What's the Difference Between `comparable` and `Ordered` Constraints in Go Generics for Type Comparisons?

Comparable Constraints vs. Ordered Operators in Go Generics

In Go generics, the comparable constraint restricts types that support equality operators (== and !=), while ordered operators (<, >, <=, and >=) require the Ordered constraint.

Consider the following code:

import "fmt"

type numbers interface {
    int | int8 | int16 | int32 | int64 | float32 | float64
}

func getBiggerNumber[T numbers](t1, t2 T) T {
    if t1 > t2 {
        return t1
    }
    return t2
}

func getBiggerNumberWithComparable[T comparable](t1, t2 T) T {
    if t1 > t2 { // Compile error
        return t1
    }
    return t2
}

func main() {
    fmt.Println(getBiggerNumber(2.5, -4.0))
    fmt.Println(getBiggerNumberWithComparable(2.5, -4.0))
}

The error in getBiggerNumberWithComparable arises because comparable does not guarantee order comparison. It includes map key types that do not support ordering.

Solution for Go 1.18 to 1.20

Prior to Go 1.21, use constraints.Ordered:

import (
    "fmt"
    "golang.org/x/exp/constraints"
)

func getBiggerNumberWithOrdered[T constraints.Ordered](t1, t2 T) T {
    if t1 > t2 {
        return t1
    }
    return t2
}

Solution for Go 1.21

In Go 1.21 and later, use cmp.Ordered:

import (
    "fmt"

    "golang.org/x/exp/constraints"
    "github.com/google/go-cmp/cmp"
)

func getBiggerNumberWithOrdered[T cmp.Ordered](t1, t2 T) T {
    if cmp.Less(t1, t2) {
        return t2
    }
    return t1
}

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