define _mb_assert_success
	@echo success: mb_assert_$(1)\; \'$(2)\'
endef
define _mb_assert_failure
	@echo failure: mb_assert_$(1)\; \'$(2)\'
	@exit 1
endef
define _mb_assert_if
	$(if $(1),$(call _mb_assert_success,$(2),$(3)),$(call _mb_assert_failure,$(2),$(3)))
endef
define _mb_assert_if_not
	$(if $(1),$(call _mb_assert_failure,$(2),$(3)),$(call _mb_assert_success,$(2),$(3)))
endef

# FIXME: Not stable
#define mb_assert_file_contains
#	$(call _mb_assert_if,$(findstring $(2),$(file < $(1))),file_contains,$(2))
#endef
define mb_assert_file_equals
	$(call _mb_assert_if_not,$(subst $(2),,$(file < $(1))),file_equals,$(2))
endef

# Assert that the variable(s) are set and doesn't have non-empty values.
# Params;
#   1. Variable name(s) to validate.
#   2. (optional) Error message to user.
define mb_assert_defined
	$(strip $(foreach 1,$1,$(call _mb_assert_defined_error,$1,$(strip $(value 2)))))
endef
define _mb_assert_defined_error
	$(if $(value $1),,$(error Undefined $1$(if $2, ($2))))
endef