Unpacking slice of slices

Question

I am curious about unpacking a slice of slices and sending them as arguments to a variadic function.

Let's say we have a function with variadic parameters:

func unpack(args ...interface{})

If we wan't to pass in a slice of interfaces it works, it doesn't matter if we unpack it or not:

slice := []interface{}{1,2,3}
unpack(slice) // works
unpack(slice...) // works

It gets tricky if we have a slice of slices. Here the compiler doesn't let us pass in an unpacked version:

sliceOfSlices := [][]interface{}{
    []interface{}{1,2},
    []interface{}{101,102},
}
unpack(sliceOfSlices) // works
unpack(sliceOfSlices...) // compiler error

The error says:

cannot use sliceOfSlices (type [][]interface {}) as type []interface {} in argument to unpack

I don't know why this happens, as we can clearly pass an []interface{} type into the function. How can I call the method unpack with the unpacked content of sliceOfSlices as arguments?

Playground example: https://play.golang.org/p/O3AYba8h4i

score 6 · Accepted Answer · edited Jun 20 '20 at 09:12

This is covered in Spec: Passing arguments to ... parameters:

If f is variadic with a final parameter p of type ...T, then within f the type of p is equivalent to type []T.

...

If the final argument is assignable to a slice type []T, it may be passed unchanged as the value for a ...T parameter if the argument is followed by .... In this case no new slice is created.

So in short: it is a compile-time error because sliceOfSlices (which is of type [][]interface{}) cannot be assigned to args (which is of type []interface{}) (proof on Playground).

In long:

In your first example when you do unpack(slice), since unpack() expects values of interface{}, therefore slice (which is of type []interface{}) will be wrapped in a new interface{} value, and it will be passed as a single argument.

When you do unpack(slice...), this will pass all the values of slice as separate values to unpack(); this is possible because type of slice is []interface{}, it matches the type of the variadic parameter (args ...interface{}).

In your second example when you do unpack(sliceOfSlices), again, sliceOfSlices will be wrapped in a new interface{} value and passed as a single argument.

But when you try unpack(sliceOfSlices...), that would want to pass each element of sliceOfSlices to unpack(), but type of sliceOfSlices (which is [][]interface{}) does not match the type of the variadic parameter, hence the compile-time error.

The only way to pass sliceOfSlices to unpack() "exploded" is to create a new slice whose type must be []interface{}, copy the elements, then you can pass it using ....

Example:

var sliceOfSlices2 []interface{}
for _, v := range sliceOfSlices {
    sliceOfSlices2 = append(sliceOfSlices2, v)
}

unpack(sliceOfSlices2...)

Try it on the Go Playground.

Let's use the following unpack() function to verify the number of arguments:

func unpack(args ...interface{}) {
    fmt.Println(len(args))
}

Running your example (and with my new slice creation), output is:

Which proves without ... only a single argument is passed (wrapped in interface{}), and using ... all elements will be passed separately.

Try this test on the Go Playground.

Unpacking slice of slices

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