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They're out of date, a pain to maintain, and most of the material is better served by the Go Tour. Fixes #2101. R=golang-dev, rsc, r, adg CC=golang-dev https://golang.org/cl/5489053
1039 lines
44 KiB
Cheetah
1039 lines
44 KiB
Cheetah
<!-- A Tutorial for the Go Programming Language -->
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{{donotedit}}
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<h2>Introduction</h2>
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<p>
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This document is a tutorial introduction to the basics of the Go programming
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language, intended for programmers familiar with C or C++. It is not a comprehensive
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guide to the language; at the moment the document closest to that is the
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<a href='/doc/go_spec.html'>language specification</a>.
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After you've read this tutorial, you should look at
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<a href='/doc/effective_go.html'>Effective Go</a>,
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which digs deeper into how the language is used and
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talks about the style and idioms of programming in Go.
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An interactive introduction to Go is available, called
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<a href='http://tour.golang.org/'>A Tour of Go</a>.
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<p>
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The presentation here proceeds through a series of modest programs to illustrate
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key features of the language. All the programs work (at time of writing) and are
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checked into the repository in the directory <a href='/doc/progs'><code>/doc/progs/</code></a>.
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<p>
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<h2>Hello, World</h2>
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<p>
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Let's start in the usual way:
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<p>
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{{code "progs/helloworld.go" `/package/` "$"}}
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<p>
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Every Go source file declares, using a <code>package</code> statement, which package it's part of.
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It may also import other packages to use their facilities.
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This program imports the package <code>fmt</code> to gain access to
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our old, now capitalized and package-qualified, friend, <code>fmt.Printf</code>.
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<p>
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Functions are introduced with the <code>func</code> keyword.
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The <code>main</code> package's <code>main</code> function is where the program starts running (after
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any initialization).
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<p>
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String constants can contain Unicode characters, encoded in UTF-8.
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(In fact, Go source files are defined to be encoded in UTF-8.)
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<p>
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The comment convention is the same as in C++:
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<p>
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<pre>
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/* ... */
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// ...
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</pre>
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<p>
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Later we'll have much more to say about printing.
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<p>
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<h2>Semicolons</h2>
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<p>
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You might have noticed that our program has no semicolons. In Go
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code, the only place you typically see semicolons is separating the
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clauses of <code>for</code> loops and the like; they are not necessary after
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every statement.
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<p>
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In fact, what happens is that the formal language uses semicolons,
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much as in C or Java, but they are inserted automatically
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at the end of every line that looks like the end of a statement. You
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don't need to type them yourself.
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<p>
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For details about how this is done you can see the language
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specification, but in practice all you need to know is that you
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never need to put a semicolon at the end of a line. (You can put
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them in if you want to write multiple statements per line.) As an
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extra help, you can also leave out a semicolon immediately before
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a closing brace.
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<p>
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This approach makes for clean-looking, semicolon-free code. The
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one surprise is that it's important to put the opening
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brace of a construct such as an <code>if</code> statement on the same line as
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the <code>if</code>; if you don't, there are situations that may not compile
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or may give the wrong result. The language forces the brace style
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to some extent.
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<p>
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<h2>Compiling</h2>
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<p>
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Go is a compiled language. At the moment there are two compilers.
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<code>Gccgo</code> is a Go compiler that uses the GCC back end. There is also a
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suite of compilers with different (and odd) names for each architecture:
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<code>6g</code> for the 64-bit x86, <code>8g</code> for the 32-bit x86, and more. These
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compilers run significantly faster but generate less efficient code
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than <code>gccgo</code>. At the time of writing (late 2009), they also have
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a more robust run-time system although <code>gccgo</code> is catching up.
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<p>
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Here's how to compile and run our program. With <code>6g</code>, say,
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<p>
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<pre>
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$ 6g helloworld.go # compile; object goes into helloworld.6
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$ 6l helloworld.6 # link; output goes into 6.out
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$ ./6.out
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Hello, world; or Καλημέρα κόσμε; or こんにちは 世界
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$
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</pre>
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<p>
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With <code>gccgo</code> it looks a little more traditional.
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<p>
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<pre>
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$ gccgo helloworld.go
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$ ./a.out
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Hello, world; or Καλημέρα κόσμε; or こんにちは 世界
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$
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</pre>
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<p>
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<h2>Echo</h2>
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<p>
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Next up, here's a version of the Unix utility <code>echo(1)</code>:
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<p>
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{{code "progs/echo.go" `/package/` "$"}}
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<p>
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This program is small but it's doing a number of new things. In the last example,
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we saw <code>func</code> introduce a function. The keywords <code>var</code>, <code>const</code>, and <code>type</code>
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(not used yet) also introduce declarations, as does <code>import</code>.
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Notice that we can group declarations of the same sort into
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parenthesized lists, one item per line, as in the <code>import</code> and <code>const</code> clauses here.
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But it's not necessary to do so; we could have said
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<p>
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<pre>
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const Space = " "
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const Newline = "\n"
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</pre>
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<p>
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This program imports the <code>"os"</code> package to access its <code>Stdout</code> variable, of type
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<code>*os.File</code>. The <code>import</code> statement is actually a declaration: in its general form,
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as used in our ``hello world'' program,
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it names the identifier (<code>fmt</code>)
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that will be used to access members of the package imported from the file (<code>"fmt"</code>),
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found in the current directory or in a standard location.
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In this program, though, we've dropped the explicit name from the imports; by default,
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packages are imported using the name defined by the imported package,
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which by convention is of course the file name itself. Our ``hello world'' program
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could have said just <code>import "fmt"</code>.
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<p>
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You can specify your
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own import names if you want but it's only necessary if you need to resolve
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a naming conflict.
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<p>
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Given <code>os.Stdout</code> we can use its <code>WriteString</code> method to print the string.
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<p>
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After importing the <code>flag</code> package, we use a <code>var</code> declaration
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to create and initialize a global variable, called <code>omitNewline</code>,
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to hold the value of echo's <code>-n</code> flag.
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The variable has type <code>*bool</code>, pointer to <code>bool</code>.
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<p>
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In <code>main.main</code>, we parse the arguments (the call to <code>flag.Parse</code>) and then create a local
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string variable with which to build the output.
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<p>
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The declaration statement has the form
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<p>
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<pre>
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var s string = ""
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</pre>
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<p>
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This is the <code>var</code> keyword, followed by the name of the variable, followed by
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its type, followed by an equals sign and an initial value for the variable.
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<p>
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Go tries to be terse, and this declaration could be shortened. Since the
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string constant is of type string, we don't have to tell the compiler that.
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We could write
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<p>
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<pre>
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var s = ""
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</pre>
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<p>
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or we could go even shorter and write the idiom
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<p>
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<pre>
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s := ""
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</pre>
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<p>
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The <code>:=</code> operator is used a lot in Go to represent an initializing declaration.
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There's one in the <code>for</code> clause on the next line:
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<p>
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{{code "progs/echo.go" `/for/`}}
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<p>
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The <code>flag</code> package has parsed the arguments and left the non-flag arguments
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in a list that can be iterated over in the obvious way.
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<p>
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The Go <code>for</code> statement differs from that of C in a number of ways. First,
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it's the only looping construct; there is no <code>while</code> or <code>do</code>. Second,
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there are no parentheses on the clause, but the braces on the body
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are mandatory. The same applies to the <code>if</code> and <code>switch</code> statements.
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Later examples will show some other ways <code>for</code> can be written.
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<p>
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The body of the loop builds up the string <code>s</code> by appending (using <code>+=</code>)
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the arguments and separating spaces. After the loop, if the <code>-n</code> flag is not
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set, the program appends a newline. Finally, it writes the result.
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<p>
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Notice that <code>main.main</code> is a niladic function with no return type.
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It's defined that way. Falling off the end of <code>main.main</code> means
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''success''; if you want to signal an erroneous return, call
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<p>
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<pre>
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os.Exit(1)
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</pre>
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<p>
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The <code>os</code> package contains other essentials for getting
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started; for instance, <code>os.Args</code> is a slice used by the
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<code>flag</code> package to access the command-line arguments.
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<p>
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<h2>An Interlude about Types</h2>
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<p>
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Go has some familiar types such as <code>int</code> and <code>uint</code> (unsigned <code>int</code>), which represent
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values of the ''appropriate'' size for the machine. It also defines
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explicitly-sized types such as <code>int8</code>, <code>float64</code>, and so on, plus
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unsigned integer types such as <code>uint</code>, <code>uint32</code>, etc.
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These are distinct types; even if <code>int</code> and <code>int32</code> are both 32 bits in size,
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they are not the same type. There is also a <code>byte</code> synonym for
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<code>uint8</code>, which is the element type for strings.
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<p>
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Floating-point types are always sized: <code>float32</code> and <code>float64</code>,
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plus <code>complex64</code> (two <code>float32s</code>) and <code>complex128</code>
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(two <code>float64s</code>). Complex numbers are outside the
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scope of this tutorial.
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<p>
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Speaking of <code>string</code>, that's a built-in type as well. Strings are
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<i>immutable values</i>—they are not just arrays of <code>byte</code> values.
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Once you've built a string <i>value</i>, you can't change it, although
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of course you can change a string <i>variable</i> simply by
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reassigning it. This snippet from <code>strings.go</code> is legal code:
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<p>
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{{code "progs/strings.go" `/hello/` `/ciao/`}}
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<p>
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However the following statements are illegal because they would modify
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a <code>string</code> value:
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<p>
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<pre>
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s[0] = 'x'
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(*p)[1] = 'y'
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</pre>
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<p>
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In C++ terms, Go strings are a bit like <code>const strings</code>, while pointers
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to strings are analogous to <code>const string</code> references.
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<p>
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Yes, there are pointers. However, Go simplifies their use a little;
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read on.
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<p>
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Arrays are declared like this:
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<p>
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<pre>
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var arrayOfInt [10]int
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</pre>
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<p>
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Arrays, like strings, are values, but they are mutable. This differs
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from C, in which <code>arrayOfInt</code> would be usable as a pointer to <code>int</code>.
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In Go, since arrays are values, it's meaningful (and useful) to talk
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about pointers to arrays.
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<p>
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The size of the array is part of its type; however, one can declare
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a <i>slice</i> variable to hold a reference to any array, of any size,
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with the same element type.
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A <i>slice
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expression</i> has the form <code>a[low : high]</code>, representing
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the internal array indexed from <code>low</code> through <code>high-1</code>; the resulting
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slice is indexed from <code>0</code> through <code>high-low-1</code>.
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In short, slices look a lot like arrays but with
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no explicit size (<code>[]</code> vs. <code>[10]</code>) and they reference a segment of
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an underlying, usually anonymous, regular array. Multiple slices
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can share data if they represent pieces of the same array;
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multiple arrays can never share data.
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<p>
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Slices are much more common in Go programs than
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regular arrays; they're more flexible, have reference semantics,
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and are efficient. What they lack is the precise control of storage
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layout of a regular array; if you want to have a hundred elements
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of an array stored within your structure, you should use a regular
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array. To create one, use a compound value <i>constructor</i>—an
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expression formed
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from a type followed by a brace-bounded expression like this:
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<p>
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<pre>
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[3]int{1,2,3}
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</pre>
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<p>
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In this case the constructor builds an array of 3 <code>ints</code>.
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<p>
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When passing an array to a function, you almost always want
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to declare the formal parameter to be a slice. When you call
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the function, slice the array to create
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(efficiently) a slice reference and pass that.
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By default, the lower and upper bounds of a slice match the
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ends of the existing object, so the concise notation <code>[:]</code>
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will slice the whole array.
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<p>
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Using slices one can write this function (from <code>sum.go</code>):
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<p>
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{{code "progs/sum.go" `/sum/` `/^}/`}}
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<p>
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Note how the return type (<code>int</code>) is defined for <code>sum</code> by stating it
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after the parameter list.
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<p>
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To call the function, we slice the array. This code (we'll show
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a simpler way in a moment) constructs
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an array and slices it:
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<p>
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<pre>
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x := [3]int{1,2,3}
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s := sum(x[:])
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</pre>
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<p>
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If you are creating a regular array but want the compiler to count the
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elements for you, use <code>...</code> as the array size:
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<p>
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<pre>
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x := [...]int{1,2,3}
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s := sum(x[:])
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</pre>
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<p>
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That's fussier than necessary, though.
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In practice, unless you're meticulous about storage layout within a
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data structure, a slice itself—using empty brackets with no size—is all you need:
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<p>
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<pre>
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s := sum([]int{1,2,3})
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</pre>
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<p>
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There are also maps, which you can initialize like this:
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<p>
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<pre>
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m := map[string]int{"one":1 , "two":2}
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</pre>
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<p>
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The built-in function <code>len</code>, which returns number of elements,
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makes its first appearance in <code>sum</code>. It works on strings, arrays,
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slices, maps, and channels.
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<p>
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By the way, another thing that works on strings, arrays, slices, maps
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and channels is the <code>range</code> clause on <code>for</code> loops. Instead of writing
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<p>
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<pre>
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for i := 0; i < len(a); i++ { ... }
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</pre>
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<p>
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to loop over the elements of a slice (or map or ...) , we could write
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<p>
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<pre>
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for i, v := range a { ... }
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</pre>
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<p>
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This assigns <code>i</code> to the index and <code>v</code> to the value of the successive
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elements of the target of the range. See
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<a href='/doc/effective_go.html'>Effective Go</a>
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for more examples of its use.
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<p>
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<p>
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<h2>An Interlude about Allocation</h2>
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<p>
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Most types in Go are values. If you have an <code>int</code> or a <code>struct</code>
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or an array, assignment
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copies the contents of the object.
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To allocate a new variable, use the built-in function <code>new</code>, which
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returns a pointer to the allocated storage.
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<p>
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<pre>
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type T struct { a, b int }
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var t *T = new(T)
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</pre>
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<p>
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or the more idiomatic
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<p>
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<pre>
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t := new(T)
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</pre>
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<p>
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Some types—maps, slices, and channels (see below)—have reference semantics.
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If you're holding a slice or a map and you modify its contents, other variables
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referencing the same underlying data will see the modification. For these three
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types you want to use the built-in function <code>make</code>:
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<p>
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<pre>
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m := make(map[string]int)
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</pre>
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<p>
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This statement initializes a new map ready to store entries.
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If you just declare the map, as in
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<p>
|
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<pre>
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var m map[string]int
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</pre>
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<p>
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it creates a <code>nil</code> reference that cannot hold anything. To use the map,
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you must first initialize the reference using <code>make</code> or by assignment from an
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existing map.
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|
<p>
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Note that <code>new(T)</code> returns type <code>*T</code> while <code>make(T)</code> returns type
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<code>T</code>. If you (mistakenly) allocate a reference object with <code>new</code> rather than <code>make</code>,
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|
you receive a pointer to a nil reference, equivalent to
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|
declaring an uninitialized variable and taking its address.
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<p>
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<h2>An Interlude about Constants</h2>
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<p>
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Although integers come in lots of sizes in Go, integer constants do not.
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|
There are no constants like <code>0LL</code> or <code>0x0UL</code>. Instead, integer
|
|
constants are evaluated as large-precision values that
|
|
can overflow only when they are assigned to an integer variable with
|
|
too little precision to represent the value.
|
|
<p>
|
|
<pre>
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|
const hardEight = (1 << 100) >> 97 // legal
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|
</pre>
|
|
<p>
|
|
There are nuances that deserve redirection to the legalese of the
|
|
language specification but here are some illustrative examples:
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|
<p>
|
|
<pre>
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|
var a uint64 = 0 // a has type uint64, value 0
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a := uint64(0) // equivalent; uses a "conversion"
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i := 0x1234 // i gets default type: int
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var j int = 1e6 // legal - 1000000 is representable in an int
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x := 1.5 // a float64, the default type for floating constants
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i3div2 := 3/2 // integer division - result is 1
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f3div2 := 3./2. // floating-point division - result is 1.5
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</pre>
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<p>
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|
Conversions only work for simple cases such as converting <code>ints</code> of one
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|
sign or size to another and between integers and floating-point numbers,
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|
plus a couple of other instances outside the scope of a tutorial.
|
|
There are no automatic numeric conversions of any kind in Go,
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|
other than that of making constants have concrete size and type when
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|
assigned to a variable.
|
|
<p>
|
|
<h2>An I/O Package</h2>
|
|
<p>
|
|
Next we'll look at a simple package for doing Unix file I/O with an
|
|
open/close/read/write interface.
|
|
Here's the start of <code>file.go</code>:
|
|
<p>
|
|
{{code "progs/file.go" `/package/` `/^}/`}}
|
|
<p>
|
|
The first few lines declare the name of the
|
|
package—<code>file</code>—and then import two packages. The <code>os</code>
|
|
package hides the differences
|
|
between various operating systems to give a consistent view of files and
|
|
so on; here we're going to use its error handling utilities
|
|
and reproduce the rudiments of its file I/O.
|
|
<p>
|
|
The other item is the low-level, external <code>syscall</code> package, which provides
|
|
a primitive interface to the underlying operating system's calls.
|
|
The <code>syscall</code> package is very system-dependent, and the way it's
|
|
used here works only on Unix-like systems,
|
|
but the general ideas explored here apply broadly.
|
|
(A Windows version is available in
|
|
<a href="progs/file_windows.go"><code>file_windows.go</code></a>.)
|
|
<p>
|
|
Next is a type definition: the <code>type</code> keyword introduces a type declaration,
|
|
in this case a data structure called <code>File</code>.
|
|
To make things a little more interesting, our <code>File</code> includes the name of the file
|
|
that the file descriptor refers to.
|
|
<p>
|
|
Because <code>File</code> starts with a capital letter, the type is available outside the package,
|
|
that is, by users of the package. In Go the rule about visibility of information is
|
|
simple: if a name (of a top-level type, function, method, constant or variable, or of
|
|
a structure field or method) is capitalized, users of the package may see it. Otherwise, the
|
|
name and hence the thing being named is visible only inside the package in which
|
|
it is declared. This is more than a convention; the rule is enforced by the compiler.
|
|
In Go, the term for publicly visible names is ''exported''.
|
|
<p>
|
|
In the case of <code>File</code>, all its fields are lower case and so invisible to users, but we
|
|
will soon give it some exported, upper-case methods.
|
|
<p>
|
|
First, though, here is a factory to create a <code>File</code>:
|
|
<p>
|
|
{{code "progs/file.go" `/newFile/` `/^}/`}}
|
|
<p>
|
|
This returns a pointer to a new <code>File</code> structure with the file descriptor and name
|
|
filled in. This code uses Go's notion of a ''composite literal'', analogous to
|
|
the ones used to build maps and arrays, to construct a new heap-allocated
|
|
object. We could write
|
|
<p>
|
|
<pre>
|
|
n := new(File)
|
|
n.fd = fd
|
|
n.name = name
|
|
return n
|
|
</pre>
|
|
<p>
|
|
but for simple structures like <code>File</code> it's easier to return the address of a
|
|
composite literal, as is done here in the <code>return</code> statement from <code>newFile</code>.
|
|
<p>
|
|
We can use the factory to construct some familiar, exported variables of type <code>*File</code>:
|
|
<p>
|
|
{{code "progs/file.go" `/var/` `/^\)/`}}
|
|
<p>
|
|
The <code>newFile</code> function was not exported because it's internal. The proper,
|
|
exported factory to use is <code>OpenFile</code> (we'll explain that name in a moment):
|
|
<p>
|
|
{{code "progs/file.go" `/func.OpenFile/` `/^}/`}}
|
|
<p>
|
|
There are a number of new things in these few lines. First, <code>OpenFile</code> returns
|
|
multiple values, a <code>File</code> and an error (more about errors in a moment).
|
|
We declare the
|
|
multi-value return as a parenthesized list of declarations; syntactically
|
|
they look just like a second parameter list. The function
|
|
<code>syscall.Open</code>
|
|
also has a multi-value return, which we can grab with the multi-variable
|
|
declaration on the first line; it declares <code>r</code> and <code>e</code> to hold the two values,
|
|
both of type <code>int</code> (although you'd have to look at the <code>syscall</code> package
|
|
to see that). Finally, <code>OpenFile</code> returns two values: a pointer to the new <code>File</code>
|
|
and the error. If <code>syscall.Open</code> fails, the file descriptor <code>r</code> will
|
|
be negative and <code>newFile</code> will return <code>nil</code>.
|
|
<p>
|
|
About those errors: The Go language includes a general notion of an error:
|
|
a pre-defined type <code>error</code> with properties (described below)
|
|
that make it a good basis for representing and handling errors.
|
|
It's a good idea to use its facility in your own interfaces, as we do here, for
|
|
consistent error handling throughout Go code. In <code>Open</code> we use a
|
|
conversion to translate Unix's integer <code>errno</code> value into the integer type
|
|
<code>os.Errno</code>, which is an implementation of <code>error</code>
|
|
<p>
|
|
Why <code>OpenFile</code> and not <code>Open</code>? To mimic Go's <code>os</code> package, which
|
|
our exercise is emulating. The <code>os</code> package takes the opportunity
|
|
to make the two commonest cases - open for read and create for
|
|
write - the simplest, just <code>Open</code> and <code>Create</code>. <code>OpenFile</code> is the
|
|
general case, analogous to the Unix system call <code>Open</code>. Here is
|
|
the implementation of our <code>Open</code> and <code>Create</code>; they're trivial
|
|
wrappers that eliminate common errors by capturing
|
|
the tricky standard arguments to open and, especially, to create a file:
|
|
<p>
|
|
{{code "progs/file.go" `/^const/` `/^}/`}}
|
|
<p>
|
|
{{code "progs/file.go" `/func.Create/` `/^}/`}}
|
|
<p>
|
|
Back to our main story.
|
|
Now that we can build <code>Files</code>, we can write methods for them. To declare
|
|
a method of a type, we define a function to have an explicit receiver
|
|
of that type, placed
|
|
in parentheses before the function name. Here are some methods for <code>*File</code>,
|
|
each of which declares a receiver variable <code>file</code>.
|
|
<p>
|
|
{{code "progs/file.go" `/Close/` "$"}}
|
|
<p>
|
|
There is no implicit <code>this</code> and the receiver variable must be used to access
|
|
members of the structure. Methods are not declared within
|
|
the <code>struct</code> declaration itself. The <code>struct</code> declaration defines only data members.
|
|
In fact, methods can be created for almost any type you name, such as an integer or
|
|
array, not just for <code>structs</code>. We'll see an example with arrays later.
|
|
<p>
|
|
The <code>String</code> method is so called because of a printing convention we'll
|
|
describe later.
|
|
<p>
|
|
The methods use the public variable <code>os.EINVAL</code> to return the (<code>error</code>
|
|
version of the) Unix error code <code>EINVAL</code>. The <code>os</code> library defines a standard
|
|
set of such error values.
|
|
<p>
|
|
We can now use our new package:
|
|
<p>
|
|
{{code "progs/helloworld3.go" `/package/` "$"}}
|
|
<p>
|
|
The ''<code>./</code>'' in the import of ''<code>./file</code>'' tells the compiler
|
|
to use our own package rather than
|
|
something from the directory of installed packages.
|
|
(Also, ''<code>file.go</code>'' must be compiled before we can import the
|
|
package.)
|
|
<p>
|
|
Now we can compile and run the program. On Unix, this would be the result:
|
|
<p>
|
|
<pre>
|
|
$ 6g file.go # compile file package
|
|
$ 6g helloworld3.go # compile main package
|
|
$ 6l -o helloworld3 helloworld3.6 # link - no need to mention "file"
|
|
$ ./helloworld3
|
|
hello, world
|
|
can't open file; err=No such file or directory
|
|
$
|
|
</pre>
|
|
<p>
|
|
<h2>Rotting cats</h2>
|
|
<p>
|
|
Building on the <code>file</code> package, here's a simple version of the Unix utility <code>cat(1)</code>,
|
|
<code>progs/cat.go</code>:
|
|
<p>
|
|
{{code "progs/cat.go" `/package/` "$"}}
|
|
<p>
|
|
By now this should be easy to follow, but the <code>switch</code> statement introduces some
|
|
new features. Like a <code>for</code> loop, an <code>if</code> or <code>switch</code> can include an
|
|
initialization statement. The <code>switch</code> statement in <code>cat</code> uses one to create variables
|
|
<code>nr</code> and <code>er</code> to hold the return values from the call to <code>f.Read</code>. (The <code>if</code> a few lines later
|
|
has the same idea.) The <code>switch</code> statement is general: it evaluates the cases
|
|
from top to bottom looking for the first case that matches the value; the
|
|
case expressions don't need to be constants or even integers, as long as
|
|
they all have the same type.
|
|
<p>
|
|
Since the <code>switch</code> value is just <code>true</code>, we could leave it off—as is also
|
|
the situation
|
|
in a <code>for</code> statement, a missing value means <code>true</code>. In fact, such a <code>switch</code>
|
|
is a form of <code>if-else</code> chain. While we're here, it should be mentioned that in
|
|
<code>switch</code> statements each <code>case</code> has an implicit <code>break</code>.
|
|
<p>
|
|
The argument to <code>file.Stdout.Write</code> is created by slicing the array <code>buf</code>.
|
|
Slices provide the standard Go way to handle I/O buffers.
|
|
<p>
|
|
Now let's make a variant of <code>cat</code> that optionally does <code>rot13</code> on its input.
|
|
It's easy to do by just processing the bytes, but instead we will exploit
|
|
Go's notion of an <i>interface</i>.
|
|
<p>
|
|
The <code>cat</code> subroutine uses only two methods of <code>f</code>: <code>Read</code> and <code>String</code>,
|
|
so let's start by defining an interface that has exactly those two methods.
|
|
Here is code from <code>progs/cat_rot13.go</code>:
|
|
<p>
|
|
{{code "progs/cat_rot13.go" `/type.reader/` `/^}/`}}
|
|
<p>
|
|
Any type that has the two methods of <code>reader</code>—regardless of whatever
|
|
other methods the type may also have—is said to <i>implement</i> the
|
|
interface. Since <code>file.File</code> implements these methods, it implements the
|
|
<code>reader</code> interface. We could tweak the <code>cat</code> subroutine to accept a <code>reader</code>
|
|
instead of a <code>*file.File</code> and it would work just fine, but let's embellish a little
|
|
first by writing a second type that implements <code>reader</code>, one that wraps an
|
|
existing <code>reader</code> and does <code>rot13</code> on the data. To do this, we just define
|
|
the type and implement the methods and with no other bookkeeping,
|
|
we have a second implementation of the <code>reader</code> interface.
|
|
<p>
|
|
{{code "progs/cat_rot13.go" `/type.rotate13/` `/end.of.rotate13/`}}
|
|
<p>
|
|
(The <code>rot13</code> function called in <code>Read</code> is trivial and not worth reproducing here.)
|
|
<p>
|
|
To use the new feature, we define a flag:
|
|
<p>
|
|
{{code "progs/cat_rot13.go" `/rot13Flag/`}}
|
|
<p>
|
|
and use it from within a mostly unchanged <code>cat</code> function:
|
|
<p>
|
|
{{code "progs/cat_rot13.go" `/func.cat/` `/^}/`}}
|
|
<p>
|
|
(We could also do the wrapping in <code>main</code> and leave <code>cat</code> mostly alone, except
|
|
for changing the type of the argument; consider that an exercise.)
|
|
The <code>if</code> at the top of <code>cat</code> sets it all up: If the <code>rot13</code> flag is true, wrap the <code>reader</code>
|
|
we received into a <code>rotate13</code> and proceed. Note that the interface variables
|
|
are values, not pointers: the argument is of type <code>reader</code>, not <code>*reader</code>,
|
|
even though under the covers it holds a pointer to a <code>struct</code>.
|
|
<p>
|
|
Here it is in action:
|
|
<p>
|
|
<pre>
|
|
$ echo abcdefghijklmnopqrstuvwxyz | ./cat
|
|
abcdefghijklmnopqrstuvwxyz
|
|
$ echo abcdefghijklmnopqrstuvwxyz | ./cat --rot13
|
|
nopqrstuvwxyzabcdefghijklm
|
|
$
|
|
</pre>
|
|
<p>
|
|
Fans of dependency injection may take cheer from how easily interfaces
|
|
allow us to substitute the implementation of a file descriptor.
|
|
<p>
|
|
Interfaces are a distinctive feature of Go. An interface is implemented by a
|
|
type if the type implements all the methods declared in the interface.
|
|
This means
|
|
that a type may implement an arbitrary number of different interfaces.
|
|
There is no type hierarchy; things can be much more <i>ad hoc</i>,
|
|
as we saw with <code>rot13</code>. The type <code>file.File</code> implements <code>reader</code>; it could also
|
|
implement a <code>writer</code>, or any other interface built from its methods that
|
|
fits the current situation. Consider the <i>empty interface</i>
|
|
<p>
|
|
<pre>
|
|
type Empty interface {}
|
|
</pre>
|
|
<p>
|
|
<i>Every</i> type implements the empty interface, which makes it
|
|
useful for things like containers.
|
|
<p>
|
|
<h2>Sorting</h2>
|
|
<p>
|
|
Interfaces provide a simple form of polymorphism. They completely
|
|
separate the definition of what an object does from how it does it, allowing
|
|
distinct implementations to be represented at different times by the
|
|
same interface variable.
|
|
<p>
|
|
As an example, consider this simple sort algorithm taken from <code>progs/sort.go</code>:
|
|
<p>
|
|
{{code "progs/sort.go" `/func.Sort/` `/^}/`}}
|
|
<p>
|
|
The code needs only three methods, which we wrap into sort's <code>Interface</code>:
|
|
<p>
|
|
{{code "progs/sort.go" `/interface/` `/^}/`}}
|
|
<p>
|
|
We can apply <code>Sort</code> to any type that implements <code>Len</code>, <code>Less</code>, and <code>Swap</code>.
|
|
The <code>sort</code> package includes the necessary methods to allow sorting of
|
|
arrays of integers, strings, etc.; here's the code for arrays of <code>int</code>
|
|
<p>
|
|
{{code "progs/sort.go" `/type.*IntSlice/` `/Swap/`}}
|
|
<p>
|
|
Here we see methods defined for non-<code>struct</code> types. You can define methods
|
|
for any type you define and name in your package.
|
|
<p>
|
|
And now a routine to test it out, from <code>progs/sortmain.go</code>. This
|
|
uses a function in the <code>sort</code> package, omitted here for brevity,
|
|
to test that the result is sorted.
|
|
<p>
|
|
{{code "progs/sortmain.go" `/func.ints/` `/^}/`}}
|
|
<p>
|
|
If we have a new type we want to be able to sort, all we need to do is
|
|
to implement the three methods for that type, like this:
|
|
<p>
|
|
{{code "progs/sortmain.go" `/type.day/` `/Swap/`}}
|
|
<p>
|
|
<p>
|
|
<h2>Printing</h2>
|
|
<p>
|
|
The examples of formatted printing so far have been modest. In this section
|
|
we'll talk about how formatted I/O can be done well in Go.
|
|
<p>
|
|
We've seen simple uses of the package <code>fmt</code>, which
|
|
implements <code>Printf</code>, <code>Fprintf</code>, and so on.
|
|
Within the <code>fmt</code> package, <code>Printf</code> is declared with this signature:
|
|
<p>
|
|
<pre>
|
|
Printf(format string, v ...interface{}) (n int, errno error)
|
|
</pre>
|
|
<p>
|
|
The token <code>...</code> introduces a variable-length argument list that in C would
|
|
be handled using the <code>stdarg.h</code> macros.
|
|
In Go, variadic functions are passed a slice of the arguments of the
|
|
specified type. In <code>Printf</code>'s case, the declaration says <code>...interface{}</code>
|
|
so the actual type is a slice of empty interface values, <code>[]interface{}</code>.
|
|
<code>Printf</code> can examine the arguments by iterating over the slice
|
|
and, for each element, using a type switch or the reflection library
|
|
to interpret the value.
|
|
It's off topic here but such run-time type analysis
|
|
helps explain some of the nice properties of Go's <code>Printf</code>,
|
|
due to the ability of <code>Printf</code> to discover the type of its arguments
|
|
dynamically.
|
|
<p>
|
|
For example, in C each format must correspond to the type of its
|
|
argument. It's easier in many cases in Go. Instead of <code>%llud</code> you
|
|
can just say <code>%d</code>; <code>Printf</code> knows the size and signedness of the
|
|
integer and can do the right thing for you. The snippet
|
|
<p>
|
|
{{code "progs/print.go" 10 11}}
|
|
<p>
|
|
prints
|
|
<p>
|
|
<pre>
|
|
18446744073709551615 -1
|
|
</pre>
|
|
<p>
|
|
In fact, if you're lazy the format <code>%v</code> will print, in a simple
|
|
appropriate style, any value, even an array or structure. The output of
|
|
<p>
|
|
{{code "progs/print.go" 14 20}}
|
|
<p>
|
|
is
|
|
<p>
|
|
<pre>
|
|
18446744073709551615 {77 Sunset Strip} [1 2 3 4]
|
|
</pre>
|
|
<p>
|
|
You can drop the formatting altogether if you use <code>Print</code> or <code>Println</code>
|
|
instead of <code>Printf</code>. Those routines do fully automatic formatting.
|
|
The <code>Print</code> function just prints its elements out using the equivalent
|
|
of <code>%v</code> while <code>Println</code> inserts spaces between arguments
|
|
and adds a newline. The output of each of these two lines is identical
|
|
to that of the <code>Printf</code> call above.
|
|
<p>
|
|
{{code "progs/print.go" 21 22}}
|
|
<p>
|
|
If you have your own type you'd like <code>Printf</code> or <code>Print</code> to format,
|
|
just give it a <code>String</code> method that returns a string. The print
|
|
routines will examine the value to inquire whether it implements
|
|
the method and if so, use it rather than some other formatting.
|
|
Here's a simple example.
|
|
<p>
|
|
{{code "progs/print_string.go" 9 "$"}}
|
|
<p>
|
|
Since <code>*testType</code> has a <code>String</code> method, the
|
|
default formatter for that type will use it and produce the output
|
|
<p>
|
|
<pre>
|
|
77 Sunset Strip
|
|
</pre>
|
|
<p>
|
|
Observe that the <code>String</code> method calls <code>Sprint</code> (the obvious Go
|
|
variant that returns a string) to do its formatting; special formatters
|
|
can use the <code>fmt</code> library recursively.
|
|
<p>
|
|
Another feature of <code>Printf</code> is that the format <code>%T</code> will print a string
|
|
representation of the type of a value, which can be handy when debugging
|
|
polymorphic code.
|
|
<p>
|
|
It's possible to write full custom print formats with flags and precisions
|
|
and such, but that's getting a little off the main thread so we'll leave it
|
|
as an exploration exercise.
|
|
<p>
|
|
You might ask, though, how <code>Printf</code> can tell whether a type implements
|
|
the <code>String</code> method. Actually what it does is ask if the value can
|
|
be converted to an interface variable that implements the method.
|
|
Schematically, given a value <code>v</code>, it does this:
|
|
<p>
|
|
<p>
|
|
<pre>
|
|
type Stringer interface {
|
|
String() string
|
|
}
|
|
</pre>
|
|
<p>
|
|
<pre>
|
|
s, ok := v.(Stringer) // Test whether v implements "String()"
|
|
if ok {
|
|
result = s.String()
|
|
} else {
|
|
result = defaultOutput(v)
|
|
}
|
|
</pre>
|
|
<p>
|
|
The code uses a ``type assertion'' (<code>v.(Stringer)</code>) to test if the value stored in
|
|
<code>v</code> satisfies the <code>Stringer</code> interface; if it does, <code>s</code>
|
|
will become an interface variable implementing the method and <code>ok</code> will
|
|
be <code>true</code>. We then use the interface variable to call the method.
|
|
(The ''comma, ok'' pattern is a Go idiom used to test the success of
|
|
operations such as type conversion, map update, communications, and so on,
|
|
although this is the only appearance in this tutorial.)
|
|
If the value does not satisfy the interface, <code>ok</code> will be false.
|
|
<p>
|
|
In this snippet the name <code>Stringer</code> follows the convention that we add ''[e]r''
|
|
to interfaces describing simple method sets like this.
|
|
<p>
|
|
A related interface is that defined by the <code>error</code> builtin type, which is just
|
|
<p>
|
|
<pre>
|
|
type error interface {
|
|
Error() string
|
|
}
|
|
</pre>
|
|
<p>
|
|
Other than the method name (<code>Error</code> vs. <code>String</code>), this looks like
|
|
a <code>Stringer</code>; the different name guarantees that types that implement <code>Stringer</code>
|
|
don't accidentally satisfy the <code>error</code> interface.
|
|
Naturally, <code>Printf</code> and its relatives recognize the <code>error</code> interface,
|
|
just as they do <code>Stringer</code>,
|
|
so it's trivial to print an error as a string.
|
|
<p>
|
|
One last wrinkle. To complete the suite, besides <code>Printf</code> etc. and <code>Sprintf</code>
|
|
etc., there are also <code>Fprintf</code> etc. Unlike in C, <code>Fprintf</code>'s first argument is
|
|
not a file. Instead, it is a variable of type <code>io.Writer</code>, which is an
|
|
interface type defined in the <code>io</code> library:
|
|
<p>
|
|
<pre>
|
|
type Writer interface {
|
|
Write(p []byte) (n int, err error)
|
|
}
|
|
</pre>
|
|
<p>
|
|
(This interface is another conventional name, this time for <code>Write</code>; there are also
|
|
<code>io.Reader</code>, <code>io.ReadWriter</code>, and so on.)
|
|
Thus you can call <code>Fprintf</code> on any type that implements a standard <code>Write</code>
|
|
method, not just files but also network channels, buffers, whatever
|
|
you want.
|
|
<p>
|
|
<h2>Prime numbers</h2>
|
|
<p>
|
|
Now we come to processes and communication—concurrent programming.
|
|
It's a big subject so to be brief we assume some familiarity with the topic.
|
|
<p>
|
|
A classic program in the style is a prime sieve.
|
|
(The sieve of Eratosthenes is computationally more efficient than
|
|
the algorithm presented here, but we are more interested in concurrency than
|
|
algorithmics at the moment.)
|
|
It works by taking a stream of all the natural numbers and introducing
|
|
a sequence of filters, one for each prime, to winnow the multiples of
|
|
that prime. At each step we have a sequence of filters of the primes
|
|
so far, and the next number to pop out is the next prime, which triggers
|
|
the creation of the next filter in the chain.
|
|
<p>
|
|
Here's a flow diagram; each box represents a filter element whose
|
|
creation is triggered by the first number that flowed from the
|
|
elements before it.
|
|
<p>
|
|
<br>
|
|
<p>
|
|
<img src='sieve.gif'>
|
|
<p>
|
|
<br>
|
|
<p>
|
|
To create a stream of integers, we use a Go <i>channel</i>, which,
|
|
borrowing from CSP's descendants, represents a communications
|
|
channel that can connect two concurrent computations.
|
|
In Go, channel variables are references to a run-time object that
|
|
coordinates the communication; as with maps and slices, use
|
|
<code>make</code> to create a new channel.
|
|
<p>
|
|
Here is the first function in <code>progs/sieve.go</code>:
|
|
<p>
|
|
{{code "progs/sieve.go" `/Send/` `/^}/`}}
|
|
<p>
|
|
The <code>generate</code> function sends the sequence 2, 3, 4, 5, ... to its
|
|
argument channel, <code>ch</code>, using the binary communications operator <code><-</code>.
|
|
Channel operations block, so if there's no recipient for the value on <code>ch</code>,
|
|
the send operation will wait until one becomes available.
|
|
<p>
|
|
The <code>filter</code> function has three arguments: an input channel, an output
|
|
channel, and a prime number. It copies values from the input to the
|
|
output, discarding anything divisible by the prime. The unary communications
|
|
operator <code><-</code> (receive) retrieves the next value on the channel.
|
|
<p>
|
|
{{code "progs/sieve.go" `/Copy.the/` `/^}/`}}
|
|
<p>
|
|
The generator and filters execute concurrently. Go has
|
|
its own model of process/threads/light-weight processes/coroutines,
|
|
so to avoid notational confusion we call concurrently executing
|
|
computations in Go <i>goroutines</i>. To start a goroutine,
|
|
invoke the function, prefixing the call with the keyword <code>go</code>;
|
|
this starts the function running in parallel with the current
|
|
computation but in the same address space:
|
|
<p>
|
|
<pre>
|
|
go sum(hugeArray) // calculate sum in the background
|
|
</pre>
|
|
<p>
|
|
If you want to know when the calculation is done, pass a channel
|
|
on which it can report back:
|
|
<p>
|
|
<pre>
|
|
ch := make(chan int)
|
|
go sum(hugeArray, ch)
|
|
// ... do something else for a while
|
|
result := <-ch // wait for, and retrieve, result
|
|
</pre>
|
|
<p>
|
|
Back to our prime sieve. Here's how the sieve pipeline is stitched
|
|
together:
|
|
<p>
|
|
{{code "progs/sieve.go" `/func.main/` `/^}/`}}
|
|
<p>
|
|
The first line of <code>main</code> creates the initial channel to pass to <code>generate</code>, which it
|
|
then starts up. As each prime pops out of the channel, a new <code>filter</code>
|
|
is added to the pipeline and <i>its</i> output becomes the new value
|
|
of <code>ch</code>.
|
|
<p>
|
|
The sieve program can be tweaked to use a pattern common
|
|
in this style of programming. Here is a variant version
|
|
of <code>generate</code>, from <code>progs/sieve1.go</code>:
|
|
<p>
|
|
{{code "progs/sieve1.go" `/func.generate/` `/^}/`}}
|
|
<p>
|
|
This version does all the setup internally. It creates the output
|
|
channel, launches a goroutine running a function literal, and
|
|
returns the channel to the caller. It is a factory for concurrent
|
|
execution, starting the goroutine and returning its connection.
|
|
<p>
|
|
The function literal notation used in the <code>go</code> statement allows us to construct an
|
|
anonymous function and invoke it on the spot. Notice that the local
|
|
variable <code>ch</code> is available to the function literal and lives on even
|
|
after <code>generate</code> returns.
|
|
<p>
|
|
The same change can be made to <code>filter</code>:
|
|
<p>
|
|
{{code "progs/sieve1.go" `/func.filter/` `/^}/`}}
|
|
<p>
|
|
The <code>sieve</code> function's main loop becomes simpler and clearer as a
|
|
result, and while we're at it let's turn it into a factory too:
|
|
<p>
|
|
{{code "progs/sieve1.go" `/func.sieve/` `/^}/`}}
|
|
<p>
|
|
Now <code>main</code>'s interface to the prime sieve is a channel of primes:
|
|
<p>
|
|
{{code "progs/sieve1.go" `/func.main/` `/^}/`}}
|
|
<p>
|
|
<h2>Multiplexing</h2>
|
|
<p>
|
|
With channels, it's possible to serve multiple independent client goroutines without
|
|
writing an explicit multiplexer. The trick is to send the server a channel in the message,
|
|
which it will then use to reply to the original sender.
|
|
A realistic client-server program is a lot of code, so here is a very simple substitute
|
|
to illustrate the idea. It starts by defining a <code>request</code> type, which embeds a channel
|
|
that will be used for the reply.
|
|
<p>
|
|
{{code "progs/server.go" `/type.request/` `/^}/`}}
|
|
<p>
|
|
The server will be trivial: it will do simple binary operations on integers. Here's the
|
|
code that invokes the operation and responds to the request:
|
|
<p>
|
|
{{code "progs/server.go" `/type.binOp/` `/^}/`}}
|
|
<p>
|
|
The type declaration makes <code>binOp</code> represent a function taking two integers and
|
|
returning a third.
|
|
<p>
|
|
The <code>server</code> routine loops forever, receiving requests and, to avoid blocking due to
|
|
a long-running operation, starting a goroutine to do the actual work.
|
|
<p>
|
|
{{code "progs/server.go" `/func.server/` `/^}/`}}
|
|
<p>
|
|
There's a new feature in the signature of <code>server</code>: the type of the
|
|
<code>service</code> channel specifies the direction of communication.
|
|
A channel of plain <code>chan</code> type can be used both for sending and receiving.
|
|
However, the type used when declaring a channel can be decorated with an arrow to
|
|
indicate that the channel can be used only to send (<code>chan<-</code>) or to
|
|
receive (<code><-chan</code>) data.
|
|
The arrow points towards or away from the <code>chan</code> to indicate whether data flows into or out of
|
|
the channel.
|
|
In the <code>server</code> function, <code>service <-chan *request</code> is a "receive only" channel
|
|
that the function can use only to <em>read</em> new requests.
|
|
<p>
|
|
We instantiate a server in a familiar way, starting it and returning a channel
|
|
connected to it:
|
|
<p>
|
|
{{code "progs/server.go" `/func.startServer/` `/^}/`}}
|
|
<p>
|
|
The returned channel is send only, even though the channel was created bidirectionally.
|
|
The read end is passed to <code>server</code>, while the send end is returned
|
|
to the caller of <code>startServer</code>, so the two halves of the channel
|
|
are distinguished, just as we did in <code>startServer</code>.
|
|
<p>
|
|
Bidirectional channels can be assigned to unidirectional channels but not the
|
|
other way around, so if you annotate your channel directions when you declare
|
|
them, such as in function signatures, the type system can help you set up and
|
|
use channels correctly.
|
|
Note that it's pointless to <code>make</code> unidirectional channels, since you can't
|
|
use them to communicate. Their purpose is served by variables assigned from bidirectional channels
|
|
to distinguish the input and output halves.
|
|
<p>
|
|
Here's a simple test. It starts a server with an addition operator and sends out
|
|
<code>N</code> requests without waiting for the replies. Only after all the requests are sent
|
|
does it check the results.
|
|
<p>
|
|
{{code "progs/server.go" `/func.main/` `/^}/`}}
|
|
<p>
|
|
One annoyance with this program is that it doesn't shut down the server cleanly; when <code>main</code> returns
|
|
there are a number of lingering goroutines blocked on communication. To solve this,
|
|
we can provide a second, <code>quit</code> channel to the server:
|
|
<p>
|
|
{{code "progs/server1.go" `/func.startServer/` `/^}/`}}
|
|
<p>
|
|
It passes the quit channel to the <code>server</code> function, which uses it like this:
|
|
<p>
|
|
{{code "progs/server1.go" `/func.server/` `/^}/`}}
|
|
<p>
|
|
Inside <code>server</code>, the <code>select</code> statement chooses which of the multiple communications
|
|
listed by its cases can proceed. If all are blocked, it waits until one can proceed; if
|
|
multiple can proceed, it chooses one at random. In this instance, the <code>select</code> allows
|
|
the server to honor requests until it receives a quit message, at which point it
|
|
returns, terminating its execution.
|
|
<p>
|
|
<p>
|
|
All that's left is to strobe the <code>quit</code> channel
|
|
at the end of main:
|
|
<p>
|
|
{{code "progs/server1.go" `/adder,.quit/`}}
|
|
...
|
|
{{code "progs/server1.go" `/quit....true/`}}
|
|
<p>
|
|
There's a lot more to Go programming and concurrent programming in general but this
|
|
quick tour should give you some of the basics.
|