merging “processes” and “restrictions”

[Ricardo Wurmus <rekado@elephly.net>]

Hi,

I think it is unfortunate that workflows have two fields for specifying
processes: one is “processes” the other is “restrictions”.  It seems to
me that we can do without “restrictions” by relaxing the requirements
for the “processes” value.

The use of both fields sometimes makes it necessary to wrap the whole
workflow definition in a let binding so that both fields can access
identical values:

  (let ((eat-fruit (eat "fruit"))
        (eat-veges (eat "vegetables")))
    (workflow
     (name "simple")
     (processes
      (list greet
            eat-fruit
            eat-veges
            sleep
            bye))
     (restrictions
      `((,eat-fruit ,greet)
        (,eat-veges ,greet)
        (,sleep ,eat-fruit ,eat-veges)
        (,bye ,sleep)))))

This looks like a minor improvement to me because the let can be where
it’s needed:

    (workflow
     (name "simple")
     (processes
      (let ((eat-fruit (eat "fruit"))
            (eat-veges (eat "veges")))
        (list (list eat-fruit greet)
              (list eat-veges greet)
              (list sleep eat-fruit eat-veges)
              (list bye sleep)))))

All of the elements of the list together is equivalent to the list of
processes.  The “processes” field now also doubles as a “restrictions”
field as the value can be an adjacency list of processes to their
dependencies.

For trivial processes where none of the processes depend on each other
it would look like this:

    (workflow
     (name "simple")
     (processes
       (list (list A)
             (list B)
             (list C))))

With just a little bit of extra processing before storing the value it
could become this instead:

    (workflow
     (name "simple")
     (processes A B C))

If you’re like me you’ll find that the restrictions syntax looks rather
verbose with all those “list”s.  Using quoting doesn’t make this any
more readable, unfortunately:

    (workflow
     (name "simple")
     (processes
      (let ((eat-fruit (eat "fruit"))
            (eat-veges (eat "veges")))
        `((,eat-fruit ,greet)
          (,eat-veges ,greet)
          (,sleep ,eat-fruit ,eat-veges)
          (,bye ,sleep)))))

Can we use macros to clarify the syntax?

    (workflow
     (name "simple")
     (processes
      (let ((eat-fruit (eat "fruit"))
            (eat-veges (eat "veges")))
        (graph (eat-fruit -> greet)
               (eat-veges -> greet)
               (sleep     -> eat-fruit eat-veges)
               (bye       -> sleep)))))

“graph” would be a macro that takes any number of node to node
associations, each of which are expected to be in the form

    (node -> nodes …)

“graph” isn’t a great name.  Maybe you can suggest a different name or
even a character…

What do you think?

--
Ricardo