Why Does Go's Lack of Array/Slice Covariance Require Workarounds for Handling Different Collection Types?

Addressing the Lack of Array/Slice Covariance in Go

In Go, the absence of array or slice covariance can pose challenges when working with collections of different types. Consider the following scenario:

func printItems(header string, items []interface{}, fmtString string) {
  // ...
}

func main() {
  var iarr = []int{1, 2, 3}
  var farr = []float{1.0, 2.0, 3.0}
  printItems("Integer array:", iarr, "")
  printItems("Float array:", farr, "")
}

Go's lack of generics prevents us from defining a generic printItems function that accepts collections of any type. Instead, we encounter the following error:

prog.go:26: cannot use iarr (type []int) as type []interface { } in function argument      
prog.go:27: cannot use farr (type []float) as type []interface { } in function argument

Interface-Based Solution

One common approach to overcome this limitation is to employ an interface that defines the required behavior for our collection. In this case, we define an List interface with two methods, At (to retrieve an item at a specific index) and Len (to get the length of the list):

type List interface {
    At(i int) interface{}
    Len() int
}

We then create concrete implementations of this interface for our integer and float lists:

type IntList []int
type FloatList []float64

func (il IntList)   At(i int) interface{} { return il[i] }
func (fl FloatList) At(i int) interface{} { return fl[i] }

func (il IntList)   Len() int { return len(il) }
func (fl FloatList) Len() int { return len(fl) }

With these implementations in place, we can now modify our printItems function to accept a List as an argument, allowing us to print both integer and float arrays:

import "fmt"

func printItems(header string, items List) {
    for i := 0; i < items.Len(); i++ {
        fmt.Print(items.At(i), " ")
    }
    fmt.Println()
}

In our main function, we can then call printItems with our integer and float lists, achieving our desired result:

func main() {
    var iarr = []int{1, 2, 3}
    var farr = []float64{1.0, 2.0, 3.0}
    printItems("Integer array:", IntList(iarr))
    printItems("Float array:", FloatList(farr))
}

By utilizing an interface to define a common set of operations for our collections, we can achieve a degree of flexibility and code reusability, despite the absence of generics in Go.

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