GNU Make user-defined functions

like this:

map = $(foreach a,$2,$(call $1,$a))

The first argument is the function to call, the second is the list to iterate over.   Here's an example use of map: we use it to iterate over a list of variable names and print out the defined value and the expanded value of each variable.

print_variable = $1 ($(value $1) -> $($1))

print_variables = $(call map,print_variable,$1)

VAR1 = foo

VAR2 = $(VAR1)

VAR3 = $(VAR2) $(VAR1)

$(info $(call print_variables,VAR1 VAR2 VAR3))

print_variable takes the name of a variable as its first and only argument and returns
a string consisting of the name of the variable, its definition and its value.   print_variables applies print_variable to a list of variables using map.

Here's the output of the above Makefile snippet:

VAR1 (foo -> foo) VAR2 ($(VAR1) -> foo) VAR3 ($(VAR2) $(VAR1) -> foo foo)

GNU Make functions can also be recursive and so it's possible to $(call) yourself.   Here's an implementation of the common reduce function using recursion.

reduce = $(if $(strip $2),$(call reduce,$1,$(wordlist 2,$(words $2),$2),$(call $1,$(firstword $2),$3)),$3)

reduce takes two arguments: a function that will be called by reduce and a list to process.  The first argument is called with two arguments: each element of the list in reduce's second argument and the result of the previous call to the function.

To see this in action we can create a list unique function that removes duplicates without reordering:

check_uniq = $(if $(filter $1,$2),$2,$2 $1)

uniq = $(call reduce,check_uniq,$1)

$(info $(call uniq,c b a a c c b a c b a))

This works because reduce will call check_uniq with each member of the input list in turn building up a new list from the result of check_uniq.  check_uniq just determines whether the element is present or not (using $(filter)) and returns the list with the element appended if it was not already seen.

Conclusion

GNU Make user-defined functions can do a lot.  If you want to see the extreme use of them examine how the GNU Make Standard Library pushed GNU Make user-defined functions to their limits.