0007

Status: Accepted

Context

Formatting was unconfigured across the set: treefmt and luaformatter were in the dev shell, but there was no treefmt config. We want one setup that runs the same way locally (nix fmt), as a pre-commit guard, and as a CI gate (treefmt --ci). Two formatter choices needed care.

• PureScript: purs-tidy is the ecosystem standard. purty is deprecated, and purescript-contrib mandates purs-tidy. The forks' .purs are vendored from upstream and are mostly already purs-tidy-formatted, so reformatting them is a small, layout-only diff (mostly import and export lists; measured at 0 changed files for arrays, 1 of 50 for prelude, more for foldable-traversable). Running purs-tidy over the upstream snapshot before an upstream sync keeps that sync free of formatting conflicts.
• Lua: StyLua is modern and lightweight but removes parentheses it considers redundant, including the wrapping parentheses the FFI reader depends on (0003). Tested on a real FFI file, StyLua turned name = (function ... end) into name = function ... end, which fails to parse. There is no StyLua option to preserve them (only per-node -- stylua: ignore, which skips formatting that node entirely). lua-format preserves the parentheses.

Decision

Configure treefmt with treefmt-nix: nixfmt (*.nix), purs-tidy with a shared .tidyrc.json (*.purs), dhall format (*.dhall), and lua-format (the FFI src/*.lua). The flake exposes formatter (so nix fmt works) and a checks.formatting (so nix flake check / treefmt --ci gates CI), and a pre-commit hook runs nix fmt. Generated and vendored trees are excluded: dist/, output/, .spago/, *.lock, .tidyrc.json.

Consequences

• One config drives local formatting, the pre-commit guard, and the CI gate.
• StyLua stays unusable until pslua's foreign reader no longer requires grouping parentheses, which is a compiler change recorded in the pslua repository.
• Vendored .purs take a small one-time reflow; future upstream syncs normalise both sides with purs-tidy to stay conflict-free.