GNU make: foreach behave different in defined and outside - makefile

Question

I am trying to write a custom "function" in GNU Makefile. However, The "$(foreach" function behaves differently between inside "defined" and outside
Makefile steps:
Define current folder is "."
Get all the sub-folders
For each sub-folder, add the suffix "-subdir"
define get_folders
dirs:=$(dir $(wildcard $(1)/*/))
test_function:=$(foreach subdir,$$(dirs), $(subdir)"-subdir")
endef
CURRENT_FOLDER:=.
dirs:=$(dir $(wildcard $(CURRENT_FOLDER)/*/))
test_outside:=$(foreach subdir,$(dirs), $(subdir)"-subdir")
.PHONNY:test_function
test_function:
$(eval $(call get_folders,$(CURRENT_FOLDER)))
#echo "in-function" $(test_function)
#echo "outside" $(test_outside)
when I run the make, following is printed out:
in-function ./header/ ./Tools/-subdir
outside ./header/-subdir ./Tools/-subdir
As you can see, the result of commands outside are as I expected but for the in-function, it seems that all sub-folders are considered one string so only one suffix was added
Can you please kindly explain me why it happened this way and how can I change it?

Answer 1

Your problem comes from the way make expands your recipe. $(eval ...) first expands its parameter ($(call get_folders,$(CURRENT_FOLDER))). If you try to imagine the result you'll understand that the first line of your macro expands as:
dirs:=./header/ ./Tools/
But consider the second line:
test_function:=$(foreach subdir,$$(dirs), $(subdir)"-subdir")
The foreach will iterate only once, on the $(dirs) token, leading to:
test_function:=$(dirs)"-subdir"
Thus the result you observe. Try this, instead:
define get_folders
dirs:=$(dir $(wildcard $(1)/*/))
test_function:=$(foreach subdir,$(dirs), $(subdir)"-subdir")
endef
Note that mixing make and shell programming in your recipes is not ideal. It is difficult to understand, maintain and debug. You could as well separate the two worlds:
define get_folders
dirs:=$$(dir $$(wildcard $(1)/*/))
test_function:=$$(foreach subdir,$$(dirs), $$(subdir)"-subdir")
endef
CURRENT_FOLDER:=.
$(eval $(call get_folders,$(CURRENT_FOLDER)))
.PHONY:test_function
test_function:
#echo "in-function" $(test_function)
The $$ in the macro are used to escape the first expansion by eval, leading to:
dirs:=$(dir $(wildcard ./*/))
test_function:=$(foreach subdir,$(dirs), $(subdir)"-subdir")
If you plan to use this macro with several CURRENT_FOLDER values you can pass it a second parameter to personalize the variable names and avoid conflicts:
# $(1): current folder
# $(2): variable names prefix
define get_folders
$(2)-dirs:=$$(dir $$(wildcard $(1)/*/))
$(2)-subdirs:=$$(foreach subdir,$$($(2)-dirs), $$(subdir)"-subdir")
endef
CURRENT_FOLDER:=.
$(eval $(call get_folders,$(CURRENT_FOLDER),test_function))
.PHONY:test_function
test_function:
#echo "in-function" $($#-subdirs)
Even better, you could turn CURRENT_FOLDER into a target-specific variable:
# $(1): current folder
# $(2): variable names prefix
define get_folders
$(2)-dirs = $$(dir $$(wildcard $(1)/*/))
$(2)-subdirs = $$(foreach subdir,$$($(2)-dirs),$$(subdir)"-subdir")
endef
$(eval $(call get_folders,$$(CURRENT_FOLDER),test_function))
.PHONY:test_function
test_function: CURRENT_FOLDER = .
test_function:
#echo "in-function" $($#-subdirs)