janus fmt — Code Formatter

janus fmt

Canonical formatter. One style. No arguments. No configuration files. Run janus fmt and your code looks like every other Janus file on the planet.

Usage

# Format a file in-place
janus fmt src/main.jan

# Check if a file is formatted (CI mode)
janus fmt --check src/main.jan

CI Mode

--check exits with code 1 if the file isn’t formatted, and prints a line-by-line diff showing exactly what would change:

src/main.jan: not formatted
  Diff in src/main.jan:
     1 - func   add( a:i64,b:i64 )->i64 do
     1 + func add(a: i64, b: i64) -> i64 do
     2 - return a+b
     2 +     return a + b

No murder mysteries. The body is right there.

What It Does

Structural Law (SPEC-017 Law 2)

The formatter enforces the structural divide — the only non-negotiable rule in Janus syntax:

Context	Delimiter
func, if, else, while, for, using, nursery	do..end
match, enum, struct, flags, literals	{ }

If you write func main() { }, the formatter rewrites it to func main() do ... end. This is the single structural correction.

Indentation

4 spaces per nesting level. No tabs. No trailing whitespace.

func process(data: i64) -> i64 do
    if data > 0 do
        return transform(data)
    end
    return 0
end

Spacing

Spaces around binary operators, after commas, after colons in type annotations:

// Before
let x=a+b*c
func add(a:i64,b:i64)->i64 do return a+b end

// After
let x = a + b * c
func add(a: i64, b: i64) -> i64 do return a + b end

Blank Lines

One blank line between top-level declarations. No double blanks. No blank lines at block boundaries.

Comments

All comments are preserved. Doc comments (///) stay above their declaration. Line comments (//) stay at end-of-line or on their own line. The formatter never removes your comments.

Idempotency

janus fmt(janus fmt(source)) == janus fmt(source) — always. Format it twice, get the same result. Every test in the formatter’s 34-test suite verifies this property.

Specification

The full formatting rules are documented in SPEC-FMT in the Janus repository at docs/SPEC-FMT.md.

Non-Goals

• No configuration. No .janusfmt.toml. One style for the entire ecosystem.
• No partial formatting. Format the whole file or nothing.
• No “minimal diff” mode. The formatter produces canonical output every time.

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“A language without fmt tolerates arguments about whitespace. Janus doesn’t.”