Seems like this would be a very common need: once you switch to a custom failure track, you want it to stay on that track until the end!!!

The problem is that in a Railway, everything automatically has 2 outputs. But we really only need one (which is exactly what Path gives us). And you end up fighting the defaults when there are the automatic 2 outputs, because you have to remember to explicitly/verbosely redirect all of those outputs or they may end up going somewhere you don't want them to go.

The default behavior of everything going to the next defined step is not helpful for doing that, and in fact is quite frustrating because you don't want unrelated steps to accidentally end up on one of the tasks in your custom failure track.

And you can't use fail for custom-track steps becase that breaks magnetic_to for some reason.

I was finding myself very in need of something like this, and was about to write my own DSL, but then I discovered this. I still think it needs a better DSL than this, but at least they provided a way to do this. Much needed.

For this example, I might write something like this:

step :decide_type, Output(Activity::Left, :credit_card) => Track(:with_credit_card)

# Create the track, which would automatically create an implicit End with the same id.
Track(:with_credit_card) do
    step :authorize
    step :charge
end

I guess that's not much different than theirs. Main improvement is it avoids ugly need to specify end_id/end_task.

But that wouldn't actually be enough either in this example, because you would actually want to have a failure track there and a path doesn't have one ... so it sounds like Subprocess and a new self-contained ProcessCreditCard Railway would be the best solution for this particular example... Subprocess is the ultimate in flexibility and gives us all the flexibility we need)

But what if you had a path that you needed to direct to from 2 different tasks' outputs?

Example: I came up with this, but it takes a lot of effort to keep my custom path/track hidden/"isolated" and prevent other tasks from automatically/implicitly going into those steps:

class Example::ValidationErrorTrack < Trailblazer::Activity::Railway
  step :validate_model, Output(:failure) => Track(:validation_error)
  step :save,           Output(:failure) => Track(:validation_error)

  # Can't use fail here or the magnetic_to won't work and  Track(:validation_error) won't work
  step :log_validation_error, magnetic_to: :validation_error,
    Output(:success) => End(:validation_error), 
    Output(:failure) => End(:validation_error) 
end

puts Trailblazer::Developer.render o
Reloading...

#<Start/:default>
 {Trailblazer::Activity::Right} => #<Trailblazer::Activity::TaskBuilder::Task user_proc=validate_model>
#<Trailblazer::Activity::TaskBuilder::Task user_proc=validate_model>
 {Trailblazer::Activity::Left} => #<Trailblazer::Activity::TaskBuilder::Task user_proc=log_validation_error>
 {Trailblazer::Activity::Right} => #<Trailblazer::Activity::TaskBuilder::Task user_proc=save>
#<Trailblazer::Activity::TaskBuilder::Task user_proc=save>
 {Trailblazer::Activity::Left} => #<Trailblazer::Activity::TaskBuilder::Task user_proc=log_validation_error>
 {Trailblazer::Activity::Right} => #<End/:success>
#<Trailblazer::Activity::TaskBuilder::Task user_proc=log_validation_error>
 {Trailblazer::Activity::Left} => #<End/:validation_error>
 {Trailblazer::Activity::Right} => #<End/:validation_error>
#<End/:success>

#<End/:validation_error>

#<End/:failure>

Now attempt to do it with Path... Does the Path() have an ID we can reference? Or maybe we just keep a reference to the object and use it directly in 2 different places?

class Example::ValidationErrorTrack::VPathHelper1 < Trailblazer::Activity::Railway
   validation_error_path = Path(end_id: "End.validation_error", end_task: End(:validation_error)) do
    step :log_validation_error
  end
  step :validate_model, Output(:failure) => validation_error_path
  step :save,           Output(:failure) => validation_error_path
end

o=Example::ValidationErrorTrack::VPathHelper1; puts Trailblazer::Developer.render o
Reloading...

#<Start/:default>
 {Trailblazer::Activity::Right} => #<Trailblazer::Activity::TaskBuilder::Task user_proc=validate_model>
#<Trailblazer::Activity::TaskBuilder::Task user_proc=validate_model>
 {Trailblazer::Activity::Left} => #<Trailblazer::Activity::TaskBuilder::Task user_proc=log_validation_error>
 {Trailblazer::Activity::Right} => #<Trailblazer::Activity::TaskBuilder::Task user_proc=save>
#<Trailblazer::Activity::TaskBuilder::Task user_proc=log_validation_error>
 {Trailblazer::Activity::Right} => #<End/:validation_error>
#<Trailblazer::Activity::TaskBuilder::Task user_proc=save>
 {Trailblazer::Activity::Left} => #<Trailblazer::Activity::TaskBuilder::Task user_proc=log_validation_error>
 {Trailblazer::Activity::Right} => #<End/:success>
#<End/:success>

#<End/:validation_error>

#<End/:failure>

It's just too bad that:

• there's not a Railway helper in case you want multiple outputs, though we could probably create one pretty easily using Path as our template
• we can't "inline" a separate Railway acitivity (Subprocess "nests" it rather than "inlines")

Tragic, but interesting and easy to remember example

This is a great example of Kohonen's Self-organizing Maps and the use of the U-Matrix. The authors were very thorough in explaining how it can be used.