Records: Projection and Update

Records are key/value structures with named fields. Rex supports:

• field projection: base.field
• record update: { base with { field = expr } }

At the value level, “record” literals are written like dicts:

{ x = 1, y = 2 }

At the type level, record types are written with ::

let p: { x: i32, y: i32 } = { x = 1, y = 2 } in
  p

Projection

type Point = Point { x: i32, y: i32 }

let p: Point = Point { x = 1, y = 2 } in p.x

Projection is accepted when the field is definitely available on the type (see Specification).

Tip: If you get a “field not definitely available” type error, it usually means the typechecker can’t prove which ADT variant you have. A match often fixes it.

Update

type Point = Point { x: i32, y: i32 }

let p: Point = Point { x = 1, y = 2 } in
  { p with { x = p.x + 10 } }

Updates can set multiple fields at once:

type Point = Point { x: i32, y: i32 }

let p: Point = Point { x = 1, y = 2 } in
  { p with { x = 100, y = 200 } }

Updating record-carrying ADT variants

This is a common pattern:

type Sum = A { x: i32 } | B { x: i32 }

let s: Sum = A { x = 1 } in
match s
  when A {x} -> { s with { x = x + 1 } }
  when B {x} -> { s with { x = x + 2 } }

The match arms refine which constructor s has, allowing the update to typecheck.