WolframLang: Rule

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

• Rule is used to represent the key/value pairs in Association (Key Value Pairs)
• Rule is used to represent named parameters (called options in WolframLang) for functions.
• 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 what it should be changed to.

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[ {a, b, c} , x_ -> x+1]
=== {1 + a, 1 + b, 1 + c}

RuleDelayed[lhs,rhs]

(Short syntax: lhs :> rhs)

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

RuleDelayed