Maru

What

Maru is a programming language. It's a self-hosting, yet tiny lisp dialect: a symbolic expression evaluator that can compile its own implementation to machine code, in about 2000 LoC altogether.

Maru is in particular trying to be malleable at the very lowest levels, so any special interest that cannot be accommodated easily within the common platform would be a strong indicator of a deficiency within the platform that should be addressed rather than disinherited. (Ian Piumarta)

This repo is also a place for exploration in the land of bootstrapping and computing system development. My personal interest is in clearly and formally expressing that which is mostly treated as black magic: the bootstrapping of a language on top of other languages (which includes the previous developmental stage of the same language).

How

Maru's architecture is described in Ian Piumarta's paper: Open, extensible composition models.

it is a sketch of how Maru's generalised eval works, which is entirely accurate in intent and approach, if a little different in some implementation details (Ian Piumarta)

The Parts - an overview

eval.c (in branch maru.0.c99) contains a reader and evaluator (interpreter) for an s-expression language, written in C (C99).
eval.l (in branch maru.1 and up) contains the same evaluator, written in (a subset of) this s-expression language that can be compiled to machine code. In other words, eval.l implementats a metacircular evaluator for the language it is written in.
emit.l contains a compiler from s-expressions to IA-32 (x86) assembly (and LLVM IR), written in the s-expression language. This compiler can be thought of as a semantics-preserving "level shift" from s-expressions to machine code, letting the metacircular evaluator in eval.l escape from the "infinite metacircular regression" to a language grounded in hardware. A possible metaphor of this is a "target universe" implemented by some electric circuits (i.e. transistors wired to each other in a CPU) that provide you a set of axiomatic foundations to build upon while compiling the abstract to the concrete; while implementing your new universe (the Maru language in this case). More details are available in the compiler's doc.
boot.l contains some basic data structures, algorithms, and paradigms that are needed by emit.l; it's written in the s-expression language.

Build instructions

To test a bootstrap cycle using one or all of the backends:

make test-bootstrap-x86    # defaults to the libc platform
make PLATFORM=[libc,linux] test-bootstrap[-llvm,-x86]

Linux

sudo apt install make time rlwrap

You will need LLVM, and/or a C compiler (any version beyond LLVM 8 should work):

sudo apt install llvm clang

For now the x86 backend only supports 32 bit mode. To use it you will need to have support for compiling and running 32 bit C code. On Debian based x86_64 systems this will install all the necessary libraries:

sudo apt install gcc-multilib

MacOS

Please note that recent MacOS versions don't support 32 bit executables anymore, but Maru's LLVM backend is expected to work fine.

Make sure XCode is installed. In a Terminal:

xcode-select --install

Install Homebrew

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"

Install LLVM using Homebrew

brew install llvm
echo export PATH="$(brew --prefix llvm)/bin:$PATH" >> ~/.bash_profile
source ~/.bash_profile

Other platforms

Currently Maru should work everywhere where there's a libc, and either the GNU toolchain, or LLVM is available.

Patches are welcome for other platforms.

Who

Initially written by Ian Piumarta, at around 2011. Full commit history is available in the piumarta branch.

This repo and readme is maintained by attila@lendvai.name.

Where

Bugs and patches: maru github page.

Discussion: maru-dev google group.

Why

Programming badly needs better foundations, and Maru is part of this exploration. The foundations should get smaller, simpler, more self-contained, and more approachable by people who set out to learn programming.
We lose a lot of value by not capturing the history of the growth of a language, including the formal encoding of its build instructions. They are useful both for educational purposes, and also for practical reasons: to have a minimal seed that is very simple to port to a new architecture, and then have a self-contained, formal bootstrap process that can automatically "grow" an entire computing system on top of that freshly laid, tiny foundation.
Maru is very small: in about 1700 lines of code in the maru.1 branch it can self-host (plus around 2300 LoC of throwaway C code for the initial step).
Ian seems to have stopped working on Maru, but it's an interesting piece of code that deserves a repo and a maintainer.
This work is full of puzzles that are a whole lot of fun to resolve!

Contribution

You are very welcome to contribute, but beware that until further notice this repo will receive forced pushes (i.e. git push -f rewriting git history (except the piumarta branch)). This will stop eventually when I settle with a build setup that nicely facilitates bootstrapping multiple, parallel paths of language development. Please make sure that you open a branch for your work, and/or that you are ready for some git fetch and git rebase.

Status

Maru's status

Backporting and bootstrapping the latest semantics from the piumarta branch is done: the eval.l in the latest branch of this repo should be semantically equivalent with the eval.l that resides in the piumarta branch, although we have arrived to this state on two different paths:

Ian, while evolving Maru, kept his eval.c and eval.l semantically in sync,
while I have bootstrapped the new features: I started out from an earlier version of the eval.l + eval.c couple (the minimal ones published on Ian's website). Then I bootstrapped the later stages of eval.l using an earlier stage of itself. I only used the C code as the initial stepping stone in the bootstrap process, and then I left it behind.

Notable new features

There are several Maru stages/branches now, introducing non-trivial new features. Some that are worth mentioning:

Introduction of platforms: they are the "holding environments" where the implementation of eval can be brought alive. Notably, besides the original libc platform, there is now a linux platform that compiles to a statically linked executable that runs directly on top of the Linux kernel, using syscalls; i.e. without linking anything from libc, or ld-linux.so. From a practical perspective this is equivalent with running directly on the bare metal (i.e. all dynamically allocated memory is provided by our own GC, etc).

List of platforms: libc, linux, metacircular (only planned: loading the evaluator's implementation into another instance of the evaluator, as opposed to compiling it to machine code).
The host and the slave are isolated while bootstrapping which makes it possible to do things like reordering types (changing their type id in the target), or changing their object layout.
Relying on this isolation, the code in eval.l now looks pretty much the same as something that is meant to be loaded into the evaluator (i.e. the function implementing car in eval.l is now called car). This paves the way for metacircularity: to be able to "bring alive" the evaluator by loading it verbatim into another instance of itself (as opposed to compiling it to machine code and giving it to a CPU to bring it alive).
The addition of an LLVM backend.

Assorted TODO:

Finish the proof of concept in tests/test-elf.l to compile the Linux plaform directly into an ELF binary.
Rewrite the build process in Maru; eliminate dependency on GNU Make.
Replace the hand-written parser in eval.l with something generated by the PEG compiler.
Implement modules and phase separation along with what is outlined in Submodules in Racket - You Want it When, Again?. Part of this is already done and is used in the bootstrap process.
Compile to, and bootstrap on the bare metal of some interesting targets. It's already demonstrated by the Linux platform. Another one could be pc-bios, because it's easily testable using QEMU. Or port it on an ARM board (like Raspberry Pi)? Or maybe even a C64 port?
Revive all the goodies in the piumarta branch, but in a structured way.
Simplify the types-are-objects part and its bootstrap, and maybe even make it optional?
Weed out some of the added bloat/complexity (e.g. compile closures instead of selectors, and use them to implement streams; write a tree shaker; etc).
Merge the language and API that the compiler and the evaluator understands; i.e. make the level-shifted code (eval.l & co.) less different than code understood by the evaluator. This would mean that we can e.g. load/compile source/buffer.l both into the level-shifted code and into the evaluator. This is slowly happening, but it's nowhere near done, and I'm not even sure what done means here.
Use LLVM's tablegen definitions to generate bytecode assemblers. It requires either the reimplementation of the tablegen parser/logic in Maru (doesn't seem to be trivial), or writing C++ code (uhh!) to compile the data to the Maru definitions implementing an assembler.
Introduce a simplified language that drops some langauge features, e.g. remove forms and the expand protocol. Make sure that this language can bootstrap itself off of C99. Then reintroduce forms and expand by using this simplified Maru as the implementation language.
Understand and incorporate François René Rideau's model of First Class Implementations: Climbing up the Semantic Tower, (see this couple of page summary, or see his page on reflection)

History and perspective

Around 2010-2013

Maru was developed as part of Alan Kay's Fundamentals of New Computing project, by the Viewpoints Research Institute. The goal of the project was to implement an entirely new, self-hosting computing system, with GUI, in 20.000 lines of code.

At some point VPRI went quiet and closed down in 2018. Much of their online content disappeared, and the team (probably) also dissolved.

Their annual reports: 2007, 2008, 2009, 2010, 2011, 2012.

This git repo

The piumarta branch of this git repo is a conversion of Ian Piumarta's Mercurial repo that was once available at http://piumarta.com/hg/maru/. To the best of my knowledge this is the latest publically available state of Ian's work. This repo was full of assorted code, probably driving the VPRI demos.

The piumarta branch will be left stale (modulo small fixes and cleanups). My plan is to eventually revive most of the goodies from this branch, but in a more organized and approachable manner, and also paying attention to the bootstrapping issues.

Ian published another Mercurial repo somewhere halfway in the commit history with only a couple of commits from around 2011. I assume that it was meant to hold the minimal/historical version of Maru that can already self-host. I started out my work from this minimal repo (hence the divergence between the piumarta and the maru.x branches in this repo).

Other instances

There are some other copies/versions of Maru. Here are the ones that I know about and contain interesting code:

github.com/melvinzhang/maru
below-the-top is some kind of generic sexp tokenizer and evaluator written in Common Lisp that can be configured so that it can bootstrap Maru. I haven't tried it myself.

Related projects

A list of projects that are worth mentioning in this context:

Project Oberon: a project which encompasses CPU, language, operating system and user interface, and which can be run on a relatively inexpensive FPGA board, and simple enough for one person to understand it all.
Seedling

jolby/maru