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I was just wondering why the string functions in Google Go are defined in a strings package as opposed to on the string data type itself. They could have easily done

func (s string) ToUpper() string {
}

instead of the current

func ToUpper(s string) string {
}

in the strings package.

My guess is that if you want to implement a custom version of ToUpper on a custom type that extends string (i.e., type MyString string), you have no way to access the builtin ToUpper anymore on that type, but I can't find any support on this.

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  • Actually if the built-in string would have methods, you would still be able to access those methods on your custom types by simply casting your custom type to string. See this example on Go Playground. Commented Jan 19, 2015 at 11:56

2 Answers 2

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The short answer is: "To keep the language simple."

Go as a language only allows methods to be defined on types that are in the same package, but because string (like other builtin types) is implemented in the language itself, there is no way to add methods to it, without complicating the language / compiler.

It's also partly, because of how Go was designed.

See this mail from Rob Pike (one of the creators of Go) for more information:

Go does not have methods on basic types because the designers of the language did not want methods defined for basic types, in part because of the knock-on effect they might have on interfaces. I believe we are all still comfortable with that decision. Others may feel differently.

-rob

And this one too:

We simply didn't understand what the implications would be; there's nothing to explain. Go was designed with caution.

In that vein, look at the size of the strings library. Making all that functionality methods on the basic type would, as Andrew said, complicate the language. Why complicate the language with such trivial things when it can be achieved by a library, which is more maintainable, easier to extend, and more flexible? The language is much simpler the way things are.

-rob

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2 Comments

Thanks for that. That answer is really in line with the spirit of Go as I got it so far :)
@rpkamp Yes you're right. I am quite uncomfortable with some choices that have been made with Go, but whenever I lwant to know more I find a clear, detailed and fully sensible explanation from the Go team about why things have been done this way. That makes me feel very comfident about the bright future of Go.
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string is one of the predeclared type in the builtin package.

Those functions in strings couldn't have been defined as method using a predeclared type as a receiver: that would have required to define a type alias (to the underlying type string, in order to attach methods to it).

A method declaration uses a receiver which has a Type, which in turn does not include any of the predeclared types (bool byte complex64 complex128 error float32 float64 int int8 int16 int32 int64 rune string uint uint8 uint16 uint32 uint64 uintptr).

Or (which is done here), one uses functions in a dedicated package 'strings'.
That seems coherent with the fact that the type string itself has no fields: its content doesn't have to "receive" the method, it can simply be used by a function.

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4 Comments

I wasn't talking about me adding it, but about it being in the language itself. However as @Justin Nuß points out this is not possible because strings are not defined in Go itself, so it would complicate the compiler to make it work.
@ScallioXTX I wasn't talking about adding it either. I have edited athe answer.
"that would have required to define a type alias". Not wanting to be annoying, but defining an alias would have been easy, no? It would just be "type mystring string" and then define "func (self mystring) ToUper() mystring { /... */ }.
@OlliP Indeed, that looks easier. But since string is a predeclared type, I guess they preferred isolating string functions in a dedicated package rather than attached to a "type" (as if string was in a package, which it is not).

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