Enums and scalars#

Enums#

Declaration#

enum Color { RED GREEN BLUE }

• members are bare CAPS identifiers, separated by whitespace/newlines — NOT commas (RED, GREEN is a syntax error)

Values#

• accessed as Color.RED — always qualified by the enum name
• the bare value (RED) is NOT in scope; an enum value is a field on the enum module (see Modules and imports), not a top-level binding — RED alone → "RED" not found, including inside case
• Color.values returns [Color!]! — all members (a real list: supports .length, .contains, indexing)
• equality compares by value; values of different enums / different members are not equal

As function arguments#

usersByStatus(status: Status.ACTIVE)

• also flows into input objects: UserSort(field: UserSortField.NAME, direction: SortDirection.DESC)

In case#

case (c) {
  Color.RED => "stop"
  Color.GREEN => "go"
}

• value patterns are the qualified members; use else for a catch-all (no static exhaustiveness check)

From strings (::)#

• "RED" :: Color! — runtime-validated coercion
• an invalid value fails at runtime: "NOPE" :: Status! → invalid enum value "NOPE" for Status

Custom scalars#

Declaration#

scalar Email
scalar JSON

Coercion#

• literal expressions auto-coerce to a compatible scalar at value-handoff boundaries (call args, typed slots, list literals, returns). This covers string literals, list literals, and backtick templates — even templates with ${...} interpolations.
• a non-literal String (a variable or a + b) does NOT auto-coerce — it needs an explicit :: cast: fetchURL(url: myUrl :: URL!), (base + domain) :: URL!. Without it: cannot use String! as URL!.
• list merging is pure / invariant: ["abc" :: ID!, "def"] → no common type between String! and ID! (the plain literal doesn't widen to ID!)
• runtime materialization only supports String -> custom scalar; e.g. 42 :: URL! is a type error (Int! vs URL!)
• scalars from GraphQL responses flow naturally; no cast needed

Treating scalars as opaque#

• equality, comparison, list membership, use in records/lists all work
• no string operations unless first cast back to String

Built-in scalars#

• Int!, Float!, String!, Boolean!, ID! (see Types and nullability)
• ID! is its own scalar, not a String alias: String! does not unify with ID! (no implicit interop) — but string literals still coerce into ID! slots (idSlot: ID! = "abc")