$(if-eq ...)

[Stephan Beal]

Hi, all!

i'm trying to implement an $(if-eq ...) function which goes a little 
something like this:

$(call if-eq,LHS,RHS,THEN[,ELSE])

The intention is of course to get an inline equivalent of:
ifeq (LHS,RHS)
...
endif

i've got it working, but of course it's missing the short-circuit 
features of built-ins, e.g. so that the $(error) in:
$(if X,,$(error this is an error))
will only be evaluated if !X. That works for built-ins of course, but i 
don't see a way to do it with add-on funcs.

The code is trivial:

define if-eq
  ifeq ($1,$2)
    $3
  else
    $4
  endif
endef

Two tests:

$(eval $(call if-eq,1,2,\
  $$(error this should not happen),\
  $$(warning this should happen)))

$(eval $(call if-eq,abc,abc,\
  $$(warning this should happen),\
  $$(error this should not happen)))


That runs correctly, but requires that i use $$ on warning/eval and then 
wrap the whole thing in an eval statement.

Does anyone know a way (or kludge) that i can use to get the 
short-circuit evaluation of the THEN/ELSE parts, such that i could do:

$(call if-eq,A,B,$(error A does not equal B),$(eval something-else))

without actually running the $(error) or $(eval something-else) when 
those parts do not apply?

One of the limitations i'm working with (against?) is that i can't use 
features which are new in 3.81 because i'll eventually be targetting 
machines where 3.80 can be expected but where an upgrade to 3.81+ 
cannot be expected.

-- 
----- stephan beal
http://www.wanderinghorse.net

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