Why Can't Go Generics' `comparable` Constraint Be Used with Ordering Operators?

Generics in Go: Order Operators Incompatible with Comparable Constraint

Introduction

When exploring Go generics, developers may encounter difficulties when attempting to utilize order operators such as > within functions that accept types satisfying the comparable constraint. This article delves into the rationale behind this incompatibility and explores potential solutions.

comparable Constraint

The comparable constraint, as defined in the Go language specifications, applies to types that support equality operators (== and !=). This includes types that can be utilized as map keys, including arrays and structs with comparable fields.

Ordering Operators

While the Go language specifications use the term "comparison operators" to encompass both equality and ordering operators, the specs clearly differentiate between the types they apply to. Specifically, ordering operators such as > and < are intended for operands that are "ordered."

Incompatibility with Comparable

Unfortunately, the comparable constraint does not imply that a type supports ordering operators. This is evident from the error message encountered: "invalid operation: cannot compare t1 > t2 (operator > not defined on T)."

Solutions

Go 1.21 and Above

For Go versions 1.21 and above, developers can leverage the cmp.Ordered type constraint introduced by the standard library. This constraint and its associated functions, Less and Compare, facilitate comparisons on ordered types.

Go 1.18 to 1.20

For versions of Go between 1.18 and 1.20, the constraints.Ordered constraint from the golang.org/x/exp package may be utilized for comparisons involving ordering operators.

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