returning struct vs. returning interface

[Jochen Voss]

Dear all,

Why does the function NewReader [1] in compress/lzw return an io.ReadCloser instead of an *lzw.Reader?  The docs say "It is guaranteed that the underlying type of the returned io.ReadCloser is a *Reader", so why not return a *Reader without wrapping?

[1] https://pkg.go.dev/compress/lzw#NewReader

Many thanks,

Jochen

[Axel Wagner]

git blame points at

https://github.com/golang/go/commit/72b501cb0326f62a20621636942e5a95fc3c1466

https://github.com/golang/go/issues/26535

That is: Originally, the type returned by NewReader was unexported, so an interface was used. Then, methods where added to that type which would be useful to be used, so the type needed to be exported. The signature of NewReader could not change, though, for backwards compatibility. Documenting that it's guarenteed to be the newly exported type makes it possible to use the new methods without breaking compatibility.

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