Why Can\'t Go Interface Methods Have Type Parameters?

Understanding Interface Method Type Parameters in Go

In Go generics, interface methods are not permitted to have type parameters. This restriction arises from fundamental design considerations outlined in the Type Parameters proposal.

Why Are Type Parameters Prohibited in Interfaces?

• Ambiguity: Type parameters in interface methods could lead to multiple interpretations, impacting implementation compatibility and code correctness.
• Performance: Allowing type parameters could require extensive tree traversal at compile time or introduce performance-intensive runtime reflection.
• Consistency: Parametrized methods would not be able to implement interfaces consistently, resulting in confusion and potential errors.

Implementing Generic Abstractions

Although type parameters are not allowed in interface methods, the language provides an alternative approach. You can move the type parameter into the interface type definition itself, as demonstrated below:

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 preserves the desired generic abstraction without violating the interface method type parameter restriction.

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