$(eval) expansion within $(eval)

[Tony Strauss]

Calling $(eval) within $(eval) can lead to very unintuitive (at least to me :) behavior.  In particular, the inner $(eval) is expanded *and* executed during the expansion of the outer $(eval).  For instance (I've also attached this example, since my email client mangled the tabs in the code below):

 

define func2
  MY_FUNC2_VAR := $$(SOME_VALUE)
  all::

       @echo "Here2: $$(MY_FUNC2_VAR)"
endef

 

define func3
  MY_FUNC3_VAR := $$(SOME_VALUE)
  all::

        @echo "Here3: $$(MY_FUNC2_VAR)"
endef

 

define func1
  SOME_VALUE := 1
  MY_FUNC1_VAR := $$(SOME_VALUE)
  all::

        @echo "Here1: $$(MY_FUNC1_VAR)"

  $(eval $(call func2))

  ifndef SOME_VALUE
     $(eval $(call func3))
  endif
endef

 

$(eval $(call func1))

all::

Running gmake (3.81) on this yields:
Here2:
Here3:
Here1: 1

 

The execution of the inner $(eval)s before the execution of the outer $(eval) means that:

1.) SOME_VALUE is not defined when either func2 or func3 is executed.

2.) func3 is expanded and executed despite being within a conditional that never will be true.

 

After encountering this, I looked this up in the O'Reilly gmake book (http://www.amazon.com/Managing-Projects-Make-Nutshell-Handbooks/dp/0596006101), and, while it did discuss it, it presented this behavior as a feature (since with it one can force early evaluation of code with an $(eval) inside an $(eval)).

 

Unfortunately, $$(eval) causes gmake to die, so there does not appear to be any way to use $(eval) within $(eval) so that it executes within the flow of the outer $(eval)'s execution.  Is there any way to do this?  One work around that I have thought of is to simply put the inner eval's function contents within a file and include the file...  Are there any plans to have some kind of $$(eval) mechanism that would defer execution inside $(eval)?  Additionally, perhaps the current $(eval) within $(eval) behavior should be documented in the gmake manual, since it really can take one by surprise the first time that it is encountered (unless it already is documented, and I missed it :).  Thank you.

 

Tony

test.mk
Description: Text document