[mtlynch]

The problem there is that you lose the guarantee that the parser validated the string value.

A caller can just say:

    // This is returning an error for some reason, so let's do it directly.
    // username, err := parsers.NewUsername(raw)
    username := parsers.Username(raw)

You also get implicit conversions in ways you probably don't want:

    var u Username
    u = "<hello>" // Implicitly converts from string to Username

[costco]

That's true I did not think of that.