Patching Records

The <| operator applies a declarative structural patch. This avoids overloading <=, which is expected to be a normal comparison operator.

The compiler enforces that the patch shape matches the target record's type, ensuring that only existing fields are updated or new fields are added according to the record's openness. When a patch path selects a Map entry, the patch applies to the value stored at that key.

updated = record <| { path: instruction }

Patching is:

• immutable
• compositional
• type-checked

Patch A is a first-class type alias for A -> A and is the canonical type for patch values. Applying a patch is done with <|.

Patch literals can be lifted into patch functions:

promote : Patch User
promote = patch { isAdmin: True }

patch { ... } produces a patch value that can be applied later with <|.

Compiler checks:

• Patch paths must resolve against the target type (unknown fields/constructors are errors).
• Predicate selectors (items[price > 80]) must type-check as Bool.
• Map key selectors (map["k"] or map[key == "k"]) must use the map's key type.
• Removing fields (-) is only allowed when the resulting record type remains valid (e.g. not removing required fields of a closed record).

5.1 Path addressing

Dot paths

user.profile.avatar.url

Traversals

items[*]

Predicates

items[price > 80]
items[id == 1]

Map key selectors

When the focused value is a Map, selectors address entries by key. After selection, the focus is the value at that key.

settings["theme"]
usersById[key == "id-1"]
rolesById[*]

In map predicates, the current element is an entry record { key, value }, so key == "id-1" is shorthand for _.key == "id-1".

Sum-type focus (prisms)

Ok.value
Some.val
Circle.radius

If the constructor does not match, the value is unchanged.

5.2 Instructions

Instruction	Meaning
value	Replace or insert
Function	Transform existing value
:= Function	Replace with function as data
-	Remove field (shrinks record type)

5.3 Replace / insert

user2 = user <| {
  name: "Grace"
  profile.avatar.url: "https://img"
}

Intermediate records are created if missing.

5.4 Transform

user3 = user <| {
  name: toUpper
  stats.loginCount: _ + 1
}

5.5 Removal

user4 = user <| {
  email: -
  preferences.notifications.email: -
}

Removal is structural and reflected in the resulting type.

5.7 Expressive Data Manipulation

Patching allows for very concise updates to deeply nested data structures and collections.

Deep Collection Updates

// Update prices of all active items in a category
store2 = store <| {
  categories[name == "Hardware"].items[active].price: _ * 1
}

users2 = usersById <| {
  ["id-1"].profile.name: toUpper
}

Complex Sum-Type Patching

// Move all shapes to the origin
scene2 = scene <| {
  shapes[*].Circle.center: origin
  shapes[*].Square.origin: origin
}

Record Bulk Update

// Set multiple fields based on previous state
user2 = user <| {
  name: toUpper
  status: if admin then SuperUser else Normal
  stats.lastVisit: now
}