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### 11.13 Variable Aliases

It is sometimes useful to make two variables synonyms, so that both variables always have the same value, and changing either one also changes the other. Whenever you change the name of a variable—either because you realize its old name was not well chosen, or because its meaning has partly changed—it can be useful to keep the old name as an alias of the new one for compatibility. You can do this with `defvaralias`.

Function: defvaralias new-alias base-variable &optional docstring

  This function defines the symbol new-alias as a variable alias for symbol base-variable. This means that retrieving the value of new-alias returns the value of base-variable, and changing the value of new-alias changes the value of base-variable. The two aliased variable names always share the same value and the same bindings.

  If the docstring argument is non-`nil`, it specifies the documentation for new-alias; otherwise, the alias gets the same documentation as base-variable has, if any, unless base-variable is itself an alias, in which case new-alias gets the documentation of the variable at the end of the chain of aliases.

  This function returns base-variable.

Variable aliases are convenient for replacing an old name for a variable with a new name. `make-obsolete-variable` declares that the old name is obsolete and therefore that it may be removed at some stage in the future.

Function: make-obsolete-variable obsolete-name current-name when &optional access-type

  This function makes the byte compiler warn that the variable obsolete-name is obsolete. If current-name is a symbol, it is the variable’s new name; then the warning message says to use current-name instead of obsolete-name. If current-name is a string, this is the message and there is no replacement variable. when should be a string indicating when the variable was first made obsolete (usually a version number string).

  The optional argument access-type, if non-`nil`, should specify the kind of access that will trigger obsolescence warnings; it can be either `get` or `set`.

You can make two variables synonyms and declare one obsolete at the same time using the macro `define-obsolete-variable-alias`.

Macro: define-obsolete-variable-alias obsolete-name current-name &optional when docstring

  This macro marks the variable obsolete-name as obsolete and also makes it an alias for the variable current-name. It is equivalent to the following:

      (defvaralias obsolete-name current-name docstring)
      (make-obsolete-variable obsolete-name current-name when)

Function: indirect-variable variable

  This function returns the variable at the end of the chain of aliases of variable. If variable is not a symbol, or if variable is not defined as an alias, the function returns variable.

  This function signals a `cyclic-variable-indirection` error if there is a loop in the chain of symbols.

    (defvaralias 'foo 'bar)
    (indirect-variable 'foo)
         ⇒ bar
    (indirect-variable 'bar)
         ⇒ bar
    (setq bar 2)
    bar
         ⇒ 2

    foo
         ⇒ 2

    (setq foo 0)
    bar
         ⇒ 0
    foo
         ⇒ 0

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