A fascinating limitation of Go is its lack of support for functions with covariant return types.

Say that you want a function which returns a Reader:

    type ReaderMaker func() io.Reader

A function which returns a concrete type which implements Reader cannot be substituted. This makes sense because interface types are always a two-word tuple, and a concrete type could be a one-word pointer, a big struct, etc.

    var invalid ReaderMaker = func() *io.PipeReader { return &io.PipeReader{} }

More subtly, a function which returns a different but compatible interface type also won't work.

    // This assignment is invalid because although anything that implements
    // ReadCloser also implements Reader, a ReadCloser itable won't necessarily
    // have Read() at the same offset as a Reader itable.
    var invalid ReaderMaker = func() io.ReadCloser { return &io.PipeReader{} }

http://play.golang.org/p/5PPpVZhap_

* edit: fixed swapped types in comment