Idea: trace transformer for debugging

[ratorx]

Not an issue, but a possibly useful transformer that might be nice to have.

I've been playing around with haumea and whilst trying to debug some dependencies had the idea for a tracing dependencies by using the following transformer:

cursor: mod: builtins.mapAttrs (n: v: builtins.trace "eval ${lib.strings.concatStringsSep "." (cursor ++ [n])}" v) mod;

This will produce a haumea tree that will output evaluation traces for all module accesses, which you could use in conjunction with nix repl to easily figure out all the dependencies of an attribute:

So if you had a haumea tree like (not actually evaluable, just shorthand):

foo = {
  bar = 5;
  baz = {
    something = root.foo.bar + 1;
  } 
}

Running nix repl -f haumea_tree.nix foo.baz.something would produce the trace of:

trace: eval foo
trace: eval foo.baz
trace: eval foo.baz.something
trace: eval foo
trace: eval foo.bar

[gyaruwizard]

Reasonable suggestion. It's even more powerful due to its invariance. This means that it can be used to visualize how other transformers are consuming modules down the pipe:

transformers = with haumea.lib.transformers; [ trace liftDefault trace prependUnderscore trace ];