fmt — formatting assembled into the caller’s buffer

Source

lib/fmt.low

Layer

L0 — pure computation (the caller’s buffer)

Capabilities

none

Prints numbers and text into a buffer the caller provides to make strings. Use it to assemble a line to output or one line of a log. This module makes no paper; it only writes on the paper handed to it (mut slice u8).

use fmt .

let p option usize fmt.put_u64 buf. 0 1234 . .
guard is_some p. . else return 1 . .

0 is pos (the next place to write), and the option u64 returned is the new pos on success, none if room ran out.

Formatting is not output. Every op only assembles bytes into the caller’s buffer and, without exception, is effects none. Actual output is done separately by a caller holding cap io, with stdio.write or outbuf. So assembly code can be called from anywhere without capabilities, tests compare buffers rather than screens, and the caller decides what to emit and when. Turning numbers into bytes needs neither kernel nor allocation, so it is a library — no core op was added, and the whole file is inside the VM/native cross-check.

Design and boundaries#

• There is one convention — every writer is (buf, pos) → new pos. Success gives some newpos, shortage gives none. It refuses with a value rather than stopping. Give the new pos to the next call as it is and the text joins up.
• All or nothing. If room runs out, not a byte is written — if “12” remains instead of “1234”, nobody notices. That is why put_u64 and put_hex count the digits first (dec_width, hex_width) and write from the front.
• A line break is one LF (put_nl). It is the same convention as take_line in io, and CRLF is neither made nor removed.
• What it does not do — floating-point formatting, padding and alignment, locale (thousands separators). If needed, measure the width with dec_width and fill spaces yourself with put_byte.
• No state. No struct, no enum. The buffer and cursor both belong to the caller, so any number of uses anywhere at once do not interfere.

Ops at a glance#

put_ in a name means “write into the buffer” and _width means “only count how many bytes are needed”. Those that change the buffer are proc, those that only count are fn, but none of the nine touches the outside.

Ops in detail#

Every writer’s first two parameters are the same. buf is where to write (received because the module does not allocate, mut because it changes it), and pos is from which place in it (the module does not remember the cursor, so passing the new pos from the previous call is the only way to say “continue”).

• put_byte — writes the single byte b to buf[pos] ('-' is 45, LF is 10). Success gives some (pos+1).
• put_str — writes the whole byte string s. s is only read, so a string literal can be given as is. none if pos + len s exceeds len buf. ..
• dec_width · hex_width — the number of digits when writing n in decimal or hex. At least 1 (0 is one digit). They precompute for all-or-nothing, but you may use them directly to measure in advance “how many bytes are enough”.
• put_u64 · put_hex — unsigned decimal and hex (lowercase, no 0x). If you need 0x, put put_str buf pos "0x" in front.
• put_i64 — signed decimal. If negative, writes - first and handles the magnitude in u64. The i64 minimum (−9223372036854775808) cannot be negated to a positive, so 0 − n is computed in u64. This boundary value prints correctly too.
• put_bool — writes "true" or "false". Not "1"/"0", so 4 or 5 bytes are needed.
• put_nl — one LF byte. Same as put_byte buf pos 10.

Using it#

Assemble purely, output once at the end with cap io. The key is passing the returned new pos on to the next call.

module report .

use fmt .
use stdio .

proc main
  input out cap io .
  input al  cap allocator .
  output u8 .
  effects alloc io .
do
  let g option mut slice u8 alloc_bytes al. capacity 64 . .
  guard is_some g. . else return 70 . .
  let buf mut slice u8 some_value g. . .

  rem assembly --- not a byte has gone out yet
  let p1 option usize fmt.put_str buf. 0 "answer=" . .
  guard is_some p1. . else return 71 . .
  let p2 option usize fmt.put_u64 buf. some_value p1. . 42 . .
  guard is_some p2. . else return 72 . .
  let p3 option usize fmt.put_str buf. some_value p2. . " hex=" . .
  guard is_some p3. . else return 73 . .
  let p4 option usize fmt.put_hex buf. some_value p3. . 255 . .
  guard is_some p4. . else return 74 . .
  let p5 option usize fmt.put_nl buf. some_value p4. . . .
  guard is_some p5. . else return 75 . .

  rem output --- only the assembled front part (0 … p5) goes out
  let w usize stdio.write out. 1 subslice buf. 0 some_value p5. . . . .
  return 0 .
end .

The output is answer=42 hex=ff and one line break. Giving each piece a different exit code tells at once where room ran out. Measuring the buffer size in advance works with the same tools.

fn need_for input n u64 . output usize . do
  let w usize fmt.dec_width n. . .
  return add add 7 w. . 1 . .
end .

Counter-examples#

proc bad input out cap io . output usize . effects none . do
  return stdio.write out. 1 "x" . .
end .

let p1 option usize fmt.put_str buf. 0 "answer=" . .
let p2 option usize fmt.put_u64 buf. 0 42 . .        rem ✗ pos should be some_value p1

Cautions#

• Running short is a normal path. It is an expected answer, not an error, so receive it with guard. If you need an upper bound, measure in advance — u64 decimal is at most 20 digits, hex at most 16, and i64 adds one for the sign.
• pos is both an index and a length. After assembly, pos equals the bytes written so far, so subslice buf. 0 pos. . is the result.
• To reuse the buffer, just reset pos to 0. Old content need not be cleared — emit only up to the current pos and the old bytes are not seen.
• Write CRLF yourself. If a protocol needs it, write put_str buf pos "\r\n".
• Every op is effects none. They can be called inside a fn or an actor handler. Only output needs cap io and effects io.
• Prefixes, signs and padding are the caller’s job. Attach 0x, +, thousands commas or filler spaces yourself with put_str and put_byte.