, OPEN, REALWORLD, REM, REMOB, SETA2, SETQ2,
| STATUS, TRACE, UNTRACE,
Limitations and Warnings
(1) General Warning:
Compiled functions do a minimum of error checking, so users are
advised to debug their programs before compiling them. Unless
the =CHECK option is used, the normal checks for undefined
variables, bad atomic, list, and numeric arguments, array
dimensions exceeding legal limits, etc., do not occur in
compiled code.
(2) Since fixed-link functions are not executed under control of
EVAL, debugging features such as TRACE, BUG, and STEP cannot be
used with them.
(3) An APPLY of a no-spread macro or LAMBDA-expression cannot be
compiled. (Use APPLY1 instead.)
(4) Functions used as macros may not be recursive. An attempt to
compile a recursive MACRO will generate an error.
86 LISP

MTS 8: LISP and SLIP in MTS
June 1976 Page Revised January 1983
(5) Each internal block must be less than one page in length. If
the compiler finds that a block is too long, it will terminate
with an error message. The user can then break the program
downinto smaller blocks and recompile. Note: Experience indi-
cates that about 50-75 lines of LISP code (uninterrupted by
internal blocks) will compile into one page.
(6) The total number of blocks in one compiled routine cannot exceed
150.
(7) The number of bytes in the first block of a program, plus the
number of special variables, times four, must be less than 4096.
(8) The number of local variables defined at one time cannot exceed
1020.
(9) The compiler occupies 40 pages of memory, and compilation of
even a small program is likely to increase storage to 50 pages.
After EXCISing (using the =EXCISE function described above) the
compiler, the user may want to compress his core usage by
performing a CHECKPOINT.
(10) Functions which depend on calls to EVAL, such as UNEVAL and
RETURN with a second argument, may not operate in the same
manner when called from a compiled program, since many function
calls are compiled directly and do not generate calls to EVAL.
(11) The compiler does not process the LABEL function, hence the use
of LABELs in compiled programs is not recommended. If a LABEL
is encountered, the compiler generates a call to the
interpreter.
Other Special Features ______________________
The functions EDIT and DEBUG provide access to a LISP editor and
debugging package. These features are documented in the sections "The
LISP Editor" and "LISP Debugging Facilities" in this volume.
LISP 87

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