creating generic functions for multi type arrays in Go - go

Question

I am trying to create a generic function that can handle actions on slices in Go... for instance, append an item of any type to a slice of that same type. This is simply a generic purpose for a more complex solution, but overall the issue boils down to this example:
package main
type car struct {
make string
color string
}
type submarine struct {
name string
length int
}
func genericAppender(thingList interface{}, thing interface{}) []interface{} {
return append(thingList, thing)
}
func main() {
cars := make([]car, 0, 10)
cars[0] = car{make: "ford", color: "red"}
cars[1] = car{make: "chevy", color: "blue"}
subs := make([]submarine, 0, 10)
subs[0] = submarine{name: "sally", length: 100}
subs[1] = submarine{name: "matilda", length: 200}
newCar := car{make: "bmw", color: "white"}
genericAppender(&cars, newCar)
}
The code playground is at this location
The above errors as follows:
prog.go:14: first argument to append must be slice; have interface {}

Answer 1

After this change you're still getting a runtime error (index out of range) however the problem is that thingList is not of type []interface{} but rather interface{} so you can't append to it. Here's an updated version of your code on playground that does a type assertion to convert it to an []interface{} in line with the append. In reality you need to do that on a separate line and check for errors.
https://play.golang.org/p/YMed0VDZrv
So to put some code here;
func genericAppender(thingList interface{}, thing interface{}) []interface{} {
return append(thingList.([]interface{}), thing)
}
will solve the basic problem you're facing. As noted, you still get runtime errors when indexing into the slice. Also, you could change the argument to avoid this by making it;
func genericAppender(thingList []interface{}, thing interface{}) []interface{} {
return append(thingList, thing)
}
Here's a complete example of the second type; https://play.golang.org/p/dIuW_UG7XY
Note I also corrected the runtime error. When you use make with 3 args they are, in this order, type, length, capacity. This means the length of the array is 0 so when you try to assign to indexes 0 and 1 it was causing a panic for IndexOutoFRange. Instead I removed the middle argument so it's make([]interface{}, 10) meaning the length is initially set to 10 so you can assign to those indexes.

Answer 2

In the answer above if you do the following then it throws error. This is what the original question was about:
//genericAppender(subs, newCar). // Throws "cannot use subs (type []submarine) as type []interface {} in argument to genericAppender"
The trick is to convert your slice of specific type into a generic []interface{}.
func convertToGeneric(thingList interface{}) []interface{} {
input := reflect.ValueOf(thingList)
length := input.Len()
out := make([]interface{},length)
for i:=0 ;i < length; i++ {
out[i] = input.Index(i).Interface()
}
return out
}
This you can call the function like this:
genericAppender(convertToGeneric(subs), newCar)
You can check modified working code here: https://play.golang.org/p/0_Zmme3c8lT

Answer 3

With Go 1.19 (Q4 2022), no need for interface, or "convert your slice of specific type into a generic []interface{}"
CL 363434 comes with a new slices packages:
// Package slices defines various functions useful with slices of any type.
// Unless otherwise specified, these functions all apply to the elements
// of a slice at index 0 <= i < len(s).
package slices
import "constraints"
// Grow increases the slice's capacity, if necessary, to guarantee space for
// another n elements. After Grow(n), at least n elements can be appended
// to the slice without another allocation. If n is negative or too large to
// allocate the memory, Grow panics.
func Grow[S ~[]T, T any](s S, n int) S {
return append(s, make(S, n)...)[:len(s)]
}
// Equal reports whether two slices are equal: the same length and all
// elements equal. If the lengths are different, Equal returns false.
// Otherwise, the elements are compared in index order, and the
// comparison stops at the first unequal pair.
// Floating point NaNs are not considered equal.
func Equal[T comparable](s1, s2 []T) bool {
if len(s1) != len(s2) {
return false
}
for i, v1 := range s1 {
v2 := s2[i]
if v1 != v2 {
return false
}
}
return true
}
// ...
Ian Lance Taylor confirms in issue 45955:
This package is now available at golang.org/x/exp/slices.
Per this thread, it will not be put into standard library until the 1.19 release.
We may of course adjust it based on anything we learn about having it in x/exp.