Interface embedding includes one interface’s method set in another by naming the embedded interface. Go uses this composition to build larger, reusable interfaces from smaller protocols. section 7.3.2 section 7.8
Building io.ReadCloser
io.ReadCloser embeds io.Reader and io.Closer, so it provides both Read and Close without redeclaring either method. This makes its requirement clear: a value must support reading and closing. The same pattern appears in ReadWriteSeeker, which combines Reader, Writer, and Seeker. Naming conventions usually apply the -er suffix once, producing ReadCloser rather than ReaderCloser. section 7.3.2
type ReadCloser interface {
Reader #1
Closer #2
}
ReadCloser’s declaration shows interface embedding directly: Reader and Closer are named inside the larger interface, whose method set includes both. section 7.3.2
Embedding is useful when a larger protocol should preserve smaller, familiar contracts:
- Avoids duplicating method declarations while clarifying which capabilities a type must provide. section 7.3.2
- Supports incremental HTTP response-body processing through Reader and explicit resource release through Closer. section 7.3.2
- Can add behavior around existing functionality, although embedding may hide methods and require type assertions to reveal optional interfaces. section 9.9
