How to describe a document in words

This guide shows you how to turn a block document into an outline of its blocks and the ways control passes between them, write that outline as lines of English, and reword the lines in your own voice.

StatifierBlocks.Describe.outline/3 answers the structure and StatifierBlocks.Describe.render/2 the lines. docs/adr/0016-document-describes-itself.md is the record; everything below is the moves.

Before you start

You need the document and the palette you compile it with. The describe never compiles: it reads the document's tree and the block types' declarations, so no compiler check runs, and a block whose type the palette cannot resolve is described as its placeholder rather than refused.

Nothing here reads a clock, starts a process, reaches a network or asks a language model, and equal input answers byte-identical output. Nothing here needs LiveView either.

The examples below are one program: later blocks use names earlier ones bound.

Outline a document

Build or load the document, then ask for its outline under your palette. This one is a library loan: a patron who owes fines is sent a notice and the loan waits for payment, the book is checked out, and the loan waits up to 21 days for its return. A renewal resumes the wait; a report that the book is lost abandons it. Either way the loan is closed.

alias StatifierBlocks.{Block, Describe, Document, Palette}
alias StatifierBlocks.Describe.{Edge, Node}

send_event = fn id, event -> Block.new("core.send", id: id, config: %{"event" => event}) end

fines =
  Block.new("core.branch",
    id: "fines",
    config: %{"arms" => [%{"slot" => "arm_owes", "cond" => "patron.fines_owed > 0"}]},
    slots: %{
      "arm_owes" => [
        send_event.("notice", "loan.fines_notice"),
        Block.new("core.await", id: "paid", config: %{"event" => "fines.paid"})
      ],
      "otherwise" => [],
      "undecided" => []
    }
  )

lending =
  Block.new("core.resumable_group",
    id: "lending",
    config: %{"history" => "shallow"},
    slots: %{
      "body" => [
        Block.new("core.await",
          id: "returned",
          config: %{"event" => "loan.returned", "timeout" => "21d"}
        )
      ],
      "interrupts" => [
        Block.new("core.on_event",
          id: "renewed",
          config: %{"event" => "loan.renewed", "outcome" => "resume"}
        ),
        Block.new("core.on_event",
          id: "lost",
          config: %{"event" => "loan.reported_lost", "outcome" => "abandon"}
        )
      ]
    }
  )

loan =
  Block.new("core.sequence",
    id: "loan",
    slots: %{
      "body" => [fines, send_event.("checkout", "loan.checked_out"), lending, send_event.("close", "loan.closed")]
    }
  )

document = Document.new(loan, id: "bdoc_library_loan", revision: 1)
outline = Describe.outline(document, Palette.core(), [])

identity = {outline.id, outline.revision}
#=> {"bdoc_library_loan", 1}

Every block is one StatifierBlocks.Describe.Node, exactly once, in StatifierBlocks.ViewModel.outline/1's reading order. A node carries the block's id, type, parent, depth, the kind of slot its parent holds it in (:step, :arm, :rail or :tray), its sentence, the outcome names its type declares, its summary chips and its fan label:

nodes = Enum.map(outline.nodes, &{&1.id, &1.parent, &1.kind})
#=> [{"loan", nil, :step}, {"fines", "loan", :step}, {"notice", "fines", :arm}, {"paid", "fines", :arm}, {"checkout", "loan", :step}, {"lending", "loan", :step}, {"returned", "lending", :step}, {"renewed", "lending", :rail}, {"lost", "lending", :rail}, {"close", "loan", :step}]

returned = Enum.find(outline.nodes, &(&1.id == "returned"))
await_outcomes = returned.outcomes
#=> ["received", "timed_out"]

Read the edges the containers draw

An edge is found inside a block of four types, from the document's structure:

Every other type is described by containment and its node's fan label alone. A :sequence or :exit edge carries every outcome its source's type declares, on the one edge.

One more kind is read across the whole document rather than inside a container. A core.send with a delay draws a :timer edge to every core.on_event and core.await whose event is the one it sends: the edge runs from the send to the rule or await, its container is the send's parent, its event is the sent event and its delay the send's delay as stored. It is not a transition; it says the send arms an event the other block waits for. A send with no delay, and any block inside a drafts shelf, draws none. Timer edges come after every other edge, and the default line reads In 24 hours, registration.deadline reaches ..., the delay in the send's own words. The loan above has no delayed send, so its outline has none.

To answer "what ends the loan early", read the interrupt edges:

interrupts = for %Edge{kind: :interrupt} = edge <- outline.edges, do: {edge.event, edge.to, edge.history}
#=> [{"loan.renewed", {:body, "lending"}, :shallow}, {"loan.reported_lost", {:exit, "lending"}, nil}]

A resume goes back into the group's body (at its history, for a core.resumable_group; a core.group restarts its body from the first step); an abandon goes to the group's exit. A branch's arms read the same way, each carrying its condition, :otherwise or :undecided. The empty otherwise arm goes straight to the branch's exit, where the arms converge, and the empty undecided arm draws no edge at all:

arms = for %Edge{kind: :branch} = edge <- outline.edges, do: {edge.condition, edge.to}
#=> [{"patron.fines_owed > 0", {:block, "notice"}}, {:otherwise, {:exit, "fines"}}]

Write the outline as lines

render/2 answers one line per node, in node order, then one line per edge, in edge order. A line is non-blank English with no newline, carriage return or tab, and a block's id never appears in a default line:

lines = Describe.render(outline, [])

node_lines = Enum.take(lines, length(outline.nodes))
#=> ["Run its steps in order", "Decide: When \"owes\", otherwise (one of)", "Send loan.fines_notice", "Wait for fines.paid", "Send loan.checked_out", "Resumable group", "Wait for loan.returned, giving up after 21d", "When loan.renewed, resume", "When loan.reported_lost, abandon", "Send loan.closed"]

interrupt_lines = Enum.filter(lines, &String.starts_with?(&1, "On "))
#=> ["On loan.renewed, When loan.renewed, resume resumes Resumable group at shallow history", "On loan.reported_lost, When loan.reported_lost, abandon abandons Resumable group"]

Reword lines with a phrasing module

Implement StatifierBlocks.Describe.Phrasing and pass the module as phrasing:. Each line is offered to the callback named for its node's kind (step/2, arm/2, rail/2, tray/2) or its edge's kind (entry/2, sequence/2, branch/2, interrupt/2, exit/2, timer/2), with the structured node or edge and the default line. Every callback is optional.

defmodule MyApp.Describing.LoanPhrasing do
  @behaviour StatifierBlocks.Describe.Phrasing

  alias StatifierBlocks.Describe.{Edge, Node}

  @impl true
  def rail(%Node{id: "renewed"}, _default), do: "A renewal"
  def rail(%Node{id: "lost"}, _default), do: "A report that the book is lost"
  def rail(%Node{}, _default), do: :default

  @impl true
  def interrupt(%Edge{event: "loan.renewed"}, _default), do: "A renewal restarts the wait for the return"
  def interrupt(%Edge{event: "loan.reported_lost"}, _default), do: "A lost book ends the lending\nearly"
  def interrupt(%Edge{}, _default), do: :default
end

reworded = Describe.render(outline, phrasing: MyApp.Describing.LoanPhrasing)

reworded_lines = Enum.filter(reworded, &String.starts_with?(&1, "A "))
#=> ["A renewal", "A report that the book is lost", "A renewal restarts the wait for the return"]

lost_line = List.last(reworded)
#=> "On loan.reported_lost, When loan.reported_lost, abandon abandons Resumable group"

The callback answers its own line or :default. An answer that is blank, carries a newline, carriage return or tab, is not a string, or comes from a callback that raises, throws or exits keeps the default line for that one node or edge, never an error. That is why the lost-book interrupt above, whose answer carries a newline, still reads as the default.

Know what a phrasing module cannot change

The seam rewords lines one at a time. It does not add, drop or reorder them, and it never sees or changes the outline: render/2 with and without a phrasing module answers the same number of lines in the same order. A node's rewording is its own line only; an edge line that names that block still quotes the block's default sentence, so reword the edge too when it should match.

same_count? = length(reworded) == length(lines)
#=> true

For the lines to stay byte-identical for equal input, each callback has to be a pure function of its arguments; that half of the contract is yours.