Writing generics function without generics

One solution would be to go generate your P() function, one for each concrete type you need to work with.
See examples in:

• "Generic programming in Go using "go generate"".
• "joeshaw/gengen"
• "cheekybits/genny"
• "clipperhouse/gen"
• "Achieving type generic functions in Go, without using reflections"

That would make calling those lib functions easier, since the concrete P () implementations generated would use the right type instead of interface{}.

What you want to do would require generics but as you already mentioned, Go does not support generic types. Therefore, you can't create a general function which would not lose the type.

You have to create such a function for each type you want to support. Note that the standard library already contains some of these going under the name MustXXX(), which you can use out of the box, for example:

template.Must(t *Template, err error) *Template

Or "similar" functions which suppress the error but if one still occurs, panics, for example:

regexp.MustCompile(str string) *Regexp (suppresses error but panics if str is not a valid regexp)

If you plan on just panicking on errors (bad idea) or logging them, then just define a function to do so and use it. E.g.

func checkErr(err error) {
    if err != nil {
        log.Println(err)
    }
}

// ...

func foo() {
    a, err := doA()
    checkErr(err)
    b, err := doB()
    checkErr(err)
    // etc.
}

The user twotwotwo has already linked to the Errors are values article that shows more examples on how to make error handling less repetitive. But I would recommend just write the whole if err != nil thing, because in my experience every third error, if not second, requires some additional handling.