Recursive $(call)

[Philip Guenther]

It would greatly increase the expressiveness of GNU make's macro
expansion if it was possible to define recursive macro functions.  For
example, here's a function that performs calculates the transitive
closure of a set of dependencies:


# If $(firstword) is car, this is cdr:
rest = $(wordlist 1,$(words ${1}),${1})

tclose = $(if ${1},$(firstword ${1}) \
                   $(call tclose,$(sort ${DEP_$(firstword ${1})} \
                                   $(call rest,${1}))))

You could then define variables ala:

DEP_foo = bar baz quux
DEP_baz = quux blarp

and expect $(sort $(call tclose,foo)) to expand to "bar baz blarp quux"

Unfortunately, the restriction on recursive macros not referencing
themselves includes references via $(call).  I would argue that GNU make
should not try to protect someone from themself if the write a recursive
$(call).  Yes, you can trivially write infinite loops if it's allowed,
but the increase in expressiveness is substantial.

(Yes, you can write a version of tclose that expands the dependencies up
to a present depth, but that's a kludge.  I would rather call out to the
shell and do the full expansion there then do that.)


Philip Guenther