File structure and lexical rules

A .vest file is a sequence of top-level definitions in any order: format definitions, constant definitions, macro definitions, and at most one byte-order (endianness) directive.

Identifiers and reserved words

Identifiers start with a letter or _ and continue with letters, digits, or _. Format names, field names, and enum variants all use this form.

These words are reserved and cannot be used as identifiers:

macro   const   enum   choose   wrap
Option  Vec     Tail   Nothing  Never
btc_varint      uleb128

Integer type names are also reserved: any u or i followed by digits, so u8, u16, u3, i32 and so on are unavailable as names.

The following identifier forms have extra syntax:

Form	Meaning
`@name`	a dependency reference — a field bound with `@`, usable in later fields
`@name.member`	dotted access into a dependency’s field, nested arbitrarily deep
`_`	the wildcard branch of a `choose`

Comments

Only line comments exist:

// this is a comment

There is no block comment syntax (/* ... */).

Byte order

!BIG_ENDIAN

!LITTLE_ENDIAN

Note:

Little-endian is the default. A file with no directive is little-endian.
The directive is file-global, wherever it appears. Putting !BIG_ENDIAN on the last line still applies it to every definition in the file.

Byte order applies only where it is meaningful: multi-byte integers. It does not affect u8, byte arrays, or an 8-bit bits block.

Integer literals

There are three forms, usable anywhere a constant integer is expected — enum values, constraints, constant fields, array lengths:

Form	Example	Notes
decimal	`15213`
hexadecimal	`0x3F`, `0x5a`	`0x` prefix required
ASCII character	`'a'`, `'\x1b'`	one byte, value 0–255

Similar to Rust, enum values may carry a type suffix that fixes the underlying representation:

kind = enum { A = 0u16, B = 1u16, }

Constant arrays

Byte-array constants take either a string or a list form, and the list form has a repeat shorthand:

const MAGIC: [u8; 4] = "vest"
const ZEROS: [u8; 4] = [0; 4]
const BYTES: [u8; 3] = [0x01, 0x02, 0x03]

Whitespace

Whitespace, including newlines, is insignificant. Fields and enum variants are comma-terminated — including the last one:

msg = {
    a: u8,
    b: u16,     // trailing comma required
}

Vim and Neovim highlighting

The repository includes vest/vest.vim. Copy it to ~/.vim/syntax/vest.vim (or ~/.config/nvim/syntax/vest.vim) and add this to your Vim configuration:

autocmd BufRead,BufNewFile *.vest setfiletype vest

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