WolframLang: Rule, RuleDelayed

A rule in WolframLang is a pair of values. It is used in:

• Rule is used as a find/replace pair for transformation of expressions, where the left-hand-side is a Pattern, and right-hand-side is replacement.

Rule is also used in:

• Rule is used to represent the key/value pairs in Association (Key Value Pairs).
• Rule is used to represent pairs of named parameter and its value (called options in WolframLang) for functions.

Rule[lhs, rhs]

🔸 SHORT SYNTAX: lhs -> rhs

A pair of values, with left-hand-side lhs and right-hand-side rhs

Both lhs and rhs is evaluated at the time the rule is defined.

Rule

ReplaceAll[ {1, {2}, 3} , {x_} -> x+1]
(* {1, 3, 3} *)

RuleDelayed[lhs, rhs]

🔸 SHORT SYNTAX: lhs :> rhs

Same as Rule, but rhs is evaluated at the time the transformation happens.

RuleDelayed