Expose topological sorting of packages via command or option

[jonrgold]

I'm hoping that we may be able to provide either a new command or a flag to ls to expose the topologically sorting of artifacts to a consumer.

Expected Behavior

• lerna ls --toposort (or similar) could return a list of artifacts but in the topologically sorted behavior

Current Behavior

Currently exec / run support topological sorting, but there doesn't seem to be an easy way to the topologically sorted dependency structure via a command.

Right now to get that structure, we're using lerna exec $LERNA_PACKAGE_NAME.

Possible Solution

• lerna ls --toposort (or similar) could return a list of artifacts but in the topologically sorted behavior
• add list command for testing flag combinations/piping to other scripts #610

Context

We have a few use cases:

1. Getting a high level view of the dependency structure in our mono-repo to assist in refactoring.
2. Piping packages from this command into our own org-specific publish script that would already be sorted.

I think this is kind of similar to #610 . Was hoping it might be somewhat simple for me to create a PR where the results of batchPackages could be used to display this to user.

[evocateur]

Yeah, totally. batchPackages is exactly the ticket, I would imagine a conditional between these two steps:

    chain = chain.then(() => getFilteredPackages(this.packageGraph, this.execOpts, this.options));

    if (this.options.toposort) {
      chain = chain.then(filteredPackages =>
        batchPackages(filteredPackages)
          // flatten batched packages for display
          .reduce((arr, batch) => arr.concat(batch), [])
      );
    }

    chain = chain.then(filteredPackages => {
      this.result = listable.format(filteredPackages, this.options);
    });

[morewry]

This would be useful for me right now :-) I'm having an issue where a build is failing. In logs it appears that lerna run is moving on / concurrently running a dependent's build before its dependency's build is complete. Be nice to verify whether the sort is right to help rule things out.

For others, this modification of the command shared by OP worked:

npx lerna exec -- echo \$LERNA_PACKAGE_NAME

[lock]

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