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GNU macro processor

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4.6 Temporarily redefining macros

It is possible to redefine a macro temporarily, reverting to the previous definition at a later time. This is done with the builtins pushdef and popdef:

 
pushdef(name [, expansion])
popdef(name)

which are quite analogous to define and undefine.

These macros work in a stack-like fashion. A macro is temporarily redefined with pushdef, which replaces an existing definition of name, while saving the previous definition, before the new one is installed. If there is no previous definition, pushdef behaves exactly like define.

If a macro has several definitions (of which only one is accessible), the topmost definition can be removed with popdef. If there is no previous definition, popdef behaves like undefine.

 
define(`foo', `Expansion one.')
=>
foo
=>Expansion one.
pushdef(`foo', `Expansion two.')
=>
foo
=>Expansion two.
popdef(`foo')
=>
foo
=>Expansion one.
popdef(`foo')
=>
foo
=>foo

If a macro with several definitions is redefined with define, the topmost definition is replaced with the new definition. If it is removed with undefine, all the definitions are removed, and not only the topmost one.

 
define(`foo', `Expansion one.')
=>
foo
=>Expansion one.
pushdef(`foo', `Expansion two.')
=>
foo
=>Expansion two.
define(`foo', `Second expansion two.')
=>
foo
=>Second expansion two.
undefine(`foo')
=>
foo
=>foo

It is possible to temporarily redefine a builtin with pushdef and defn.

The macros pushdef and popdef are recognized only with parameters.


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  Copyright 2003   by The Free Software Foundation     Updated Jun 2003