Records, enums and newtypes

A record declaration gives each field a name and a type.

An enum declaration lists named variants with optional positional payload types.

A newtype declaration names one base type after =.

A nominal derives clause follows the declaration name and type parameters. It precedes the body of a record or enum, or = and the base type of a newtype.

A newtype declaration may request the same derived protocols as a record or enum. Its base type supplies the member capability checks. Derivation is explicit; the base type does not implicitly give the newtype a protocol conformance. The clause precedes = and accepts the same conditional generic binder.

A record field default is an expression written after its type and =.

Declaring the same field name twice in one record produces TPZ5008.

A named record construction without a spread produces TPZ5004 when it omits a field with no default.

A named record construction may omit a field that has a declared default.

Supplying the same field twice in a named record construction produces TPZ5008.

Supplying a field absent from the declared record produces TPZ5006.

A named construction checks each uniquely supplied field against its resolved declared type after substituting record type arguments.

Each construction evaluates defaults for omitted fields after its explicit initializers, in field declaration order. A supplied field or a spread supplies the field instead. Defaults refer to names in their declaration environment.

A record field default must satisfy the declared field type. A string default for an int field is a type error.

The name in a named record construction denotes a declared record.

A spread in a named record construction must appear first and occur at most once.

A named record construction checks the spread value against the constructed record type.

A spread on a record value update produces TPZ5001.

A structural record literal encloses at least one named field initializer in braces and has no nominal record name.

In an expression, empty braces form a block.

A record update evaluates its base once before explicit field initializers. Initializers run in source order. A spread supplies omitted fields without reevaluating their defaults. The result has new record storage and retains the field values from the base.

In a structural record update value { field: expression }, value denotes an existing structural record.

For a field supplied once, a structural record update checks its value against the base record field type.

A structural record value update has the type of its base record.

Value-update syntax requires a structural record. Update a nominal record with its declared name and a spread, as in Point { ...point, x: 2 }.

Each explicit field name in a structural record update must occur once. Repeating a field is rejected with TPZ5008 even when an earlier initializer has an incompatible type. A field copied from the base may be replaced by one explicit initializer.

Calling a qualified enum constructor without payloads, such as Choice.Empty(), produces a value of the enum type.

A payload-bearing enum variant is a constructor function from its substituted payload types to the enum type.

Enum variants may be named None, Some, Ok, or Err. Construct them through the enum name, such as Reply.None() or Reply.Ok(value). The prelude Ok(value) constructs a Result value. An unqualified constructor pattern such as None() resolves through the normalized scrutinee type. Prelude spelling does not take priority. For a union, collect the constructor declarations with that head before checking payload arity. One declaration is required. Multiple instantiations of that same declaration project the union of the corresponding payload types. Distinct declarations with the same head are ambiguous even when their payload shapes agree. Narrow with an existing typed pattern and match the narrowed value. Ambiguity is a TPZ5001 pattern-type error. Wrong payload arity remains TPZ5004. Constructor patterns use unqualified heads.

An enum declaration may have no variants. Nominal type names are predeclared, so enum payloads may refer to their own enum or another declared enum, including through Array. This admits the declaration; it does not create a value for an enum with no finite constructor path. Payloads follow the same union type rules as other type positions. A union payload may contain enum types.

Each variant name must be unique within its enum. The retained checker diagnoses repeated declarations with TPZ5008; current SELF appends them without that guard. The intended rejection remains a product expectation and the missing SELF check is an implementation limitation.

A newtype constructor checks its single positional argument against the resolved base type and returns the newtype.

A newtype call with an argument count other than one or a spread argument produces TPZ5004. Named arguments follow the ordinary call argument rule and are rejected during parsing with TPZ2001.

Record, enum, and newtype type parameters are comma-separated names inside angle brackets after the declaration name.

Generic nominal construction context

Construct a generic record, enum, or newtype in a context that supplies the same nominal declaration and all its type arguments. The expected arguments may themselves be enclosing generic parameters. Field or payload values alone do not supply omitted nominal arguments; a missing matching context is a type-argument arity error (TPZ5022). The declaration origin is part of that match; the current record-context retargeting defect is an implementation limitation.

A binding annotation, a declared argument type, a declared return or block-tail type, and parentheses can carry a complete nominal expectation to construction. Once a record is instantiated, its substituted declared field types provide expectations for nested constructions. A generic newtype uses the substituted base type from its matching expected newtype. Collection nesting follows the collection contextual-checking rule and preserves the matching nominal identity requirement.

Explicit call type-argument lists belong to generic call targets. Nominal constructors do not contribute declaration parameters to that call-parameter list: a newtype or enum constructor call with explicit type arguments is rejected as a non-generic call (TPZ5512). Put the full nominal type in the surrounding annotation or other expected context instead. Record construction keeps its brace syntax; this rule makes no spacing-dependent promise about parser lookahead.