schema-types.md

summary	Named field-type shortcuts for inline schema frontmatter values — the registered names, the canonical CUE each one resolves to, and example usage.

Schema field types

A field-type shortcut is a short name a schema frontmatter value can use instead of spelling out a CUE expression. created: date is shorter and harder to typo than created: '=~"^\\d{4}-\\d{2}-\\d{2}$"', and every project using the shortcut gets the same regex — no three projects land on three slightly different ISO patterns.

Shortcuts work the same way for inline schemas (kinds.<name>.schema:) and for proto.md frontmatter. The library ships embedded in the mdsmith binary; no network access is needed to resolve a shortcut.

How a value is interpreted

A frontmatter value (right-hand side of key: value in the schema's frontmatter: block) is classified by shape:

Shape	Treatment
Bare identifier in the shortcut registry	Substituted with the canonical CUE expression
Bare identifier that is a CUE built-in type	Passes through verbatim (string, int, bool, float, bytes, …)
Bare identifier that is neither	Config error naming the field and the unknown name
Anything else (operators, quotes, …)	Passes through verbatim as raw CUE

A "bare identifier" is a YAML scalar matching [a-zA-Z_][a-zA-Z0-9_-]* — letters, digits, underscores, and hyphens, starting with a letter or underscore. Anything containing whitespace, operators, quotes, brackets, or other punctuation is treated as raw CUE.

The strict third row catches typos at config-load time. A schema that writes created: iso-date errors with unknown shortcut "iso-date" instead of sliding through to an undefined-reference error deep in CUE evaluation.

Registered shortcuts

The following names are accepted in a schema frontmatter value:

Name	Canonical CUE	Accepts	Rejects
date	=~"^\\d{4}-\\d{2}-\\d{2}$"	2024-05-01	2024-5-1
datetime	=~"^\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}(Z|[+-]\\d{2}:\\d{2})?$"	2024-05-01T12:30:00Z	2024-05-01 12:30:00
time	=~"^\\d{2}:\\d{2}(:\\d{2})?$"	12:30	12:3
email	=~"^[^@\\s]+@[^@\\s]+\\.[^@\\s]+$"	user@example.com	user@@example
url	=~"^https?://"	https://example.com	ftp://example.com
filename	=~"^[A-Za-z0-9._-]+\\.md$"	notes.md	notes.txt
nonEmpty	string & !=""	hello	""

The regex bodies are shape-only. date accepts 2024-02-30 even though February never has 30 days; semantic validation is out of scope.

Inline schema example

kinds:
  rfc:
    schema:
      frontmatter:
        created: date
        modified: datetime
        contact: email
        homepage: url
      require:
        filename: "RFC-[0-9][0-9][0-9][0-9].md"

mdsmith check validates every file in the rfc kind: created must match the ISO-date regex, contact must look like an email, and so on.

proto.md example

The same shortcuts work in a proto.md schema's YAML frontmatter:

---
created: date
homepage: url
---
# ?

## Goal

## Tasks

Substitution happens at schema-load time. After that the validator treats the canonical CUE the same way whether a user typed it out or the loader expanded the shortcut.

Composing a shortcut with extra constraints

A value that needs both a shortcut and an extra constraint is not a bare name. Write the canonical CUE directly:

frontmatter:
  created: '=~"^\\d{4}-\\d{2}-\\d{2}$" & >="2020-01-01"'

The shortcut form is only triggered when the value is exactly the bare name. Anything more complex passes through as raw CUE.

Adding a shortcut

The library lives at cue/types/types.cue in this repository. The runtime registry is in internal/schema/shortcuts.go; a drift test pins the two to each other. User-defined shortcuts are not yet a configuration surface — a project that needs a custom shortcut should write the canonical CUE in a shared proto.md (or wait for a follow-up plan if the need recurs).

See also

• Schemas — the broader story on inline schemas, proto.md, and per-scope rule overrides.
• MDS020 — the rule that surfaces schema diagnostics.
• File kinds — how kinds attach schemas (and other rule config) to file groups.