(m4.info)Conditionals

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6 Conditionals, loops, and recursion
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Macros, expanding to plain text, perhaps with arguments, are not quite
enough.  We would like to have macros expand to different things, based
on decisions taken at run-time.  For that, we need some kind of
conditionals.  Also, we would like to have some kind of loop construct,
so we could do something a number of times, or while some condition is
true.

Ifdef

Testing if a macro is defined

Ifelse

If-else construct, or multibranch

Shift

Recursion in 'm4'

Forloop

Iteration by counting

Foreach

Iteration by list contents

Stacks

Working with definition stacks

Composition

Building macros with macros