ArgParser: lift option and defaulting over records and structural unions

[Smaug123]

A field whose type is another argument record, or a union of alternative argument sets, may now be wrapped in option or Choice<'a, 'a>: a whole group of arguments which need not be supplied.

type Notify = { Email : string ; Subject : string option }

[<ArgParser>]
type Args =
    {
        Verbose : bool
        Notify : Notify option
    }

The group is present exactly when at least one argument beneath it was supplied — the same rule which chooses a union's case — and its own required arguments are then enforced as usual, so --subject=hi alone is an error demanding --email rather than a quietly absent group.

Previously a record or union was recognised only as a field's exact outer type, so one wrapper layer sent the field to the leaf machinery, which cannot parse a structural type and gave up with "Could not decide how to parse arguments".

How it works

One new generator-only ParseTree.Container node, which erases to a two-case Sum whose second case is the empty product. No runtime change at all: "which case was selected" already means "was anything beneath supplied".

It is a node of its own rather than a Sum because it is not an alternation the author wrote, and help text must not present it as one — it prints the ordinary group header with (optional), and a union nested beneath keeps its own "exactly one of" framing.

A payload which is itself satisfiable with no arguments is refused. Nothing the user could type would distinguish "the group was supplied, and everything in it took its default" from "the group was never mentioned", and for Choice the two can disagree in value, so this is a real modelling question the generated parser should not answer by silently preferring one reading.

That gate is also what makes the erasure sound: the empty case can never be touched, and the payload is never empty-satisfiable, so exactly one case is always selectable and none of the runtime's three selection errors can arise. The synthetic case names are therefore unreachable in any message.

Deliberately out of scope

SomeArgs list (a repeated group), Choice<'a list, 'a list> and Choice<Map, Map>. The first has no defensible delimitation rule on a flat command line; the other two turned out confusing enough to want separate thought. A design note covering all three, and the Accumulation flattening they would need, is written up separately.

Along the way

Two bugs, both pre-existing and both independently valid:

• 'a list list / 'a option list produced an assertion phrased as an internal error ("WoofWare.Myriad invariant violated") despite being reachable from ordinary source. The list arm of createParseFunction checked only for maps, while every sibling arm rejects each nesting it cannot express; the [<PositionalArgs>] path had always rejected them properly, so both paths now give the same message.
• The Default-prefixed member name was emitted bare. A field named space in name takes its default from Defaultspace in name , so the generated file did not parse. Three sites emit that call — a leaf's applied default, the help text, and (new here) a defaulted group's instantiation.

Testing

The reference oracle could not previously generate a Sum case with no leaves at all — chooseCuts gives every case at least one named id — so the exact shape Container introduces was untested despite being representable. Three properties now cover it, including one which confirms the gate is necessary rather than merely conservative: without it, the empty command line genuinely is ambiguous.

Each commit was reviewed individually against its parent.

🤖 Generated with Claude Code