In April I wrote about how I use the platform_define feature of rebar to make deprecated types work without removing warnings_as_errors. I have switched to erlang.mk since then, but needed a similar feature for one of my libraries.

Since erlang.mk only uses make, adding support for a platform_define-like feature is very straightforward.

otp_release = $(shell erl -noshell -eval 'io:format("~s", [erlang:system_info(otp_release)]), init:stop()')
otp_17plus = $(shell echo $(otp_release) | grep -q -E "^[[:digit:]]+$$" ; echo $$?)

The first variable (otp_release) will contain the OTP version (e.g. 17). The second variable (otp_17plus) will be either 0 or 1, depending on wether or not otp_release matches a regular expression. The regular expression checks if otp_release is just a number (e.g. 17) or not (e.g. R16B03-1).

That’s enough to conditionally add options to erlc.

ifeq ($(otp_17plus),0)
	ERLC_OPTS += -Dnamespaced_types=1
	TEST_ERLC_OPTS += -Dnamespaced_types=1
endif

This will define namespaced_types only on Erlang 17+, allowing us to use the same ifdef-switch from April.

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-ifdef(namespaced_types).
-type xxx_dict() :: dict:dict().
-else.
-type xxx_dict() :: dict().
-endif.