Can You Constrain Types to Support Indices in Go 1.18 Generics?

Implementing Indices Constraints in Go 1.18 Generics

With the introduction of generics in Go 1.18, developers can define types with type constraints, ensuring that only specific types are allowed. This flexibility becomes crucial when implementing algorithms that rely on types supporting indices.

Can You Constrain Types with an Index Method?

To constrain types with indices, one might consider using the indexability property. However, the only meaningful constraint you can create is type Indexable interface { ~[]byte | ~string }. This constraint limits the types to byte slices and strings.

Why is a Union Constraint Limited?

The operations allowed on union-constrained types are restricted to those supported by all types in the constraint set. To support indexing, types in the union must have identical key and element types.

Additionally, map types in the union must have identical key types. And for arrays, their length is part of the type, making it impractical to define a union for all possible lengths.

Practical Implementation for Indexing

Therefore, the only union that satisfies the indexing requirement is []byte | string. Since byte is an alias for uint8, you can also use []uint8 with this constraint.

Limitations of Union Constraint for Indexing

While the union constraint []byte | string supports indexing, it does not support range. This is because the union doesn't have a core type.

In conclusion, constraining types to those that support indices in Go generics is limited to the specific constraint type Indexable interface { ~[]byte | ~string }. Other than this, there's no alternative way to define a constraint that encompasses all indexable types.

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