How to Pass a Slice of One Interface to a Function Expecting a Slice of a Different, Compatible Interface in Go?

Passing a Slice of Interface to a Different Compatible Interface in Go

When working with interfaces in Go, you may encounter situations where you need to pass a slice of one interface to a function that expects a slice of a different interface, despite the first interface including the second.

Consider the following example:

<code class="go">type A interface {
    Close() error
    Read(b []byte) (int, error)
}

type Impl struct {}
func (I Impl) Read(b []byte) (int, error) {
    fmt.Println("In read!")
    return 10, nil
}
func (I Impl) Close() error {
    fmt.Println("I am here!")
    return nil
}

func single(r io.Reader) {
    fmt.Println("in single")
}

func slice(r []io.Reader) {
    fmt.Println("in slice")
}

im := &Impl{}
single(im) // works
list := []A{im}
slice(list) // fails: cannot use list (type []A) as type []io.Reader</code>

While passing an individual item of type A to the function single with an interface parameter io.Reader succeeds, attempting to pass a slice of A to the function slice expecting a slice of io.Reader fails.

Resolution:

Unfortunately, this issue is a limitation in Go. To work around it, you must create a new slice of the desired interface type and populate it with elements from the existing slice.

<code class="go">newList := make([]io.Reader, len(list))
for i, elem := range list {
    newList[i] = elem
}

slice(newList) // now works</code>

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