Ruby's Kernel#sprintf / format / String#% in pure Go — MRI-compatible, no cgo.
🌐 Website · 📚 Documentation
go-ruby-format is a pure-Go (no cgo) reimplementation of Ruby's format-string engine — Kernel#sprintf, Kernel#format and String#%. It parses an MRI format string and renders it against its arguments, matching reference Ruby byte-for-byte: every conversion (d i f e g s p x o b c %), every flag (- + space 0 #), width and precision including the * argument form, named references (%<name>s and %{name}), absolute n$ argument indexing, arbitrary-precision Bignum and hex-float (%a) output. It was extracted from rbgo's prelude/internals into a reusable standalone library: no dependency on the Ruby runtime, the dependency runs the other way. It is the format-string backend for go-embedded-ruby, bound by rbgo as a native module just like go-ruby-regexp and go-ruby-erb — differential-tested against MRI, 100% coverage, CI green across 6 arches and 3 OSes.
| Repo | What it is |
|---|---|
| format | the library: Ruby's format-string engine in pure Go |
| docs | MkDocs Material documentation, versioned with mike, served at /docs/ |
| go-ruby-format.github.io | the Hugo landing page |
| brand | logos and brand assets |
- Pure Go, zero cgo. Cross-compiles and embeds anywhere; a static binary by default.
- Extracted from rbgo, reusable by anyone. A standalone library lifted from
rbgo's prelude/internals;
rbgobinds it as a native module, the same pattern as go-ruby-regexp and go-ruby-erb. - MRI byte-exact. Output matches reference Ruby exactly, validated by a
differential oracle against the
rubybinary. - Standalone & reusable. No dependency on the Ruby runtime; the dependency runs the other way.
- 100% test coverage is the target, enforced as a CI gate.
Complete — MRI byte-exact. Faithful port of MRI's sprintf.c: all conversions (d i f e g s p x o b c %), all flags (- + space 0 #), width and precision including the * argument form, named references (%<name>s / %{name}), n$ indexing, Bignum and hex-float (%a). Validated by a differential oracle against the system ruby — rendered output compared byte-for-byte — at 100% coverage, gofmt + go vet clean, CI green across the six 64-bit Go targets and three OSes.
Differential-tested against MRI at 100% coverage, gofmt + go vet clean, CI
green across the six 64-bit Go targets (amd64, arm64, riscv64, loong64, ppc64le,
s390x) and three operating systems. It is bound into the
go-embedded-ruby (rbgo) ecosystem
as a native module.
BSD-3-Clause.