SchOS/docs/plan/finch.md

Finch, for SchOS's own programs

Two things, and only one of them is later.

• Bringing Finch into SchOS (below) is later — behind 1.0.md's freeze like everything else here that is not named there.
• Finishing Finch itself, and making it fully self-hosting, is work on the Finch repository and does not touch this tree. It can proceed now; see the gap.

Finch is the author's own language: https://pandor.pl/git/nexoniarz/Finch (GitHub mirror: Nexoniarz/Finch). The plan is to write SchOS's own applications and tools in it, in place of C, and so leave behind what C does badly: silent narrowing, signed/unsigned mixing, undefined behaviour on an out-of-range index, a forgotten free.

What it is today (version 2.4.0)

Measured from the repository, not from its README alone:

• LLVM front end, C-like syntax, no semicolons, no headers, no preprocessor. It compiles through LLVM's -O2 pipeline to native code; there is no garbage collector.
• Values, not references: lists, text and structs copy on assignment and are freed at the end of their block. Out-of-range indexes, division by zero and null pointers stop with a runtime error rather than undefined behaviour.
• Errors as values (int(s) or 0, try read_file(p)), maps, methods, modules.
• C interop through libclang: import "foo.h" plus link "foo" calls any C library, structs by value and callbacks included. That is the way in to everything SchOS already has — musl, the /dev/* interfaces, Alpine's libraries.
• Targets: Linux, macOS, Windows, on x86-64 and ARM64 (src/platform.fch; the System V, AAPCS64 and Microsoft x64 ABIs are in src/gen.fch). ARM64 and Windows programs are run under qemu and Wine in the tests; macOS is IR-checked.
• Language: enums, match, generics, errors as values, maps, methods, defer, and a standard library in Finch (json, path, time, proc).
• Tooling: a language server (finch lsp, written in Finch), VS Code and Kate support, 107 tests.
• Written in itself: the whole compiler is Finch (about 8,500 lines in src/, no C++) and builds itself from a committed seed to a byte-identical result — bootstrap/build.sh.

What SchOS would need to do

1. A schos target. finch picks a triple, a C compiler and a linker per target. SchOS runs unmodified Alpine binaries and Alpine's musl (see alpine.md), so the first target is the existing x86_64-unknown-linux-musl, and a Finch program built on the host should run on SchOS before anything in Finch is made SchOS-specific. Test that before writing a line of target code.
2. The compiler on SchOS. Needs LLVM 21 and libclang, both of which Alpine packages. That is a question of apk and memory, not of SchOS.
3. The Finch-written compiler on SchOS. It emits LLVM IR as text and needs only clang and libclang, no LLVM C++ libraries, so it is the compiler that would ship in the image. It needs a seed made for the SchOS target.
4. Bindings for SchOS's own interfaces (/dev/pci, /dev/netdev, schinit's job protocol, the framebuffer) — generated by the C importer from the headers in include/uapi/, so they are not hand-written and cannot drift from the kernel.

Where self-hosting stands

The compiler is written in Finch and nothing else. bootstrap/build.sh builds it from the seed (its own IR, Linux x86-64 and ARM64) and then from itself, and checks the two make identical IR. It passes the 107 tests: programs, refusals, -g builds, a shared library and the language server. The work is on the finish-finch branch of github.com/Nexoniarz/Finch; before-bootstrap there is the all-C++ compiler, kept as the backup. main is untouched.

Left: the compiler does not run on macOS or Windows yet (it needs a seed made with those systems' headers; programs for them are built from Linux with --target), and main is switched to this branch only when the author says so, after merging their local embed work.

Rules

• Inference, not an instruction: the kernel stays in C. Finch is for applications and tools; the user said "our apps or stuff like that".
• No new capability starts until the register of unfinished work is empty; the SchOS half of this document is a plan, not an exception to that.