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5.6 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:

Builtin: pushdef (name, [expansion]
Builtin: popdef (name…)

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.

The expansion of both pushdef and popdef is void. The macros pushdef and popdef are recognized only with parameters.

define(`foo', `Expansion one.')
⇒
foo
⇒Expansion one.
pushdef(`foo', `Expansion two.')
⇒
foo
⇒Expansion two.
pushdef(`foo', `Expansion three.')
⇒
pushdef(`foo', `Expansion four.')
⇒
popdef(`foo')
⇒
foo
⇒Expansion three.
popdef(`foo', `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. However, POSIX allows other implementations that treat define as replacing an entire stack of definitions with a single new definition, so to be portable to other implementations, it may be worth explicitly using popdef and pushdef rather than relying on the GNU behavior of define.

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

Local variables within macros are made with pushdef and popdef. At the start of the macro a new definition is pushed, within the macro it is manipulated and at the end it is popped, revealing the former definition.

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


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