Head First Go(golang early release) pdf.pdf-资料库

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READY, SET, GO! Back in 2007, the search engine Google had a problem. They had to maintain programs with millions of lines of code. Before they could test new changes, they had to compile the code into a runnable form, a process which at the time took the better part of an hour. Needless to say, this was bad for developer productivity. So Google engineers Robert Griesemer, Rob Pike, and Ken Thompson sketched out some goals for a new language: • Fast compilation • Less cumbersome code • Unused memory freed automatically (garbage collection) • Easy to write software that does several operations simultaneously (concurrency) • Good support for processors with multiple cores After a couple years of work, Google had a language that was fast to write code for and produced programs that were fast to compile and run. The project switched to an open source license in 2009. Go is now free for anyone to use. And you should use it! Go is rapidly gaining popularity thanks to its simplicity and power. If you're writing a commandline tool, Go can produce executable files for Windows, Mac, and Linux, all from the same source code. If you're writing a web server, Go can help you handle many users connecting at once. And no matter what you're writing, it will help you ensure that your code is easier to maintain and add to. Ready to learn more? Let's Go!

THE GO PLAYGROUND The easiest way to try Go is to visit https://play.golang.org in your web browser. There, the Go team has set up a simple editor where you can enter Go code and run it on their servers. The result is displayed right there in your browser. (Of course, this only works if you have a stable Internet connection. If you don't, see page 25 to learn how to download and run the Go compiler directly on your computer. Then run the following examples using the compiler instead.) Let's try it out now! Open http://play.golang.org in your browser. (Don't worry if what you see doesn't quite match the screenshot; it just means they've improved the site since this book was printed!)

Delete any code that's in the editing area, and type this instead: Click the "Format" button, which will automatically reformat your code according to Go conventions. Click the "Run" button. You should see "Hello, Go!" displayed at the bottom of the screen. Congratulations, you've just run your first Go program! Turn the page, and we'll explain what we just did... WHAT DOES IT ALL MEAN? You've just run your first Go program! Now let's look at the code and figure out what it actually means... Every Go file starts with a package declaration. A package is a group of code that all does similar things, like formatting strings or drawing images. The package declaration gives the name of the package that this file's code will become a part of. In this case, we use the special package main, which is required if this code is going to be run directly (usually from the terminal). Next, Go files almost always have one or more import statements. Each file needs to import other packages before its code can use the code those other packages contain. Loading all the Go code on your computer at once would result in a big, slow program, so instead you specify only the packages you need by importing them.

The last part of every Go file is the actual code, which is often split up into one or more functions. A function is a group of one or more lines of code that you can call (run) from other places in your program. When a Go program is run, it looks for a function named main and runs that first, which is why we named this function main. Don't worry if you don't understand all this right now! We'll look at everything in more detail in the next few pages. The typical Go file layout You'll quickly get used to seeing these three sections, in this order, in almost every Go file you work with: 1. The package declaration 2. Any import statements 3. The actual code

The saying goes, "a place for everything, and everything in its place." Go is a very consistent language. This is a good thing: you'll often find you just know where to look in your project for a given piece of code, without having to think about it! THERE ARE NO DUMB QUESTIONS Q: My other programming language requires that each statement end with a semicolon. Doesn't Go? A: You can use semicolons to separate statements in Go, but it's not required (in fact, it's generally frowned upon). Q: Why did we run the automatic reformatting tool on our code? A: Whenever you share your code, other Go developers will expect it to be in the standard Go format. That means that things like indentation and spacing will be formatted in a standard way, making it easier for everyone to read. Where other languages achieve this by relying on people manually reformatting their code to conform to a style guide, with Go all you have to do is run the standard formatting tool, and it will automatically fix everything for you. We ran the formatter on every example we created for this book, and you should run it on all your code, too! WHAT IF SOMETHING GOES WRONG? Go programs have to follow certain rules to avoid confusing the compiler. If we break one of these rules, we'll get an error message. Suppose we forgot to add parentheses on our call to the Println function on line 6: If we try to run this version of the program, we get an error:

Go tells us the name of the source code file and the line number where there's a problem. (The Go Playground saves the contents of the online editor to a temporary file before running it, which is where the "main. go" file name comes from.) Then it gives a description of the error. In this case, because we deleted the parenthesis, Go can't tell we're trying to call the Println function, so it can't understand why we're putting "Hello, Go" at the end of line 6. BREAKING STUFF IS EDUCATIONAL! We can get a feel for the rules Go programs have to follow by intentionally breaking our program in various ways. Take this code sample, try making one of the changes below, and run it. Then undo your change, and try the next one. See what happens! If you do this... ...it will fail because... Delete the package declaration... package main Every Go file has to begin with a package declaration.

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Head First Go(golang early release) pdf.pdf

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