1.8 Loose Ends

There is a lot more to Go than we’ve covered in this quick introduction. Here are some topics we’ve barely touched upon or omitted entirely, with just enough discussion that they will be familiar when they make brief appearances before the full treatment.

Control flow: We covered the two fundamental control-flow statements, if and for, but not the switch statement, which is a multi-way branch. Here’s a small example:

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switch coinflip() {
case "heads":
    heads++
case "tails":
    tails++
default:
    fmt.Println("landed on edge!")
}

The result of calling coinflip is compared to the value of each case. Cases are evaluated from top to bottom, so the first matching one is executed. The optional default case matches if none of the other cases does; it may be placed anywhere. Cases do not fall through from one to the next as in C-like languages (though there is a rarely used fallthrough statement that overrides this behavior).

A switch does not need an operand; it can just list the cases, each of which is a boolean expression:

func Signum(x int) int {
    switch {
    case x > 0:
        return +1
    default:
        return 0
    case x < 0:
        return -1
    }
}

This form is called a tagless switch; it’s equivalent to switch true.

Like the for and if statements, a switch may include an optional simple statement—a short variable declaration, an increment or assignment statement, or a function call—that can be used to set a value before it is tested.

The break and continue statements modify the flow of control. A break causes control to resume at the next statement after the innermost for, switch, or select statement (which we’ll see later), and as we saw in Section 1.3, a continue causes the innermost for loop to start its next iteration. Statements may be labeled so that break and continue can refer to them, for instance to break out of several nested loops at once or to start the next iteration of the outermost loop. There is even a goto statement, though it’s intended for machine-generated code, not regular use by programmers.

Named types: A type declaration makes it possible to give a name to an existing type. Since struct types are often long, they are nearly always named. A familiar example is the definition of a Point type for a 2-D graphics system:

type Point struct {
    X, Y int
}
var p Point

Type declarations and named types are covered in Chapter 2.

Pointers: Go provides pointers, that is, values that contain the address of a variable. In some languages, notably C, pointers are relatively unconstrained. In other languages, pointers are disguised as “references,” and there’s not much that can be done with them except pass them around. Go takes a position somewhere in the middle. Pointers are explicitly visible. The & operator yields the address of a variable, and the * operator retrieves the variable that the pointer refers to, but there is no pointer arithmetic. We’ll explain pointers in Section 2.3.2.

Methods and interfaces: A method is a function associated with a named type; Go is unusual in that methods may be attached to almost any named type. Methods are covered in Chapter 6. Interfaces are abstract types that let us treat different concrete types in the same way based on what methods they have, not how they are represented or implemented. Interfaces are the subject of Chapter 7.

Packages: Go comes with an extensive standard library of useful packages, and the Go community has created and shared many more. Programming is often more about using existing packages than about writing original code of one’s own. Throughout the book, we will point out a couple of dozen of the most important standard packages, but there are many more we don’t have space to mention, and we cannot provide anything remotely like a complete reference for any package.

Before you embark on any new program, it’s a good idea to see if packages already exist that might help you get your job done more easily. You can find an index of the standard library packages at https://golang.org/pkg and the packages contributed by the community at https://godoc.org. The go doc tool makes these documents easily accessible from the command line:

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$ go doc http.ListenAndServe
package http // import "net/http"

func ListenAndServe(addr string, handler Handler) error

    ListenAndServe listens on the TCP network address addr and then
    calls Serve with handler to handle requests on incoming connections.
...

Comments: We have already mentioned documentation comments at the beginning of a program or package. It’s also good style to write a comment before the declaration of each function to specify its behavior. These conventions are important, because they are used by tools like go doc and godoc to locate and display documentation (§10.7.4).

For comments that span multiple lines or appear within an expression or statement, there is also the /* ... */ notation familiar from other languages. Such comments are sometimes used at the beginning of a file for a large block of explanatory text to avoid a // on every line. Within a comment, // and /* have no special meaning, so comments do not nest.