Parse Declarations (:!)

The :! annotation domain declares how the parser should handle a user-defined operator — specifically its precedence and associativity. The parser pre-scans the source for :! declarations before parsing expressions, so they take effect immediately.

Syntax

(op) :! {prec = N; assoc = Left}

• prec — an integer precedence level. Higher values bind more tightly.
• assoc — one of Left, Right, or None. Determines how chains of the same operator group:
  • Left: a op b op c parses as (a op b) op c
  • Right: a op b op c parses as a op (b op c)
  • None: chaining is a parse error; explicit parentheses are required.

Example

(<=>) :! {prec = 30; assoc = Left}
(<=>) a b = if (a == b) 0 (if (a > b) 1 (0 - 1))

cmp1 = 5 <=> 3
cmp2 = 3 <=> 3
cmp3 = 1 <=> 5

build =  {target = c, os = linux, arch = x86_64}
<=> = <closure>
cmp1 = 1
cmp2 = 0
cmp3 = -1

Built-in Operator Precedences

For reference, the approximate precedence levels of built-in operators:

User-defined operators without a :! declaration receive a default precedence. Define :! to override this and integrate your operator naturally with built-in ones.

Metaprogramming Hook

Because :! declarations are processed during parsing (before evaluation), they serve as a metaprogramming hook — they let you reshape how the parser reads subsequent expressions. Combined with user-defined operators, this gives you control over the syntactic structure of your programs.