What Does the Tilde (~) Symbol Mean in Go Generics?

Understanding the Meaning of ~ in Go Generics

In Go generics, the tilde token (~) introduces a new concept known as the "approximation element." The expression ~T denotes the set of all types whose underlying type is T.

Understanding Underlying Types

The definition of "underlying types" is crucial to comprehending the purpose of the approximation element. The language specification states that each type has an underlying type, which is itself if the type is a predeclared type (e.g., int, float64, string), a type literal, or refers to a type with a bound identifier.

Practical Implications

The approximation element allows interface constraints to include types beyond just those with exact matches. For example, an interface constraint like ExactSigned = int | int8 | ... | int64 would not allow a type like MyInt8, which is defined over int8. However, the constraint constraints.Signed, which includes approximation elements like ~int8, allows the inclusion of MyInt8.

Using Approximation Elements

Approximation elements can be used in unions (e.g., constraints.Signed = ~int8 | ~int16 | ...), anonymous constraints with or without syntactic sugar, and even with single-element approximation elements.

Common Use Cases

A common use case for approximation elements is with composite types (slices, structs, etc.) that need methods. By binding the identifier of a composite type, you can allow for its inclusion in a constraint using the approximation element.

Note: Approximation elements cannot be used with type parameters in interfaces.

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