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Simon Brooke a0fc8a3e67 Documentation changes only, chiefly spelling/formatting

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beowulf

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LISP 1.5 is to all Lisp dialects as Beowulf is to English literature.

What this is
Installation
- Input/output
  - SYSOUT
  - SYSIN
Learning Lisp 1.5
Other Lisp 1.5 resources
- Other implmentations
- History resources
License

Table of contents generated with markdown-toc

What this is

A work-in-progress towards an implementation of Lisp 1.5 in Clojure. The objective is to build a complete and accurate implementation of Lisp 1.5 as described in the manual, with, in so far as is possible, exactly the same behaviour — except as documented below.

BUT WHY?!!?!

Because.

Because Lisp is the only computer language worth learning, and if a thing is worth learning, it's worth learning properly; which means going back to the beginning and trying to understand that.

Because there is, so far as I know, no working implementation of Lisp 1.5 for modern machines.

Because I'm barking mad, and this is therapy.

Status

Working Lisp interpreter, but some key features not yet implemented.

Project Target

The project target is to be able to run the Wang algorithm for the propositional calculus given in chapter 8 of the Lisp 1.5 Programmer's Manual. When that runs, the project is as far as I am concerned feature complete. I may keep tinkering with it after that and I'll certainly accept pull requests which are in the spirit of the project (i.e. making Beowulf more usable, and/or implementing parts of Lisp 1.5 which I have not implemented), but this isn't intended to be a new language for doing real work; it's an educational and archaeological project, not serious engineering.

Some readline-like functionality would be really useful, but my attempt to integrate JLine has not (yet) been successful.

An in-core structure editor would be an extremely nice thing, and I may well implement one.

You are of course welcome to fork the project and do whatever you like with it!

Invoking

Invoke with

java -jar target/uberjar/beowulf-0.3.0-standalone.jar --help

(Obviously, check your version number)

Command line arguments as follows:

  -h, --help                               Print this message
  -p PROMPT, --prompt PROMPT               Set the REPL prompt to PROMPT
  -r INITFILE, --read SYSOUTFILE           Read Lisp sysout from the file SYSOUTFILE 
                                           (defaults to `resources/lisp1.5.lsp`)
  -s, --strict                             Strictly interpret the Lisp 1.5 language, 
                                           without extensions.

To end a session, type STOP at the command prompt.

Building and Invoking

Build with

lein uberjar

Reader macros

Currently SETQ and DEFUN are implemented as reader macros, sort of. It would now be possible to reimplement them as FEXPRs and so the reader macro functionality will probably go away.

Functions and symbols implemented

Function	Type	Signature	Implementation	Documentation
NIL	Lisp variable	?		The canonical empty list. See manual pages 22, 69s
T	Lisp variable	?		The canonical true value. See manual pages 22, 69
F	Lisp variable	?		The canonical false value. See manual pages 22, 69
ADD1	Host lambda function	x:number		Add one to the number `x`.
AND	Host lambda function	expr*	PREDICATE	`T` if and only if none of my `args` evaluate to either `F` or `NIL`, else `F`. In `beowulf.host` principally because I don't yet feel confident to define varargs functions in Lisp.
APPEND	Lisp lambda function	?		see manual pages 11, 61
APPLY	Host lambda function	?		Apply this `function` to these `arguments` in this `environment` and return the result. For bootstrapping, at least, a version of APPLY written in Clojure. All args are assumed to be symbols or `beowulf.cons-cell/ConsCell` objects. See page 13 of the Lisp 1.5 Programmers Manual.
ASSOC	Lisp lambda function, Host lambda function	a:list	?	If a is an association list such as the one formed by PAIRLIS in the above example, then assoc will produce the first pair whose first term is x. Thus it is a table searching function. All args are assumed to be `beowulf.cons-cell/ConsCell` objects. See page 12 of the Lisp 1.5 Programmers Manual. NOTE THAT this function is overridden by an implementation in Lisp, but is currently still present for bootstrapping.
ATOM	Host lambda function	x:expr	PREDICATE	Returns `T` if and only if the argument `x` is bound to an atom; else `F`. It is not clear to me from the documentation whether `(ATOM 7)` should return `T` or `F`. I'm going to assume `T`.
CAR	Host lambda function	list		Return the item indicated by the first pointer of a pair. NIL is treated specially: the CAR of NIL is NIL.
CAAAAR	Lisp lambda function	list	?	?
CAAADR	Lisp lambda function	list	?	?
CAAAR	Lisp lambda function	list	?	?
CAADAR	Lisp lambda function	list	?	?
CAADDR	Lisp lambda function	list	?	?
CAADR	Lisp lambda function	list	?	?
CAAR	Lisp lambda function	list	?	?
CADAAR	Lisp lambda function	list	?	?
CADADR	Lisp lambda function	list	?	?
CADAR	Lisp lambda function	list	?	?
CADDAR	Lisp lambda function	list	?	?
CADDDR	Lisp lambda function	list	?	?
CADDR	Lisp lambda function	list	?	?
CADR	Lisp lambda function	list	?	?
CDAAAR	Lisp lambda function	list	?	?
CDAADR	Lisp lambda function	list	?	?
CDAAR	Lisp lambda function	list	?	?
CDADAR	Lisp lambda function	list	?	?
CDADDR	Lisp lambda function	list	?	?
CDADR	Lisp lambda function	list	?	?
CDAR	Lisp lambda function	list	?	?
CDDAAR	Lisp lambda function	list	?	?
CDDADR	Lisp lambda function	list	?	?
CDDAR	Lisp lambda function	list	?	?
CDDDAR	Lisp lambda function	list	?	?
CDDDDR	Lisp lambda function	list	?	?
CDDDR	Lisp lambda function	list	?	?
CDDR	Lisp lambda function	list	?	?
CDR	Host lambda function	list		Return the item indicated by the second pointer of a pair. NIL is treated specially: the CDR of NIL is NIL.
CONS	Host lambda function	expr, expr		Construct a new instance of cons cell with this `car` and `cdr`.
CONSP	Host lambda function	o:expr	?	Return `T` if object `o` is a cons cell, else `F`. NOTE THAT this is an extension function, not available in strict mode. I believe that Lisp 1.5 did not have any mechanism for testing whether an argument was, or was not, a cons cell.
COPY	Lisp lambda function	?		see manual pages 62
DEFINE	Host lambda function	?	PSEUDO-FUNCTION	Bootstrap-only version of `DEFINE` which, post boostrap, can be overwritten in LISP. The single argument to `DEFINE` should be an association list of symbols to lambda functions. See page 58 of the manual.
DIFFERENCE	Host lambda function	x:number, y:number		Returns the result of subtracting the number `y` from the number `x`
DIVIDE	Lisp lambda function	x:number, y:number		see manual pages 26, 64
DOC	Host lambda function	?	?	Open the page for this `symbol` in the Lisp 1.5 manual, if known, in the default web browser. NOTE THAT this is an extension function, not available in strct mode.
EFFACE	Lisp lambda function	?	PSEUDO-FUNCTION	see manual pages 63
ERROR	Host lambda function	?	PSEUDO-FUNCTION	Throw an error
EQ	Host lambda function	?	PREDICATE	Returns `T` if and only if both `x` and `y` are bound to the same atom, else `NIL`.
EQUAL	Host lambda function	?	PREDICATE	This is a predicate that is true if its two arguments are identical S-expressions, and false if they are different. (The elementary predicate `EQ` is defined only for atomic arguments.) The definition of `EQUAL` is an example of a conditional expression inside a conditional expression. NOTE: returns `F` on failure, not `NIL`
EVAL	Host lambda function	?		Evaluate this `expr` and return the result. If `environment` is not passed, it defaults to the current value of the global object list. The `depth` argument is part of the tracing system and should not be set by user code. All args are assumed to be numbers, symbols or `beowulf.cons-cell/ConsCell` objects. However, if called with just a single arg, `expr`, I'll assume it's being called from the Clojure REPL and will coerce the `expr` to `ConsCell`.
FACTORIAL	Lisp lambda function	?	?	?
FIXP	Host lambda function	?	PREDICATE	?
GENSYM	Host lambda function	?		Generate a unique symbol.
GET	Host lambda function	?		From the manual: '`get` is somewhat like `prop`; however its value is car of the rest of the list if the `indicator` is found, and NIL otherwise.' It's clear that `GET` is expected to be defined in terms of `PROP`, but we can't implement `PROP` here because we lack `EVAL`; and we can't have `EVAL` here because both it and `APPLY` depends on `GET`. OK, It's worse than that: the statement of the definition of `GET` (and of) `PROP` on page 59 says that the first argument to each must be a list; But the in the definition of `ASSOC` on page 70, when `GET` is called its first argument is always an atom. Since it's `ASSOC` and `EVAL` which I need to make work, I'm going to assume that page 59 is wrong.
GREATERP	Host lambda function	?	PREDICATE	?
INTEROP	Host lambda function	?	?	?
INTERSECTION	Lisp lambda function	?	?	?
LENGTH	Lisp lambda function	?		see manual pages 62
LESSP	Host lambda function	?	PREDICATE	?
MAPLIST	Lisp lambda function	?	FUNCTIONAL	see manual pages 20, 21, 63
MEMBER	Lisp lambda function	?	PREDICATE	see manual pages 11, 62
MINUSP	Lisp lambda function	?	PREDICATE	see manual pages 26, 64
NOT	Lisp lambda function	?	PREDICATE	see manual pages 21, 23, 58
NULL	Lisp lambda function	?	PREDICATE	see manual pages 11, 57
NUMBERP	Host lambda function	?	PREDICATE	?
OBLIST	Host lambda function	?		Return a list of the symbols currently bound on the object list. NOTE THAT in the Lisp 1.5 manual, footnote at the bottom of page 69, it implies that an argument can be passed but I'm not sure of the semantics of this.
ONEP	Lisp lambda function	?	PREDICATE	see manual pages 26, 64
OR	Host lambda function	?	PREDICATE	`T` if and only if at least one of my `args` evaluates to something other than either `F` or `NIL`, else `F`. In `beowulf.host` principally because I don't yet feel confident to define varargs functions in Lisp.
PAIR	Lisp lambda function	?		see manual pages 60
PAIRLIS	Lisp lambda function, Host lambda function	?	?	This function gives the list of pairs of corresponding elements of the lists `x` and `y`, and APPENDs this to the list `a`. The resultant list of pairs, which is like a table with two columns, is called an association list. Essentially, it builds the environment on the stack, implementing shallow binding. All args are assumed to be `beowulf.cons-cell/ConsCell` objects. See page 12 of the Lisp 1.5 Programmers Manual. NOTE THAT this function is overridden by an implementation in Lisp, but is currently still present for bootstrapping.
PLUS	Host lambda function	?		?
PRETTY		?	?	?
PRINT		?	PSEUDO-FUNCTION	see manual pages 65, 84
PROG	Host nlambda function	?		The accursed `PROG` feature. See page 71 of the manual. Lisp 1.5 introduced `PROG`, and most Lisps have been stuck with it ever since. It introduces imperative programming into what should be a pure functional language, and consequently it's going to be a pig to implement. Broadly, `PROG` is a variadic pseudo function called as a `FEXPR` (or possibly an `FSUBR`, although I'm not presently sure that would even work.) The arguments, which are unevaluated, are a list of forms, the first of which is expected to be a list of symbols which will be treated as names of variables within the program, and the rest of which (the 'program body') are either lists or symbols. Lists are treated as Lisp expressions which may be evaulated in turn. Symbols are treated as targets for the `GO` statement. GO: A `GO` statement takes the form of `(GO target)`, where `target` should be one of the symbols which occur at top level among that particular invocation of `PROG`s arguments. A `GO` statement may occur at top level in a PROG, or in a clause of a `COND` statement in a `PROG`, but not in a function called from the `PROG` statement. When a `GO` statement is evaluated, execution should transfer immediately to the expression which is the argument list immediately following the symbol which is its target. If the target is not found, an error with the code `A6` should be thrown. RETURN: A `RETURN` statement takes the form `(RETURN value)`, where `value` is any value. Following the evaluation of a `RETURN` statement, the `PROG` should immediately exit without executing any further expressions, returning the value. SET and SETQ: In addition to the above, if a `SET` or `SETQ` expression is encountered in any expression within the `PROG` body, it should affect not the global object list but instead only the local variables of the program. COND: In strict mode, when in normal execution, a `COND` statement none of whose clauses match should not return `NIL` but should throw an error with the code `A3`... except that inside a `PROG` body, it should not do so. sigh. Flow of control: Apart from the exceptions specified above, expressions in the program body are evaluated sequentially. If execution reaches the end of the program body, `NIL` is returned. Got all that? Good.
PROP	Lisp lambda function	?	FUNCTIONAL	see manual pages 59
QUOTE	Lisp lambda function	?		see manual pages 10, 22, 71
QUOTIENT	Host lambda function	?		I'm not certain from the documentation whether Lisp 1.5 `QUOTIENT` returned the integer part of the quotient, or a realnum representing the whole quotient. I am for now implementing the latter.
RANGE	Lisp lambda function	?	?	?
READ	Host lambda function	?	PSEUDO-FUNCTION	An implementation of a Lisp reader sufficient for bootstrapping; not necessarily the final Lisp reader. `input` should be either a string representation of a LISP expression, or else an input stream. A single form will be read.
REMAINDER	Host lambda function	?		?
REPEAT	Lisp lambda function	?	?	?
RPLACA	Host lambda function	?	PSEUDO-FUNCTION	Replace the `CAR` pointer of this `cell` with this `value`. Dangerous, should really not exist, but does in Lisp 1.5 (and was important for some performance hacks in early Lisps)
RPLACD	Host lambda function	?	PSEUDO-FUNCTION	Replace the `CDR` pointer of this `cell` with this `value`. Dangerous, should really not exist, but does in Lisp 1.5 (and was important for some performance hacks in early Lisps)
SEARCH	Lisp lambda function	?	FUNCTIONAL	see manual pages 63
SET	Host lambda function	?	PSEUDO-FUNCTION	Implementation of SET in Clojure. Add to the `oblist` a binding of the value of `var` to the value of `val`. NOTE WELL: this is not SETQ!
SUB1	Lisp lambda function, Host lambda function	?		?
SUB2	Lisp lambda function	?	?	?
SUBLIS	Lisp lambda function	?		see manual pages 12, 61
SUBST	Lisp lambda function	?		see manual pages 11, 61
SYSIN	Host lambda function	?	?	Read the contents of the file at this `filename` into the object list. If the file is not a valid Beowulf sysout file, this will probably corrupt the system, you have been warned. File paths will be considered relative to the filepath set when starting Lisp. It is intended that sysout files can be read both from resources within the jar file, and from the file system. If a named file exists in both the file system and the resources, the file system will be preferred. NOTE THAT if the provided `filename` does not end with `.lsp` (which, if you're writing it from the Lisp REPL, it won't), the extension `.lsp` will be appended. NOTE THAT this is an extension function, not available in strct mode.
SYSOUT	Host lambda function	?	?	Dump the current content of the object list to file. If no `filepath` is specified, a file name will be constructed of the symbol `Sysout` and the current date. File paths will be considered relative to the filepath set when starting Lisp. NOTE THAT this is an extension function, not available in strict mode.
TERPRI		?	PSEUDO-FUNCTION	see manual pages 65, 84
TIMES	Host lambda function	?		?
TRACE	Host lambda function	?	PSEUDO-FUNCTION	Add this `s` to the set of symbols currently being traced. If `s` is not a symbol or sequence of symbols, does nothing.
UNION	Lisp lambda function	?	?	?
UNTRACE	Host lambda function	?	PSEUDO-FUNCTION	Remove this `s` from the set of symbols currently being traced. If `s` is not a symbol or sequence of symbols, does nothing.
ZEROP	Lisp lambda function	?	PREDICATE	see manual pages 26, 64

Functions described as 'Lisp function' above are defined in the default sysout file, resources/lisp1.5.lsp, which will be loaded by default unless you specify another initfile on the command line.

Functions described as 'Host function' are implemented in Clojure, but if you're brave you can redefine them in Lisp and the Lisp definitions will take precedence over the Clojure implementations.

Architectural plan

Not everything documented in this section is yet built. It indicates the direction of travel and intended destination, not the current state.

resources/lisp1.5.lsp

The objective is to have within resources/lisp1.5.lsp, all those functions defined in the Lisp 1.5 Programmer's Manual which can be implemented in Lisp.

This means that, while Beowulf is hosted on Clojure, all that would be required to rehost Lisp 1.5 on a different platform would be to reimplement

bootstrap.clj
host.clj
read.clj

The objective this is to make it fairly easy to implement Lisp 1.5 on top of any of the many Make A Lisp implementations.

beowulf/boostrap.clj

This file is essentially Lisp as defined in Chapter 1 (pages 1-14) of the Lisp 1.5 Programmer's Manual; that is to say, a very simple Lisp language, which should, I believe, be sufficient in conjunction with the functions provided by beowulf.host, to bootstrap the full Lisp 1.5 interpreter.

In addition it contains the function INTEROP, which allows host language functions to be called from Lisp.

beowulf/host.clj

This file provides Lisp 1.5 functions which can't be (or can't efficiently be) implemented in Lisp 1.5, which therefore need to be implemented in the host language, in this case Clojure.

beowulf/read.clj

This file provides the reader required for boostrapping. It's not a bad reader - it provides feedback on errors found in the input - but it isn't the real Lisp reader.

Intended deviations from the behaviour of the real Lisp reader are as follows:

It reads the meta-expression language MEXPR in addition to the symbolic expression language SEXPR, which I do not believe the Lisp 1.5 reader ever did;
It treats everything between a double semi-colon and an end of line as a comment, as most modern Lisps do; but I do not believe Lisp 1.5 had this feature.

Commentary

What's surprised me in working on this is how much more polished Lisp 1.5 is than legend had led me to believe. The language is remarkably close to Portable Standard Lisp which is in my opinion one of the best and most usable early Lisp implementations.

What's even more surprising is how faithful a reimplementation of Lisp 1.5 the first Lisp dialect I learned, Acornsoft Lisp, turns out to have been.

I'm convinced you could still use Lisp 1.5 for interesting and useful software (which isn't to say that modern Lisps aren't better, but this is software which is almost sixty years old).

Installation

Download the latest release 'uberjar' and run it using:

    java -jar <path name of uberjar>

Or clone the source and build it using:

    lein uberjar`

To build it you will require to have Leiningen installed.

Input/output

Lisp 1.5 greatly predates modern computers. It had a facility to print to a line printer, or to punch cards on a punch-card machine, and it had a facility to read system images in from tape; but there's no file I/O as we would currently understand it, and, because there are no character strings and the valid characters within an atom are limited, it isn't easy to compose a sensible filename.

I've provided two functions to work around this problem.

SYSOUT

SYSOUT dumps the global object list to disk as a single S Expression (specifically: an association list). This allows you to persist your session, with all your current work, to disk. The function takes one argument, expected to be a symbol, and, if that argument is provided, writes a file whose name is that symbol with .lsp appended. If no argument is provided, it will construct a filename comprising the token Sysout, followed by the current date, followed by .lsp. In either case the file will be written to the directory given in the FILEPATH argument at startup time, or by default the current directory.

Obviously, SYSOUT may be called interactively (and this is the expected practice).

SYSIN

SYSIN reads a file from disk and overwrites the global object list with its contents. The expected practice is that this will be a file created by SYSOUT. A command line flag --read is provided so that you can specify

Learning Lisp 1.5

The Lisp 1.5 Programmer's Manual is still in print, ISBN 13 978-0-262-13011-0; but it's also available online.

Other Lisp 1.5 resources

The main resource I'm aware of is the Software Preservation Society's site, here. It has lots of fascinating stuff including full assembler listings for various obsolete processors, but I failed to find the Lisp source of Lisp functions as a text file, which is why resources/lisp1.5.lsp is largely copytyped and reconstructed from the manual.

Other implementations

There's an online (browser native) Lisp 1.5 implementation here (source code here). It even has a working compiler!

History resources

I'm compiling a list of links to historical documents on Lisp 1.5.

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