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Comment by vog | Hacker News Reader

> As a shameless plug, I built a tool similar to Make and redo

Also be sure to have a look at tup, which operates vastly more efficient by simply walking the DAG in the opposite direction:

http://gittup.org/tup/

That is, instead of looking what you want to build and checking all timestamps of dependency files, it can use e.g. inotify, then know exactly which files changed, and rebuilds everything that depends on those files.

Moreover, it performs the modification check only once, at the beginning. During the build, it doesn't need to re-check everything, because it already knows which files were recreated.

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Good idea, but it misses some clever tricks. What happens when a dependency does change, but not in a way that actually matters? Buck has a nice feature where Java libraries are only fully recompiled when the API of their dependencies change, since everything is dynamically linked. https://www.youtube.com/watch?v=uvNI_E0ZgZU

> What happens when a dependency does change, but not in a way that actually matters

I believe that tup does have some features in that direction, but I may be mistaken.

tup can be made to ignore unimportant changes. I had to look up the syntax, hopefully I got it correct.

I have foo.c, bar.c and a rule like:

  : foreach foo.c bar.c |> ^o^ gcc -c %f -o %o |> %B.o {objs}
  : {objs} |> gcc %f -o %o |> baz

The ^o^ part tells it to not trigger the next rules if there's been no change to the output. So if you just change your source files to use an updated license, reformatted it for clarity, etc., then nothing else will happen.

I used this with some of my literate code projects where I had tup running org-tangle for me (via an emacs script). If I'd only updated the documentation, and the code hadn't changed, nothing else would build. If I'd only changed unimportant parts of the code that generated the same object files, no new binary or library would be built.

This looks awesome! Definitely very similar to walk(1). Thanks for sharing!

I can't recall what precisely, but I tried tup with a tiny project to build, and I found it couldn't handle the dependency structure. And apparently (as of 1 years ago) it wasn't supposed to be handled.

That sounds strange, especially for a tiny project. What kind of dependency structure did you have?

I really can't recall but it was a very stiff roadblock. You just couldn't depend on some file or variable ..

Maybe someone who used tup more recently can pitch in

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