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Lyth

ADR 0028: Generic declarations are Go generics

Status: Accepted; type parameters and arguments respelled [T] by 0031

Context

Section titled “Context”

Lyth could call generic Go functions and use instantiated Go types, but not declare its own. Without that, helpers such as a typed Page<T> response or a maxOf had to be written per type, and collection operations (ADR 0029, ADR 0030) could not be written in Lyth at all. The syntax (fun <T: C>, struct Page<T>) was specified early so that editor support could rely on it.

Decision

Section titled “Decision”
  • Functions, structs, and interfaces declare type parameters: fun <T: cmp.Ordered> maxOf(a: T, b: T): T, struct Page<T>, interface Container<T>. They lower to Go type parameters one to one, and each Lyth type parameter is a go/types type parameter, so instantiation and constraint satisfaction are exactly Go’s.
  • Constraints are interfaces: Any by default, a predeclared Comparable, Go constraints such as cmp.Ordered, or method interfaces. Interfaces that Go allows only as constraints are rejected elsewhere.
  • Type arguments are inferred from the arguments, the expected result type, and lambda results, or given in full (zero<String>()). Construction infers a struct’s type arguments from its fields. The generated Go names type arguments only where Go cannot infer them (a type parameter that occurs only in the result, a composite literal).
  • A generic struct’s methods name its type parameters in the receiver (fun Page<T>.First()); methods cannot declare their own, as in Go.
  • The zero value of a type parameter is Go’s, even for type arguments that have no zero value in Lyth.

Alternatives

Section titled “Alternatives”
  • Monomorphize in the Lyth compiler: would allow generic methods and T?, but produces Go that no Go programmer would write, bloats binaries, and diverges from Go’s semantics and tooling.
  • Kotlin-style variance (in/out) and reified type parameters: neither exists in Go, and both need a runtime representation Lyth does not have.
  • Reject var z: T without an initializer: sound, but (T, Bool) and (T, error) results, Go’s idiom for “not found”, would need contortions.

Consequences

Section titled “Consequences”

Generic Lyth code is generic Go code, callable from Go with Go’s syntax. Lyth inherits Go’s limits: no generic methods (extension functions fill the gap), no specialization. Nullable type parameters (T?) need a representation that works for every type argument and remain open. Named arguments and default values on generic functions are not supported yet.