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 primary drive with Maru is to clearly and formally express 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).

Meta

This document aims to present an overview of Maru at its latest developmental stage.

In the earlier developmental stages (i.e. in the other git branches) this file contains notes relevant only to that specific stage.

There are various documents in the doc/ directory that discuss some topics in further detail.

The documentation generated by deepwiki is also not useless.

How

Maru's architecture is described in doc/how.md.

Build instructions

Table of supported (tested) environments:

To test a bootstrap cycle:

./build.sh bootstrap llvm linux
./build.sh bootstrap [llvm,arm,x86] [libc,posix,linux]

To cross compile to an aarch64 OpenWrt:

./build.sh target/arch=aarch64 ld.so=/lib/ld-musl-aarch64.so.1 bootstrap arm linux

This should even run on an x86_64 host through qemu-user with binfmt misc:

apt install qemu-user-static gcc-aarch64-linux-gnu
./build.sh target/arch=aarch64 bootstrap arm linux

And even with aarch64 musl libc inside qemu (build.sh sets a reasonable QEMU_LD_PREFIX):

apt install qemu-user-static gcc-aarch64-linux-gnu musl
./build.sh target/arch=aarch64 ld.so=/lib/ld-musl-aarch64.so.1 bootstrap arm linux

Note that build.sh is just a thin wrapper for calling functions defined in build.l.

sudo apt install make time rlwrap

sudo apt install llvm clang

xcode-select --install

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

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

Who

Originally written by Ian Piumarta at around 2011, as part of a Viewpoints Research Institute project. Full commit history is retained in the piumarta branch.

The current gardener is attila@lendvai.name.

Where

Bugs and patches: Maru's 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 for people who set out to learn programming. This is also necessary for more trust in our computing environment.

• I'm fascinated by bootstrapping issues. 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.

• Ian seems to have abandoned Maru, and his published archive couldn't be run as-is. But this is an interesting piece of code that deserves a repo and a maintainer to keep bitrot at bay.

• This work is full of puzzles that are a whole lot of fun to solve!

Contribution

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

Status

Maru's status

There's the piumarta branch preserving the old state and accumulating some fixes.

There's the maru.1 line of development that started from the minimal version of Maru and properly bootstrapped the extra features of the piumarta branch, i.e. without evolving eval.c in parallel. I only use the 2300 LoC of throwaway C code as the initial stepping stone in the bootstrap process (branch maru.1.c99), but once the first step is made, the C code is left behind for good. The head of the maru.3 branch should be semantically equivalent to the eval.l that resides in the piumarta branch.

After that I started to further evolve Maru's implementation in the maru.4 branch and beyond. Each branch contains a readme explaining what's relevant for that stage.

Notable new features

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

• Introduction of platforms, and notably the linux platform that compiles to a statically linked executable that only uses Linux kernel syscalls; From a practical perspective this is almost equivalent with running directly on the bare metal, minus dealing with the hardware drivers (i.e. all dynamically allocated memory needs to be managed by our own GC, all IO behind our own abstractions, etc). Other platforms: libc (functional), posix (extends libc), and metacircular (only a placeholder for now).

• New compiler backends:

  • x86-64 and aarch64 that directly generate Elf64 binary files in a single pass.

  • An LLVM backend that emits .ll LLVM IR text files.

• With the x86-64 and aarch64 backends on the Linux platform it's truly freestanding: there is no external dependency beyond a bootstrap binary that animates the codebase; i.e. no make, no assemblers, not even a linker.

• 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 runtime 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 animate it).

Notable features

• A statistical profiler scanning the backtrace.

• A bootstrapped PEG parser.

• A proper C groveller that uses a DSL implemented through a PEG grammar (see source/c/). It generates and compiles C code, that is then run to print the grovelled C values.

Future plans

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 publicly 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

• https://github.com/kstephens/maru seems to have 3 versions captured: 1.0, 2.1, 2.4

• 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 relevant in this context:

• ribbit is a bootstrapped, portable, very compact, and extensible Scheme implementation by Mathis Laroche and Marc Feeley (of Gambit fame). Can compile to dozens of languages, and has an FFI to the base languages. It has flexible mechanisms to extend the VM with new primitives.

• Robert van Engelen's tinylisp (99 lines of C with GC and a REPL), his paper, and its big brother, which is 1k LoC of heavily commented C. No compiler, they are not self-hosting.

• Loko Scheme is a self-hosting scheme that runs on the bare metal, i.e. some hardware drivers included.

• sectorlisp (github): LISP with GC in 436 bytes. It doesn't have a compiler, i.e. it cannot self-host. It only has a C implementation, an x86 assembly implementation (in the form of a boot sector), and John McCarthy's Lisp in Lisp evaluator. It would be an interesting project to add a compiler to it and see how the end result compares to Maru. Or to start growing a language as demonstrated in this repo, but starting out from sectorlisp. Note that sectorlisp is not equivalent to the first stage of Maru (the maru.1 git branches), because that can already self-host, i.e. it can bootstrap itself off of the C implementation.

• Seedling: a ladder of languages, with a minimalistic core language at the bottom called Seed (it's a Forth like). Seed can self-host in less than 1k LoC. The higher level languages above Seed are (going to be) extensions of it, and are implemented on top of Seed. Porting to a new architecture will be trivial. And an interesting tidbit: the initial bootstrap was done not by using another programming language/compiler, but by pen and paper!

• bootstrappable.org: a community around bootstrapping, and making/keeping projects bootstrapable. It brings together many interesting projects: stage0 (~500 byte self-hosting hex assembler), live-bootstrap, GNU Mes (Scheme + C, mutually self-hosting each other), m2-planet (a tiny C compiler).

• Kalyn: a subset of Haskell semantics (mostly; not lazy), but with Lisp syntax. Entirely (!) self-hosting over x86-64 in 4-5 kLoC. The project feels of high standard, including its documentation.

• nanohs: a tiny self-hosting subset of Haskell. It doesn't have any type-check implemented, but it can parse (and ignore) type annotations. This setup enables very simple self-hosting, while makes it possible to also type-check the source using a fully implemented Haskell compiler.

• PEG-based tree rewriter: runnable code to accompany Ian Piumarta's paper called PEG-based tree rewriter provides front-, middle- and back-end stages in a simple compiler. Ian wrote this before Maru, and there are several similarities between the two. See the mailing list thread.

• blynn's Haskell compiler: bootstrap a Haskell compiler incrementally from C, with extensive documentation.

• ichbins is a minimal self-hosting compiler of a Lisp dialect to C in 6 pages of code. elv is a VM built on it for very basic Lisp + Erlang-style processes (abandoned).

• RefPerSys: a mostly symbolic artificial intelligence long-term project, with ambitious Artificial General Intelligence goals. It contains interesting and relevant ideas, e.g. in refpersys-design.pdf.

• 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.

• tort: Inspired by Ian Piumarta's idst, maru and other small runtimes. Core is approx. 5000 lines of C.

• kernel: "Kernel is a conservative, Scheme-like dialect of Lisp in which everything is a first-class object." (including special forms) You may want to also see this blog.

• Compiling a Lisp: Overture: Educational article series about constructing a simple Lisp compiler, implemented in C.

• D. F. Hendry's MINT3: Supposedly something interesting from the past, but I haven't researched it myself yet.

• Meta II (1954) - a compiler compiler, or a language to write compilers in. See this tutorial.

  META II is not intended as a standard language which everyone will use to write compilers. Rather, it is an example of a simple working language which can give one a good start in designing a compiler-writing compiler suited to his own needs. Indeed, the META II compiler is written in its own language, thus lending itself to modification.”

• mu: I haven't played with this yet, but it seems to be a very self-contained and low-level language/VM whose compiler can target the bare Linux kernel, or the bare machine (x86). The readme has a collection of links to docs on low-level stuff. Mu could be another stepping stone to bootstrap Maru.