Kind

A kind is the "sort" of a type-level thing — what classifies a type the way a type classifies a value.

Kind	Classifies	Members
Type	the type of a value	Int, User, `[ :a \	:b ],fn() -> Unit`
Row	a structural channel fragment	`:a \	:b, the row variable'r`
Signature	a complete function descriptor	a sig 'f binder
Constructor (Type -> Type, …)	a type still needing arguments	Vector, Option, Map

• An ordinary type variable 'a has kind Type (the zero-ceremony default).

• A row variable 'r has kind Row; it can never unify with a Type.

• Keep kind (what sort a variable is) separate from constraint (what it can do, e.g. an inline 'a: Ordered function binder).

• A row is at kind Row; wrapping it in [ … ] produces a kind-Type open variant.

• open 'r on a binder constrains 'r to instantiate to an open-variant Type (not that 'r has kind Row in the binder list) — see open-binder.md. The same open shape can be inferred from an open-only use such as Timed('r).

• Ordinary relative callbacks need no explicit effect variable. APIs that deliberately transform channels may bind rows or a complete sig explicitly; see effect-row.md and ../10-services.md.

  See variant.md and ../02-type-inference.md.