Hi, On Mon, Jun 14, 2021 at 2:24 AM Amit Saha <amitsaha...@gmail.com> wrote:
> Hi - My main motivation to understand this is i always had to google > this - how to convert a string to a byte slice. > > Is []byte a type that has been defined in the compiler? > No, but `byte` is. It is a predeclared identifier <https://golang.org/ref/spec#Predeclared_identifiers>, referring to a builtin integer type <https://golang.org/ref/spec#Numeric_types> (it's an alias for `uint8`). So the compiler knows what a `byte` is and what a slice is, so it knows what a slice of byte is. The conversion between a slice of `byte`s and a `string` is then defined in the spec <https://golang.org/ref/spec#Conversions>: A non-constant value x can be converted to type T in any of these cases: > > - […] > > > - x is an integer or a slice of bytes or runes and T is a string type. > > > - x is a string and T is a slice of bytes or runes. > > This means the compiler, when seeing a conversion, explicitly tests for these two cases. How the conversion then actually works, you can see in the compiler explorer <https://godbolt.org/z/6vn3rnfvn>. The compiler emits a call into a function called `runtime.stringtoslicebyte`. You can actually find that function in the source code of the runtime <https://github.com/golang/go/blob/14305bf0b9ab87bcaca11416ab61e7a4ba09690d/src/runtime/string.go#L165> then (the same function also contains other functions implementing similar conversions). Really, it just allocates a new `[]byte` of the appropriate size and then copies over the bytes from the string. Or, is that an internal level detail that an earlier stage (parsing) > You can actually mess with this a little bit to show that it's not done in the parsing stage, but that the compiler actually knows if a type is a slice of bytes or not. Because `byte` is a predeclared identifier, it can be shadowed, like every other identifier. That is, you can declare your *own* type called `byte`: https://play.golang.org/p/6vDjw9eWX9s You can also define your own alias for `uint8` and then convert that: https://play.golang.org/p/nvseU7ofRru Or you can do both - first shadown the builtin `byte` alias and *then* create your own: https://play.golang.org/p/1Y80stIxa5v You can also define your own type called `uint8` and then define `byte` as an alias to that and see that you can no longer convert: https://play.golang.org/p/xUQrAmMm5Km All of this shows that the compiler really can't just rely on parsing and the name. It really needs to have a notion of whether something is a slice of a specific pre-declared type `uint8`, no matter what it is called in the source code. It does that by creating a "virtual" package builtin <https://golang.org/pkg/builtin/> inside the compiler and then synthesizing type-definitions in that package. The code for that is here <https://github.com/golang/go/blob/7677616a263e8ded606cc8297cb67ddc667a876e/src/cmd/compile/internal/gc/universe.go> . But it should be noted that this package doesn't really exist and behaves significantly different from "normal" packages - not just because it is implemented entirely in the compiler/runtime, but also because it's identifier are defined "outside" any package, in the universe block <https://golang.org/ref/spec#Blocks>. I assume this covers all questions :) Let us know if you have follow-ups :) Axel takes care of when the compiler sees that statement? > > Thanks, > Amit > > -- > 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/CANODV3nBYmshDLFwUUdnnyVuvpjhWnwBJOb%3DwrZKEHXmtBgbSg%40mail.gmail.com > . > -- 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/CAEkBMfHLRuoQtV3uyvF8vHsJUpL5jEFcBJNs88PicZSs3%2Bhpsw%40mail.gmail.com.
