Go: go1.15.8 darwin/amd64 OS: MacOS 11.2.1 It makes sense to me that a function can have arguments that are interfaces - so long as the argument provides the methods that the function requires, it will be happy. However, I’m having a difficult time understanding functions that return an interface. I posted code here <https://play.golang.org/p/ByjPODn-hhV> for context. Note that I’m not concerned about the deadlock that occurs in the playground - it works fine on my Mac.
The code comes from the example provided in the Cmd.StdoutPipe documentation <https://golang.org/pkg/os/exec/#Cmd.StdoutPipe>, which indicates: func (c *Cmd) StdoutPipe() (io.ReadCloser, error) The example uses a “short assignment” statement, so it’s not obvious what “concrete” type c.StdoutPipe() returns, but according to the io.ReadCloser documentation <https://golang.org/pkg/io/#ReadCloser>, it will have to provide these methods: type ReadCloser interface { Reader Closer } My (clearly flawed) interpretation of interface types suggested that I could assign the first result from c.StdoutPipe() to anything that implements io.ReadCloser. The io.PipeReader documentation <https://golang.org/pkg/io/#PipeReader> indicates that it provides both a Read() and a Close() function with the appropriate signatures, so I figured I’d try connecting up both ends of a pipe: rPipe, _ := io.Pipe() p, err := cmd.StdoutPipe() if err != nil { log.Fatal(err) } rPipe = p The first clue that I was misguided was when the compiler indicated that I needed a type assertion. I put one in to see what would happen: rPipe = p.(*io.PipeReader) Everything compiled, but not surprising, it panics: panic: interface conversion: io.ReadCloser is *os.File, not *io.PipeReader I suppose it makes sense that an assignment operator requires more than simply interface compatibility. A couple of questions: 1. How is one supposed to know what type is required in a situation like this? Is a type assertion panic the only way to discover that a *os.File type is required?! 2. Any suggestions on how I can get a io.PipeReader to connect with the cmd.StdoutPipe()? -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/d7c51270-5e2c-439d-a1bb-d78fd65a6a25n%40googlegroups.com.
