assigns rhs to be the value of lhs, and associates the assignment with the symbol f.


  • TagSet defines upvalues or downvalues as appropriate.
  • The symbol f in f/:lhs=rhs must appear in lhs as the head of lhs, the head of the head, one of the elements of lhs, or the head of one of the elements.
  • A common case is f/:h[f[args]]=rhs.
  • You can see all the rules associated with a particular symbol by typing ?symbol.
  • If f appears several times in lhs, then f/:lhs=rhs associates the assignment with each occurrence.
  • When it appears in unevaluated symbolic form, TagSet is treated as a scoping construct so that variables in nested occurrences are renamed if necessary.


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Basic Examples  (1)

Define a symbolic property of a symbol:

Scope  (3)

Different Kinds of Values  (3)

For ownvalues, the tag is redundant:

For downvalues, the tag is redundant:

Use a tag to define upvalues:

Applications  (1)

Build a database of exchange rates:

Properties & Relations  (5)

^= defines upvalues in the same way as using a tag does:

The right side of an immediate definition is evaluated when the definition is made:

The right side of a delayed definition is evaluated each time the definition is used:

Definitions with the same left side overwrite earlier ones:

Definition prints definitions associated with a symbol:

Information prints various information about a symbol, including any definitions:

UpValues returns a list of rules corresponding to any upvalues defined:

Use =. to clear definitions with a particular left-hand side:

Clear all definitions:

Possible Issues  (2)

In the presence of global variables, pattern variables may show unexpected behavior:

Delayed assignments behave as expected:

Use upvalues to assign to subscripted variables:

Subscripted variables are distinct from indexed variables:

Introduced in 1988