Makefile won't assign variable values in `ifeq` statement within function definition - makefile

Question

I'm trying to assign a value to a variable in make when a condition is met, inside a function:
Without $() around the ifeq statements:
VARIABLE=true
define test_ifeq
FOO := foo
ifeq (${VARIABLE}, true)
FOO := true
else
FOO := false
endif
echo "value: ${FOO}"
endef
all:
$(call test_ifeq)
Result:
FOO := foo
make: FOO: No such file or directory
make: *** [Makefile:15: all] Error 127
With $() around the ifeq and variable assignment statements:
VARIABLE=true
define test_ifeq
$(FOO := foo)
$(ifeq (${VARIABLE}, true)
FOO := true
else
FOO := false
endif)
echo "value: ${FOO}"
endef
all:
$(call test_ifeq)
Result:
echo "value: "
value:
Why does this not work?

Answer 1

The ifeq and the likes are evaluated at the moment when make reads the makefile, that means there is no actual ifeq present in the assigned value of your test_ifeq variable.
To achieve what you need you have to use $(if ...) and $(filter ...) built in functions along with $(eval ...).
It should look something like this:
VARIABLE=true
define test_ifeq
$(eval FOO := $(if $(filter $(VARIABLE),true), true, false))
echo "value: $(FOO)"
endef
all:
$(call test_ifeq)
NOTE: I haven't tested it, wrote it straight out of head...

Answer 2

To do it with Make:
VARIABLE=true
define test_ifeq
FOO := foo
ifeq (${VARIABLE}, true)
FOO := true
else
FOO := false
endif
$$(info "value: $${FOO}")
endef
$(eval $(call test_ifeq))
To do it on the command line (assuming bash):
VARIABLE=true; FOO=foo; if [ $VARIABLE = true ]; then FOO=true; else FOO=false; fi; echo $FOO
To do it within a rule in a makefile:
all:
VARIABLE=true; \
FOO=foo; \
if [ $$VARIABLE = true ]; then \
FOO=true; else \
FOO=false; \
fi; \
echo $$FOO
To do it within a rule, using a function:
VARIABLE=true
define test_ifeq2
FOO=foo; \
if [ $(VARIABLE) = true ]; then \
FOO=true; else \
FOO=false; \
fi; \
echo $$FOO
endef
all:
$(call test_ifeq2)