Operators cheat sheet

A quick reference for precedence, associativity, and common parsing pitfalls.

Precedence (higher binds tighter)

• 1100 xfy: ;
• 1050 xfy: ->
• 1000 xfy: ,
• 900 xfx: CLP(FD) reification #<=>, #==>, #<==
• 700 xfx: =, \=, ==, \==, =.., is, =:=, =\=, <, =<, >, >=, in, @<, @=<, @>, @>=
• 500 yfx: +, -
• 400 yfx: *, /, //, mod
• 200 fy: unary -, unary +
• 300 fy: @ (JSON constants)

Associativity

• xfy (right‑assoc): A -> B -> C is A -> (B -> C)
• yfx (left‑assoc): 1 - 2 - 3 is (1 - 2) - 3
• xfx (non‑assoc): cannot chain without parentheses

Non‑associative examples

?- X = Y = Z.
% error: use parentheses
?- X = (Y = Z).

Operator vs separator commas

• a, b uses , as a control operator (conjunction)
• p(a, b, c) uses commas as argument separators (not operators)

Prefix operators

• -3 + 2 parses as (-3) + 2
• \+ Goal is negation as failure (see Control)
• @/1 writes classic JSON constants: @(null), @(true), @(false)

Term order operators

• @<, @=<, @>, @>= compare terms by standard order (not numeric/semantic comparisons)

If–then–else

• A -> B ; C parses as (A -> B) ; C
• Parenthesize when mixing with , and ;: (A, B -> C ; D)

CLP(FD)

• Comparisons: #=, #\=, #<, #=<, #>, #>=
• Reification: #<=>, #==>, #<== (xfx at 900)

See also: Basics → Operators, Reference → Operators table