How to Unpack a Slice of Slices into a Variadic Function in Go?

Variadic Function and Unpacked Slice of Slices

When working with a variadic function that accepts multiple arguments, handling a slice of slices can present a challenge. Here's a specific instance where unpacking a slice of slices can cause a compiler error:

Consider the following variadic function:

<code class="go">func unpack(args ...interface{})</code>

If we have a slice of interfaces, we can pass it directly to the function without any issues:

<code class="go">slice := []interface{}{1, 2, 3}
unpack(slice) // works
unpack(slice...) // also works</code>

However, if we have a slice of slices of interfaces, attempting to pass an unpacked version results in a compilation error:

<code class="go">sliceOfSlices := [][]interface{}{
    []interface{}{1, 2},
    []interface{}{101, 102},
}
unpack(sliceOfSlices) // works
unpack(sliceOfSlices...) // compiler error</code>

The error message is:

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

To understand this behavior, we need to delve into the specification for passing arguments to variadic parameters in Go:

"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, the variadic parameter is ...interface{}, and a direct assignment of sliceOfSlices (which is of type [][]interface{}) to args (of type []interface{}) is not valid.

So, how can we send the unpacked contents of sliceOfSlices to unpack()? The solution lies in creating a new slice of type []interface{}, copying the elements, and then passing it using ....

<code class="go">var sliceOfSlices2 []interface{}
for _, v := range sliceOfSlices {
    sliceOfSlices2 = append(sliceOfSlices2, v)
}

unpack(sliceOfSlices2...)</code>

This method ensures that each inner slice is represented as an element in a single slice, making it compatible with the ...interface{} variadic parameter.

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