Can Go Generics Handle Type Parameters in Interface Method Definitions?

Type Parameters in Interface Methods: A Go Generics Quandary

In Go's pursuit of generics, a peculiar stumbling block emerges: the prohibition of type parameters within interface method definitions. This restriction, encountered by a developer attempting to abstract a key/value store, leaves us wondering why and if a solution exists.

Rationale Behind the Restriction

The Go core team has made a deliberate design decision to disallow type parameters in interfaces for several reasons:

1. Interpretational Ambiguity: Type parameters in interfaces could lead to multiple interpretations of methods, making it difficult to establish consistent implementation rules.
2. Compile-Time Performance: Traversing the entire type system hierarchy at compile time to resolve type parameters could significantly impact performance.
3. Runtime Reflection: Allowing type parameters in interfaces would require runtime reflection to determine the actual type parameters, slowing down execution.
4. Implementation Conflicts: Parametrized methods are essentially independent functions, which could clash with the notion of implementing interfaces.

Circumventing the Restriction

While the restriction can be frustrating, it's not insurmountable. The solution proposed in the Type parameters proposal is to move the type parameter into the interface type definition itself:

type Reader[V Unmarshaler] interface {
    Read(bucket []byte, k ...[]byte) ([][]byte, error)
    ReadDoc(bucket []byte, factory func() (V, error), k ...[]byte) ([]V, error)
}

type Unmarshaler interface {
    UnmarshalKV(v []byte) error
}

This approach allows for type-safe generic interfaces while preserving the design constraints of the language.

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