Operators¶

PyLog supports standard Prolog operators with defined precedence and associativity. The reader parses infix/prefix forms into canonical structures.

Main categories¶

Arithmetic: +, -, *, /, //, mod
Arithmetic compare: =:=, =\=, <, =<, >, >=
Term equality/identity: =, \=, ==, \==, =..
Control: , (conjunction), ; (disjunction), ->/2 (if‑then)
Negation: \+/1 (prefix)
CLP(FD): #=, #\=, #<, #=<, #>, #>= and reification #<=>, #==>, #<==

Precedence and associativity¶

Higher precedence binds tighter. Associativity determines grouping at the same precedence.

; (1100, xfy) and -> (1050, xfy) are right‑associative control constructs.
, (1000, xfy) is conjunction and is right‑associative.
Comparisons (700, xfx) are non‑associative: you cannot chain them without parentheses.
is/2 and arithmetic comparisons are at precedence 700.
+ and - (500, yfx) are left‑associative; *, /, //, mod (400, yfx) bind tighter than +/-.
Prefix -, + have high precedence (200, fy): -3 + 2 parses as (-3) + 2.
Term order comparisons @<, @=<, @>, @>= compare terms by standard order.

Examples¶

?- 1 - 2 - 3.
% parses as (1 - 2) - 3 (left‑assoc)

?- A -> B ; C.
% parses as (A -> B) ; C (right‑assoc)

?- X = 1, Y = 2, (X =:= Y ; X < Y).
X = 1, Y = 2.

Non‑associative operators (xfx)¶

Operators with type xfx cannot be chained at the same precedence. Use parentheses to group.

?- X = Y = Z.
% error: non‑associative operator requires parentheses
?- X = (Y = Z).

Prefix operators¶

Unary - and + apply to numeric expressions.
\+/1 is negation as failure; see Basics → Control for semantics and caveats.
@/1 is recognized for classic JSON constants: @(null), @(true), @(false) (see Reference → JSON).

Comma: operator vs separator¶

, is a control operator in goals: a, b means run a then b.
Inside argument lists, commas separate arguments and are not operators: p(a, b, c) is a single goal with three arguments.

Quoting operator atoms¶

Operators are atoms in canonical form and can be used as functors when quoted: '+'(1,2) is the same as 1 + 2. This is rarely needed but useful for meta‑programming.

CLP(FD) reification¶

The reader supports #<=>, #==>, #<== as non‑associative (xfx) at precedence 900. See CLP(FD) → Reification for details.