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RuleDelayed

lhs :> rhs or lhs

rhs 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.

lhs

rhs 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.

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]=

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