Rule¶

Rule[a, b] creates an immediate replacement rule (a -> b). RuleDelayed[a, b] creates a delayed rule (a :> b) whose right-hand side is re-evaluated every time the rule fires.

$ wo 'Rule[a, b]'
a -> b

$ wo 'RuleDelayed[x, Random[]]'
x :> Random[]

//, /. and //. all bind looser than ->, so a rule written as an argument or a list item can be handed to a postfix function or replaced in:

$ wo 'ToString[f[a -> 5 // Head], InputForm]'
f[Rule]

$ wo 'ToString[{a -> 5 /. 5 -> 6}, InputForm]'
{a -> 6}

A rule answers to a head replacement the way any other expression does:

$ wo 'ToString[(1 -> 2) /. Rule -> List, InputForm]'
{1, 2}

$ wo 'ToString[{a -> 1} /. Rule -> ff, InputForm]'
{ff[a, 1]}