This is documentation for Mathematica 3, which was
based on an earlier version of the Wolfram Language.
 RuleDelayed lhs :> rhs or lhsrhs represents a rule that transforms lhs to rhs, evaluating rhs only after the rule is used. The character can be entered as :> or \[RuleDelayed]. RuleDelayed has the attribute HoldRest. You can apply rules using Replace. The assignment lhs := rhs specifies that the rule lhs :> rhs should be used whenever it applies. You can use Condition to specify when a particular rule applies. lhsrhs can be entered as lhs \[RuleDelayed] rhs or lhs :> rhs. In StandardForm, RuleDelayed is printed using . See notes for Rule. See the Mathematica book: Section 2.4.1, Section A.5.5. See also: Replace, SetDelayed, Rule. Further Examples The right-hand side of this rule is evaluated when you give the rule. In[1]:= Out[1]= A rule like this is probably not particularly useful. In[2]:= Out[2]= Here, the right-hand side of the rule is maintained in an unevaluated form, to be evaluated every time the rule is used. In[3]:= Out[3]= Applying the rule causes the expansion to be done. In[4]:= Out[4]=