prettify

Grammar-derived pretty-printers for structured languages. Each language is defined by a BBNF grammar file — #[derive(Parser)] generates the parser, @pretty directives control formatting, and pprint renders the final output.

Usage

use prettify::{prettify_json, prettify_css, PrinterConfig};

let config = PrinterConfig { max_width: 80, indent: 2, use_tabs: false };

let formatted_json = prettify_json(input, &config);
let formatted_css = prettify_css(input, &config);

Partial formatting via source range:

use prettify::{prettify_json_range, PrinterConfig};

let config = PrinterConfig { max_width: 80, indent: 2, use_tabs: false };
let formatted = prettify_json_range(input, 100..200, &config);

Languages

Language	Tests	Grammar source	Entry point
JSON	7	`json-pretty.bbnf`	`prettify_json()`
CSS	7	`css-stylesheet-pretty.bbnf`	`prettify_css()`
EBNF	4	`ebnf-pretty.bbnf`	`prettify_ebnf()`
BNF	5	`bnf-pretty.bbnf`	`prettify_bnf()`
BBNF	5	`bbnf-pretty.bbnf`	`prettify_bbnf()`

All 28 tests pass. Idempotency verified: prettify(prettify(x)) == prettify(x).

Architecture

Grammar (.bbnf)
  → #[derive(Parser)]     proc-macro codegen (bbnf-derive)
    → AST (enum)           typed parse tree
      → to_doc()           @pretty-directed Doc emission
        → Doc tree         pprint intermediate representation
          → pprint()       Wadler-Lindig pretty-printer
            → String       formatted output

The @pretty directives annotate grammar rules with formatting hints:

Hint	Effect
`group`	Wrap in `Doc::Group` — try flat, break if too wide
`block`	Hardline-separated items
`indent`	Indent children
`blankline`	Double hardline between items
`softbreak`	Softline separators
`nobreak`	Space-only separators (no line breaks)
`fast`	`Join` instead of `SmartJoin` — linear, no DP justification

range_to_doc() enables partial formatting: nodes outside the target range emit verbatim source text, nodes inside emit formatted Doc trees.

Performance

Phase-split CSS throughput (normalize.css + app.css):

Phase	Throughput
Parse	~31 MB/s
to_doc	~28 MB/s
pprint render	~94 MB/s

End-to-end cached throughput:

Benchmark	Throughput
JSON data.json	~116 MB/s
JSON canada.json	~50 MB/s
CSS normalize.css	~29 MB/s
CSS app.css	~28 MB/s

Dependencies

All path dependencies — no registry crates at runtime:

parse_that — parser combinators
bbnf-derive — #[derive(Parser)] proc-macro
bbnf — grammar analysis
pprint — Wadler-Lindig pretty-printer

Dev: bencher for [[bench]] harness.

Build

cargo test                      # 28 tests
cargo bench --bench prettify    # 12 benchmarks (JSON 6 + CSS 6)
cargo clippy -- -D warnings

Requires Rust nightly (#![feature(cold_path)]).