How to Convert a Slice of One Interface Type to Another in Go?

Type Conversion of Slices Containing Different Interfaces

In Go, it is possible to encounter a scenario where you need to pass a slice of one interface to a function that expects a slice of a different compatible interface. Consider the following example:

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

type B interface {
    Close() error
}

type Impl struct {}

// Implementation of interface A and B
func (I Impl) Close() error {...}
func (I Impl) Read(b []byte) (int, error) {...}</code>

Interface Compatibility

In this example, interface A includes interface B, i.e., every type that implements A also implements B. As a result, a concrete implementation of A, such as Impl, satisfies both A and B.

Passing Individual Values

If we attempt to pass individual items across functions, it funktioniert as expected:

<code class="go">im := &Impl{}
single(im) // works</code>

Passing Slices

However, when trying to pass slices, we encounter an error:

<code class="go">list := []A{t}
slice(list) // FAILS!</code>

The error is: cannot use list (type []A) as type []io.Reader in argument to slice

Solution

To resolve this issue, we can manually create a new slice of the desired interface type:

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

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

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