The other conditional,
ifelse, is much more powerful. It can be
used as a way to introduce a long comment, as an if-else construct, or
as a multibranch, depending on the number of arguments supplied:
Used with only one argument, the
ifelse simply discards it and
produces no output.
If called with three or four arguments,
ifelse expands into
equal, if string-1 and string-2 are equal (character
for character), otherwise it expands to not-equal. A final fifth
argument is ignored, after triggering a warning.
If called with six or more arguments, and string-1 and
string-2 are equal,
ifelse expands into equal-1,
otherwise the first three arguments are discarded and the processing
ifelse is recognized only with parameters.
Using only one argument is a common
m4 idiom for introducing a
block comment, as an alternative to repeatedly using
special usage is recognized by GNU
m4, so that in this
case, the warning about missing arguments is never triggered.
ifelse(`some comments') ⇒ ifelse(`foo', `bar') error→m4:stdin:2: Warning: too few arguments to builtin `ifelse' ⇒
Using three or four arguments provides decision points.
ifelse(`foo', `bar', `true') ⇒ ifelse(`foo', `foo', `true') ⇒true define(`foo', `bar') ⇒ ifelse(foo, `bar', `true', `false') ⇒true ifelse(foo, `foo', `true', `false') ⇒false
Notice how the first argument was used unquoted; it is common to compare the expansion of a macro with a string. With this macro, you can now reproduce the behavior of blind builtins, where the macro is recognized only with arguments.
define(`foo', `ifelse(`$#', `0', ``$0'', `arguments:$#')') ⇒ foo ⇒foo foo() ⇒arguments:1 foo(`a', `b', `c') ⇒arguments:3
For an example of a way to make defining blind macros easier, see Composition.
ifelse can take more than four arguments. If given more
than four arguments,
ifelse works like a
statement in traditional programming languages. If string-1 and
string-2 are equal,
ifelse expands into equal-1, otherwise
the procedure is repeated with the first three arguments discarded. This
calls for an example:
ifelse(`foo', `bar', `third', `gnu', `gnats') error→m4:stdin:1: Warning: excess arguments to builtin `ifelse' ignored ⇒gnu ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth') ⇒ ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth', `seventh') ⇒seventh ifelse(`foo', `bar', `3', `gnu', `gnats', `6', `7', `8') error→m4:stdin:4: Warning: excess arguments to builtin `ifelse' ignored ⇒7
Naturally, the normal case will be slightly more advanced than these
examples. A common use of
ifelse is in macros implementing loops
of various kinds.