A file that declares its own language, then uses it

Metaxis is a rewriter. A .mx file has two halves: a header of directives that say what the second half's syntax is and what to turn it into, and a body written in that syntax. The tool reads the header, builds a reader for the language it describes, reads the body with it, and prints what the templates say. Nothing is built in, not +, not if, not that a number is a number, so the file that declares + could have declared anything else, and the tool never quietly prefers the language it was written in.

@token  name   "[A-Za-z_][A-Za-z0-9_]*"
@token  number "[0-9]+"
@comment "#" eol
@separator ";" => ";\n"

@syntax a "=" b   10 right  => "{a} = {b}"
@syntax a "+" b   60        => "add({a}, {b})"
@syntax "twice" e           => "({e} * 2)"
@end
x = 1;              # nothing here is built in
twice x + 2;

$ mx examples/first.mx
x = 1;
(add(x, 2) * 2)

One rule

Everything a directive says about foreign text is inside a string. A quoted word in a pattern is text the body must contain; a quoted template is text to write. Everything outside the quotes is Metaxis's own fixed vocabulary, which no file can change: a directive name, a hole, a level, a bracket. That is what lets a directive mention if without being an if statement, and it is why a .mx file can read a language with keywords without ever having a keyword list: then is a word where a rule quoted it and a name everywhere else, by position.

What it has been pointed at

Five translators, each taken far enough to be compiled and run by the test suite, and each picked because it would ask the tool for something:

• Pascal → C, the mechanics, checked by a C compiler and a number.
• C → arm64 assembly, a target that is a sequence rather than a tree.
• Python → C, a language whose blocks are indentation; the output is run under python3 as well, and the two answers compared.
• BASIC → C, a source that declares nothing, so the head of the output is the aggregate of its body.
• C → C, the tool rewriting its own front end in text mode, with the result compiled and run.

Where to read

• Tutorial: one concept per section, each with a file you can run and the output it gives. Every transcript is checked by the suite.
• Glossary: the concepts in the order they depend on each other, then every term of art, for a reader meeting Pratt parsers, attribute grammars or hygiene here first.
• Reference: what every part of a .mx file means, exhaustively, with a contents table and an index.
• Examples: every example in the tree with its source and recorded output side by side.
• Notation: why it is shaped this way and what that costs, argued rather than stated.
• Direction: where it could go, and which futures are declined.
• Prior art: the tools that do something like this, and how this one scores against them.
• Languages: which languages the tool can be pointed at and which it cannot, each verdict resting on a stated property.
• Changelog and Roadmap: when things shipped, and what is not built and why.

Getting it

git clone https://github.com/hansolovkarlsson/Metaxis
cd Metaxis && make && make check
bin/mx examples/first.mx

C11 and make, plus POSIX <regex.h>. Nothing else. The suite runs on every push, on Linux and macOS.