Can Go Generics Effectively Enforce Indexing Constraints Beyond `[]byte` and `string`?

Constraining Types to Indexable Types in Go Generics

In Go 1.18, the introduction of generics has opened up new possibilities for type safety. One commonly encountered scenario is the need to restrict the allowed types to those that support indexing, such as arrays, slices, maps, and strings.

Is Indexability a Constraint?

While indexing is a desirable feature for many types, it is not itself a constraint that can be directly specified. Constraints in Go generics are specified using type sets, which define the allowed types.

Meaningful Indexing Constraint

The only meaningful indexing constraint using a union type set is Indexable, which restricts allowed types to either []byte or string. This constraint enables the definition of generic functions that accept both types and perform indexing operations.

Limitations of Indexing Constraints

Other potential indexable types, such as arrays and maps, cannot be meaningfully combined in a union type set. The operations allowed on unioned types are only those supported by all member types, and the constraints require equal key and element types for indexing.

Practical Implementation

As a result, the only practical indexing constraint is Indexable for []byte and string. This allows for generic functions that operate on both types, such as the GetAt function provided in the answer, which returns the byte at a specified index.

Additional Notes

It's important to note that the Indexable constraint does not support range operations because it lacks a core type. However, the types []byte and string both have range capability and can be used where range operations are required.

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