how to conditionally pass a function to a function

[Michael Ploujnikov]

Hi,
I'm trying to write a function (rule-generator), which would generate actual 
Make rules based on functions/macros passed to it. In case when the user of 
rule-generator does not specify functions, default ones should be used. 
Otherwise, rule-generator should make sure to call the user-specified functions 
instead of the defaults. I've decided that rule-generator would be called 
within an $(eval $(call ...)) construct while the rule-generator itself will 
simply $(call ...) the other functions. Without the conditional choice between 
default and user-supplied functions my code looks like:

default-start-fun=echo "starting" && \
echo "started"
default-end-fun=echo "ending" && \
echo "ended"

address@hidden "uh, oh, we are doing something different!" &&\
echo "starting ... started"

define rule-generator
$(1):
        @echo "$(1) generating $(2)" && \                                       
               
        $(call $(3)) && \                                                       
               
        $(call $(4))
endef

$(eval $(call 
rule-generator,my-rule1,package-name,default-start-fun,default-end-fun))

Running this makefile results in:

$ make -f /tmp/simple.mk
my-rule1 generating package-name
starting
started
ending
ended

Now, I would like to move the burden of specifying the default functions from 
the user of rule-generator to the actual rule-generator "internals". This way 
the user can either omit passing arguments to rule-generator like:

$(eval $(call rule-generator,my-rule1,package-name,,))

Or the user can specify custom functions like:

$(eval $(call rule-generator,my-rule1,package-name,custom-start-fun,))

I've tried a number of things that failed for various reasons. I've tried 
inserting conditionals into the body of rule-generator, but Make choked on the 
line continuation syntax:

define rule-generator
$(1):
        @echo "$(1) generating $(2)" && \                                       
               
ifdef 3
        $(call $(3)) && \                                                       
               
else
        $(call default-star-fun) && \                                           
               
endif
        $(call $(4))
endef

This resulted in a confusion between make and shell commands:

$ make -f /tmp/simple.mk
my-rule1 generating package-name
/bin/sh: ifdef: command not found
make: *** [my-rule1] Error 127

If I delete the " && \" after the first echo line (which I need in the final 
solution), make also chokes:

$ make -f /tmp/simple.mk
/tmp/simple.mk:21: *** missing `endif'.  Stop.

I tried setting variables inside the rule-generator, but outside of its target 
body. However, it seems that variable assignment is deferred until I don't need 
them any more:

define rule-generator
ifdef 3
start-fun=$(3)
else
start-fun=default-star-fun
endif
$(1):
        @echo "$(1) generating $(2)" && \                                       
               
        $(call $(start-fun)) && \
        $(call $(4))
endef

$(eval $(call rule-generator,my-rule1,package-name,custom-start-fun))

$ make -f /tmp/simple.mk
/bin/sh: -c: line 0: syntax error near unexpected token `&&'
/bin/sh: -c: line 0: `echo "my-rule1 generating package-name" &&  && echo 
"ending" && echo "ended"'
make: *** [my-rule1] Error 2

I've also tried $(eval ...) calls inside the if-statements but those seem to be 
executed regardless of the condition so the last one always overrides the 
previous occurrences.

I would like to know if someone has found a solution in GNU Make for this kind 
of problem.

Thanks.

P.S. Please cc me since I'm not subscribed to the list.