How Can Interfaces Solve Go's Array/Slice Covariance Problem?

Utilizing Interfaces to Address Array/Slice Covariance Deficiency in Go

In Go, the lack of array/slice covariance presents challenges when working with arrays or slices 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, "")
}

This code attempts to print an integer array and a float array using a generic printItems function. However, it encounters compilation errors due to Go's restriction on collection covariance.

Solution Using Interfaces

An idiomatic approach to overcome this limitation is to employ interfaces. By defining an interface that encapsulates the size and indexing of a list, we can create types that implement it for different data types:

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

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) }

Usage

With these types and the interface defined, we can now generically access and print the arrays:

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

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))
}

Conclusion

While generics offer a more elegant solution, using interfaces remains a robust approach to handle collection covariance in Go, allowing us to work with different data types in a generic manner.

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