Makefile function example

1-simple.mk

PATH = /tmp/

define myfn
  processed_input=${PATH}/$(2)
  $(1) := $$(processed_input)
endef

mytarget:
	$(eval FILE=myfile)
	$(eval $(call myfn,abs_filename,$(FILE)))
	/bin/echo ${abs_filename}

2-wrong-complex.mk

PATH = /tmp/

define myfn
  return_var=${PATH}/$(2)
  /bin/echo complex # we used an echo function here for simplicity
                    # but instead it could be "if condition", "for loop" or any further shell expression
  $(1) := $$(return_var)
endef

mytarget:
	$(eval FILE=myfile)
	# using value here to avoid unwanted expansion will result in the return not being correctly assigned
	# And if we didn't use value at all, ```echo complex``` will be treated as a target without previous tabs
	#                                    if we added the tabs Make will complain because ```echo complex``` is now considered a target body without a previous target
	#                                    if we added a target "without tabs" then added ```echo complex```, it renders useless in generalization cause the make target won't give us output when we want
	#                                    in the 3-correct-complex.mk it will be clearer how the define is treated as another Makefile
	$(eval $(value $(call myfn,abs_filename,$(FILE)))
	/bin/echo ${abs_filename}

3-correct-complex.mk

PATH = /tmp/

define myfn
  processed_input=${PATH}/$(2)
  processed_more=$(shell echo $(processed_input))
               # could be "if condition", "for loop" or any further shell expression
               # Notice that: the output of the shell function is the stdout/stderr, for more advanced usage
  	       # Either use a file communication, or better, another scriptfile (shell, python or whatever)
  $(1) := $$(processed_more)
# NOTE: Uncommenting dummy_target will fail the main my_target unless $(value ..) got used between the $(eval ..) and $(call ..)
# NOTE: we should not use the $(value ..) here to avoid incorrect assignment 
#dummy_target:
	# further targets could be added here
	# Notice that, ```define``` keyword defines a multi-line expression that is very like another makefile 
	# with it's own variables and targets
endef

mytarget:
	$(eval FILE=myfile)
	# using value here to avoid unwanted expansion will result in the return not being correctly assigned
	$(eval $(call myfn,abs_filename,$(FILE)))
	/bin/echo ${abs_filename} # Path2 will be printed

README.md

Usage:

This is an example for a function myfn that:

• Takes a local input file name FILE
• Returns an abs_filename, created by using the global variable PATH

Notes:

• We couldn't replace myfn with another make target as it wouldn't give us a clear return value
• We had to use ($value ...) to avoid the unwanted expansion described in Eval Function and Value Function
• When we miss the value function, we get "Missing Separator Stop error"
• Commenting an ($eval ($call ... ) expression is not possible (even the syntax highlighting showed so -_-).
• Use this way only if the function expression is complicated and needs multiple lines, otherwise, one could use filters and similar stuff

References:

• StackOverflow Answer.
• Eval Function.
• Value Function.
• Shell Function.
• Define the multi-line-expression-enabler.

Thanks for the nice example.
One remark: 1-simple.mk line 10: closing parenthesis is missing

Thanks for the hint!
Now it's fixed ^_^