Re: $(call ...) versus (macro-name ...)

[Ken Smith]

I have found value in calling functions which take no arguments because,
as Paul quoted from the manual, $(0) is defined.  The appended makefile
demonstrates two such uses.  One use is that your function can run
differently based on how it is called.  The other use I demonstrate here
is that you can "namespace" variables by prepending $(0) to them.  The
variables are still global but you can ignore them because it is
improbable that you'll have other variables with similar names.

  Ken Smith

# Implemented with GNU Make 3.81beta4

override EMPTY :=
SPACE := $(EMPTY) $(EMPTY)

function-variant1 = $(function-implementation)
function-variant2 = $(function-implementation)

function-implementation = \
$(strip \
  $(if $(0),,$(error This function must be called)) \
\
  $(info 0="$(0)") \
\
  $(eval $(0).variant := $(subst -,$(SPACE),$(0))) \
  $(eval $(0).variant := $(word 2,$($(0).variant))) \
\
  $(if $(filter variant1,$($(0).variant)), \
    $(info Hi!), \
    $(if $(filter variant2,$($(0).variant)), \
      $(info Hello.), \
      $(error Unrecognized variant "$($(0).variant)".) \
     ) \
   ) \
 )

$(call function-variant1)
$(call function-variant2)

# Generates an error.
# $(call function-implementation)

# Generates an error.
# $(function-implementation)

# Generates an error.
# $(function-variant1)

# Generates an error.
# $(function-variant1)

.PHONY: all
all:

On Tue, Feb 14, 2006 at 07:53:13PM +0100, John Graham-Cumming wrote:
> Paul D. Smith wrote:
> >However, you're correct that in the special case where call is invoked
> >and not passed any arguments, the variable is simply expanded.  So,
> >saying "$(call FOO)" is exactly identically equivalent to saying
> >"$(FOO)" (except a tiny bit slower).
> 
> Actually there is one small difference.  When you do $(call FOO) GNU Make's 
> code resets the counter used to detect a variable that refers to itself and 
> hence it's possible to use $(call FOO) to avoid a 'variable refers to itself' 
> error if you really know what you are doing.
> 
> I recently used this quirk to add interactive breakpoints to my GNU Make 
> Debugger project.
> 
> John.
> -- 
> John Graham-Cumming
> address@hidden
> 
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