Adding a Built-in Language

Fresh supports a language through two independent systems — syntax highlighting and auto-indentation. You can add either on its own. Both are connected through the grammar catalog (GrammarRegistry), which maps a file to a language; detection (extension, filename, glob, shebang, configured default) lives in crates/fresh-editor/src/primitives/detected_language.rs.

System	What powers it
Highlighting	a syntect (TextMate/Sublime) grammar, or a language pack
Auto-indent	the regex indent-rules tier (language families)

Adding a language

Pick only the pieces you need.

Highlighting

1. Syntect grammar. Add a self-contained .sublime-syntax file under crates/fresh-editor/src/grammars/ and register it with the catalog (primitives/grammar/loader.rs).
2. Language pack. Ship the same grammar as an installable pack — no recompile and no core change. See Language Packs.

Auto-indentation

1. Map to a family. If the language fits an existing family (below), that's a one-line addition to the family table in indent_rules.rs.
2. Custom rules from config. Users (and language packs) can define or tune a language's indent rules in config — no recompile. Full reference: Configuration → Customize Auto-Indentation.
3. New family. If the language's block syntax matches none of the existing ones, add a family to indent_rules.rs.

Detection and LSP

• Detection (extensions, filenames, globs) comes from the catalog entry or a [languages.<id>] config block.
• LSP is orthogonal to both systems: a server config under [lsp.<id>] (or a pack's fresh.lsp).

How auto-indentation works

When you press Enter or type a closing bracket, Fresh chooses an indent through a tiered fallback:

1. Regex indent-rules tier — the primary path. Each language belongs to a family of simple rules (primitives/indent_rules.rs).
2. Tree-sitter AST — used for the few languages that still ship a bundled grammar (primitives/indent.rs).
3. Generic bracket heuristic — a language-agnostic fallback for unknown files (primitives/indent_pattern.rs).

The rules tier applies scope masking: before matching, it blanks out comment and string spans (reusing the highlighter's existing output), so a bracket or keyword inside a string or comment doesn't trigger an indent.

Language families

A family is a shared set of indent rules describing one class of block syntax. Most languages can be pointed at an existing family rather than needing bespoke logic. The families (defined in indent_rules.rs) are:

• CurlyBrace — { } [ ] ( ) block structure (C, Rust, JS/TS, Go, JSON, CSS…).
• Python — layout-defined: : opens a block, indentation is the structure.
• RubyLike — def…end, do…end, with midblock keywords.
• LuaLike — function…end, if…then…end, for…do…end.
• BashLike — if…then…fi, for…do…done, case…esac.
• PascalLike — begin…end, case…of…end.

A family captures the usual signals: what opens a deeper level, what closes one, one-shot indent/dedent for things like a braceless if or a Python return, and a "self-close" rule so one-liners (def f; end) don't over-indent. The exact patterns are data in indent_rules.rs; the user-facing equivalents are documented in Configuration → Customize Auto-Indentation.

A note on tree-sitter

Fresh leans toward the syntect and indent-rules paths above rather than tree-sitter. Each tree-sitter grammar adds a sizable parse table to the binary (around 18 MB was reclaimed by dropping the ones that weren't essential), so it's reserved for languages syntect can't render, and a new language usually doesn't need a tree-sitter indent query — the rules tier covers it.

Where things live

Concern	Location
Indent families & rules	crates/fresh-editor/src/primitives/indent_rules.rs
Generic bracket fallback	crates/fresh-editor/src/primitives/indent_pattern.rs
Tree-sitter indent	crates/fresh-editor/src/primitives/indent.rs
Syntect grammars	crates/fresh-editor/src/grammars/ + primitives/grammar/loader.rs
Language detection / catalog	crates/fresh-editor/src/primitives/detected_language.rs, primitives/grammar/
User-facing indent config	Configuration guide
Language packs (no recompile)	Language Packs

Background

The design rationale and the per-language support matrix live in the repo's internal docs: indentation-rules-design.md and language-support-review.md.