beowulf
LISP 1.5 is to all Lisp dialects as Beowulf is to Emglish literature.
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 bahaviour - except as documented below.
Status
Boots to REPL, but few functions yet available.
Building and Invoking
Build with
lein uberjar
Invoke with
java -jar target/uberjar/beowulf-0.2.1-SNAPSHOT-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 INITFILE Read Lisp functions from the file INITFILE
-s, --strict Strictly interpret the Lisp 1.5 language, without extensions.
To end a session, type STOP at the command prompt.
Functions and symbols implemented
The following functions and symbols are implemented:
| Symbol | Type | Signature | Documentation |
|---|---|---|---|
| NIL | ? | null | ? |
| T | ? | null | ? |
| F | ? | null | ? |
| ADD1 | Host function | ([x]) | ? |
| AND | Host function | ([& args]) | 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 | Host function | ([x y]) | Append the the elements of y to the elements of x. All args are assumed to be beowulf.cons-cell/ConsCell objects. See page 11 of the Lisp 1.5 Programmers Manual. |
| APPLY | Host function | ([function args environment depth]) | 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. |
| ATOM | Host function | ([x]) | 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 | ? | null | ? |
| CDR | ? | null | ? |
| CONS | ? | null | ? |
| COPY | Lisp function | (X) | ? |
| DEFINE | Host function | ([args]) | Bootstrap-only version of DEFINE which, post boostrap, can be overwritten in LISP. The single argument to DEFINE should be an assoc list which should be nconc’ed onto the front of the oblist. Broadly, (SETQ OBLIST (NCONC ARG1 OBLIST)) |
| DIFFERENCE | Host function | ([x y]) | ? |
| DIVIDE | Lisp function | (X Y) | ? |
| ERROR | Host function | ([& args]) | Throw an error |
| EQ | Host function | ([x y]) | Returns T if and only if both x and y are bound to the same atom, else NIL. |
| EQUAL | Host function | ([x y]) | 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 function | ([expr] [expr env depth]) | 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. |
| FIXP | Host function | ([x]) | ? |
| GENSYM | Host function | ([]) | Generate a unique symbol. |
| GET | Lisp function | (X Y) | ? |
| GREATERP | Host function | ([x y]) | ? |
| INTEROP | Host function | ([fn-symbol args]) | Clojure (or other host environment) interoperation API. fn-symbol is expected to be either 1. a symbol bound in the host environment to a function; or 2. a sequence (list) of symbols forming a qualified path name bound to a function. Lower case characters cannot normally be represented in Lisp 1.5, so both the upper case and lower case variants of fn-symbol will be tried. If the function you’re looking for has a mixed case name, that is not currently accessible. args is expected to be a Lisp 1.5 list of arguments to be passed to that function. Return value must be something acceptable to Lisp 1.5, so either a symbol, a number, or a Lisp 1.5 list. If fn-symbol is not found (even when cast to lower case), or is not a function, or the value returned cannot be represented in Lisp 1.5, an exception is thrown with :cause bound to :interop and :detail set to a value representing the actual problem. |
| INTERSECTION | Lisp function | (X Y) | ? |
| LENGTH | Lisp function | (L) | ? |
| LESSP | Host function | ([x y]) | ? |
| MEMBER | Lisp function | (A X) | ? |
| MINUSP | Lisp function | (X) | ? |
| NULL | Lisp function | (X) | ? |
| NUMBERP | Host function | ([x]) | ? |
| OBLIST | Host 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 function | (X) | ? |
| PAIR | Lisp function | (X Y) | ? |
| PLUS | Host function | ([& args]) | ? |
| PRETTY | ? | null | ? |
| ? | null | ? | |
| PROP | Lisp function | (X Y U) | ? |
| QUOTIENT | Host function | ([x y]) | 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. |
| READ | Host function | ([] [input]) | 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 function | ([x y]) | ? |
| REPEAT | Lisp function | (N X) | ? |
| RPLACA | Host function | ([cell value]) | 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 function | ([cell value]) | 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) |
| SET | Host function | ([symbol val]) | 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 function | (N) | ? |
| SYSIN | Host function | ([filename]) | 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. |
| SYSOUT | Host function | ([] [filepath]) | 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. |
| TERPRI | ? | null | ? |
| TIMES | Host function | ([& args]) | ? |
| TRACE | ? | null | ? |
| UNTRACE | ? | null | ? |
| ZEROP | Lisp function | (N) | ? |
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
MEXPRin addition to the symbolic expression languageSEXPR, 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.
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.
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
At present, clone the source and build it using
lein uberjar.
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.
I’m not at this time aware of any other working Lisp 1.5 implementations.
License
Copyright © 2019 Simon Brooke. Licensed under the GNU General Public License, version 2.0 or (at your option) any later version.