[[!meta title="Typesetting scenario steps based on matched bindings"]]
[[!tag pondering]]
[[!meta date="2019-11-30 09:34"]]

We're working on typesetting scenarios. The first iteration of this
aims at the following:

* Subplot reads the markdown input, and a YAML file with bindings. The
  YAML file is named in the markdown's metadata block (a Pandoc
  extension to markdown).
* The bindings may capture parts of a step using regular expressions.
* Each scenario snippet is typeset separately.
* Each step in a snippet is typeset separately.
* There is no support for "and" or "but". The keyword must be one of
  "given", "when", or "then".
* The keyword of a step is typeset in italics.
* Captured parts of a step are typeset in bold face.
* The rest of the step is typeset in a normal font.
* Steps that do not match a binding, or have an unknown keyword, or
  otherwise are problematic, are typeset in monospace and have a
  question mark prepended to them. A descriptive error message is
  added to the document as well.

To implement this, I'm going to want a couple of abstractions:

* A snippet parser, as an iterator over unparsed steps (string
  slices).
* A step parser, which takes an unparsed step (a string slice), and
  returns a parsed step.

A parsed step is matched against all known bindings. It can either be
a successful match, or an unsuccessful one.

A successful match consists of a step kind (keyword), and a list of
partial steps. A partial step is either captured or uncaptured parts
of the step.

Given a binding file like this:

```yaml
- given: I am (?P<name>\S+)
- when: I declare myself (king|queen)
- then: everyone agrees
```

And a scenario snippet like this:

```
given I am Tomjon
when I declare myself king
then there is applause
```

This would result in the following parse results:

* A matched step, with keyword _given_, an uncaptured part "I am " and
  a captured part "Tomjon"
* A matched step, with keyword _when_, and an uncaptured part "I
  declare myself king"
* An unmatched step, with text "then there is applause"