After a pretty large amount of Make I've concluded that one of the things that tends to make make confusing is the fact that it uses linear order-dependent code to express a DAG which isn't linear at all. A Makefile is just be a database of facts about a DAG, so order should be strictly a matter of style, not semantics.
One example problem area is with include, where it frequently turns out to be the case that a given include of e.g. generic_build_rules.mk has to be included somewhere in the middle of a Makefile, since some code needs to go before the include and other code after. What I would like to do is achieve a style in which the order of variable and rule declarations isn't relevant at all. Rules for coming close as far as I can see: 1. Never use simply expanded vars 2. Never use if etc, instead use $(if 3. Never use ?= (corollary to 2) 4. Enable .SECONDEXPANSION, and always use $$(VAR) in depends to get lazy eval Are the any non-obvious bad consequences to these (particularly 4)? The last part of the puzzle is what to do about targets themselves. So far as I can see there's no way to achieve late expansion of vars in targets. So if you have generic_build_rules.mk, and you want to introduce a target-specific variable for a rule with a variable in it's target in a Makefile that includes that file, you have an unavoidable order-dependency between the include and the target-specific variable declaration. Is there a way to avoid this that I'm missing? Static pattern rules are another sad case of inevitable order-dependence. They're more robust and yield must less confusing error messages than implicit rules, so it's sad no to be able to use them in an order-independent Makefile. Would it possible to have something like .SECONDEXPANSION that applied to targets and static pattern lists as well (or instead)? This is an area I'd be willing to try to help with if it's deemed possible. Thanks, Britton _______________________________________________ Help-make mailing list Help-make@gnu.org https://lists.gnu.org/mailman/listinfo/help-make
