prettyprint

prettyprint — go-ruby-prettyprint

A pure-Go (no cgo) reimplementation of Ruby's prettyprint standard library — the Wadler/Lindig pretty-printing engine that lays out a stream of text, breakable separators and groups into a width-constrained, nicely indented document. It is a faithful, byte-identical port of MRI 4.0.5's prettyprint.rb: each group prints on one line when it fits within the maximum width and is otherwise broken at its breakable points, with nesting preserved — without any Ruby runtime.

It is the layout backend for go-embedded-ruby (rbgo), a standalone, reusable module with no dependency on the Ruby runtime — a sibling of go-ruby-yaml, go-ruby-regexp and go-ruby-marshal.

What it is — and isn't. This is the layout engine only — the deterministic group/breakable/indent/width-fitting algorithm (the buffer, the group stack and the depth-bucketed group queue). The pp object inspector that uses it — walking an object graph and emitting text/group/breakable calls — is the host's job and stays in rbgo. This library is the standalone Go backend pp binds to.

Features

Faithful port of PrettyPrint, validated against the ruby binary on every supported platform:

• Groups that print flat when they fit and break at their breakables when they overflow maxwidth — the exact break_outmost_groups / depth-bucketed GroupQueue algorithm MRI uses, so nested groups break outermost-first.
• Breakables — breakable(sep, width) line-break hints that emit their separator when the line is not broken, with the width argument for multibyte or proportional separators.
• Nesting / indentation — nest(indent) and the group(indent, …) indent argument, with a pluggable genspace block for the indentation string.
• Open/close text — group(indent, open, close) wrapping the block in bracketing text counted toward the fit decision.
• Fill mode — fill_breakable, where each break is decided individually.
• The single-line formatter — singleline_format, where breakables become their separator text and nothing ever breaks.
• Custom newline and custom maxwidth, matching MRI's PrettyPrint.new / PrettyPrint.format signatures.

CGO-free, dependency-free, 100% test coverage, gofmt + go vet clean, and green across the six 64-bit Go targets (amd64, arm64, riscv64, loong64, ppc64le, s390x) and three operating systems (Linux, macOS, Windows).

Install

go get github.com/go-ruby-prettyprint/prettyprint

Usage

package main

import (
	"fmt"

	"github.com/go-ruby-prettyprint/prettyprint"
)

func main() {
	// PrettyPrint.format(''.dup, 10) { |q| ... }
	out := prettyprint.Format(10, "\n", nil, func(q *prettyprint.PrettyPrint) {
		q.Group(2, "[", 1, "]", 1, func() {
			q.TextString("111")
			q.BreakableString()
			q.TextString("222")
			q.BreakableString()
			q.TextString("333")
		})
	})
	fmt.Printf("%q\n", out) // "[111\n  222\n  333]"

	// The single-line formatter never breaks: breakables become their separator.
	flat := prettyprint.SingleLineFormat(func(q *prettyprint.SingleLine) {
		q.GroupDefault(func() {
			q.TextString("[")
			q.Breakable("", 0)
			q.TextString("1")
			q.BreakableString()
			q.TextString("2")
			q.TextString("]")
		})
	})
	fmt.Printf("%q\n", flat) // "[1 2]"
}

API map (MRI → Go)

The default genspace is DefaultGenSpace (n ASCII spaces); pass nil to New / Format to use it.

Tests & coverage

The suite is 100% statement-covered and combines two layers:

• Deterministic, ruby-free tests — golden programs with byte-exact expected output (verified against ruby -rprettyprint), plus white-box tests for the group-queue terminal arms. These alone hold the 100% gate on every platform, including Windows and the qemu cross-arch lanes where no ruby is present.
• MRI differential oracle — oracle_test.go runs a shared corpus through both this package and a generated PrettyPrint.format / singleline_format script and asserts the output is identical. It binmodes stdout/stdin (so Windows text-mode never rewrites the bytes), skips when ruby is absent, and gates on RUBY_VERSION >= "4.0".

go test -race -coverprofile=cover.out ./...
go tool cover -func=cover.out | tail -1   # total: (statements) 100.0%

License

BSD-3-Clause — see LICENSE. Copyright the go-ruby-prettyprint/prettyprint authors.

WebAssembly

Being pure Go (CGO=0), this library also compiles to WebAssembly — both GOOS=js GOARCH=wasm (browser / Node.js) and GOOS=wasip1 GOARCH=wasm (WASI). CI builds both targets on every push, alongside the six 64-bit native/qemu arches.

GOOS=js     GOARCH=wasm go build ./...   # browser / Node
GOOS=wasip1 GOARCH=wasm go build ./...   # WASI (wasmtime, wasmer, wasmedge, …)