mirror of
https://github.com/golang/go
synced 2024-11-12 00:30:22 -07:00
Russ Cox
parent
e434f1a763
commit
898714a92f
@ -38,9 +38,8 @@ our old, now capitalized and package-qualified friend, <code>fmt.Printf</code>.
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<p>
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Function declarations are introduced with the <code>func</code> keyword.
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<p>
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Notice that string constants can contain Unicode characters, encoded in UTF-8.
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Go is defined to accept UTF-8 input. Strings are arrays of bytes, usually used
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to store Unicode strings represented in UTF-8.
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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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@ -92,7 +91,7 @@ Next up, here's a version of the Unix utility <code>echo(1)</code>:
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09 "flag"; // command line option parser
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10 )
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<p>
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12 var n_flag = flag.Bool("n", false, "don't print final newline")
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12 var omitNewline = flag.Bool("n", false, "don't print final newline")
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<p>
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14 const (
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15 Space = " ";
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@ -108,7 +107,7 @@ Next up, here's a version of the Unix utility <code>echo(1)</code>:
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25 }
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26 s += flag.Arg(i)
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27 }
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28 if !*n_flag {
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28 if !*omitNewline {
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29 s += Newline
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30 }
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31 os.Stdout.WriteString(s);
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@ -149,7 +148,7 @@ a naming conflict.
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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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Having imported the <code>flag</code> package, line 12 creates a global variable to hold
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the value of echo's <code>-n</code> flag. The variable <code>n_flag</code> has type <code>*bool</code>, pointer
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the value of echo's <code>-n</code> flag. The variable <code>omitNewline</code> has type <code>*bool</code>, pointer
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to <code>bool</code>.
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<p>
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In <code>main.main</code>, we parse the arguments (line 20) and then create a local
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@ -179,10 +178,6 @@ or we could go even shorter and write the idiom
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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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(For those who know Limbo, its <code>:=</code> construct is the same, but notice
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that Go has no colon after the name in a full <code>var</code> declaration.
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Also, for simplicity of parsing, <code>:=</code> only works inside functions, not at
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the top level.)
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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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<pre> <!-- progs/echo.go /for/ -->
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@ -211,7 +206,7 @@ It's defined that way. Falling off the end of <code>main.main</code> means
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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 an array used by the
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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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@ -225,7 +220,7 @@ 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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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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<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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@ -255,11 +250,11 @@ read on.
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Arrays are declared like this:
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<p>
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<pre>
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var array_of_int [10]int;
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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>array_of_int</code> would be usable as a pointer to <code>int</code>.
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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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@ -275,7 +270,7 @@ an underlying, often 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 actually much more common in Go programs than
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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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@ -284,7 +279,7 @@ array.
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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, take the address of the array and Go will automatically
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the function, take the address of the array and Go will
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create (efficiently) a slice reference and pass that.
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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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@ -307,10 +302,10 @@ and invoke it like this:
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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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The expression <code>[3]int{1,2,3}</code> -- a type followed by a brace-bounded expression
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-- is a constructor for a value, in this case an array of 3 <code>ints</code>. Putting an <code>&</code>
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The expression <code>[3]int{1,2,3}</code> - a type followed by a brace-bounded expression
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- is a constructor for a value, in this case an array of 3 <code>ints</code>. Putting an <code>&</code>
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in front gives us the address of a unique instance of the value. We pass the
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pointer to <code>sum()</code> by (automatically) promoting it to a slice.
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pointer to <code>sum()</code> by (implicitly) promoting it to a slice.
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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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@ -334,15 +329,15 @@ There are also maps, which you can initialize like this:
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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, and maps.
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slices, maps, and channels.
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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. To allocate something on the stack,
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just declare a variable. To allocate it on the heap, use <code>new()</code>, which
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copies the contents of the object.
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To allocate a new variable, use <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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@ -373,7 +368,7 @@ If you just declare the map, as in
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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 to an
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you must first initialize the reference using <code>make()</code> or by assignment of 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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@ -390,7 +385,7 @@ can overflow only when they are assigned to an integer variable with
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too little precision to represent the value.
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<p>
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<pre>
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const hard_eight = (1 << 100) >> 97 // legal
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const hardEight = (1 << 100) >> 97 // legal
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</pre>
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<p>
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There are nuances that deserve redirection to the legalese of the
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@ -431,10 +426,10 @@ sort of open/close/read/write interface. Here's the start of <code>file.go</cod
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15 }
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</pre>
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<p>
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The first line declares the name of the package -- <code>file</code> --
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and then we import two packages. The <code>os</code> package hides the differences
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The first few lines declare the name of the package - <code>file</code> -
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and then import two packages. The <code>os</code> package hides the differences
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between various operating systems to give a consistent view of files and
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so on; here we're only going to use its error handling utilities
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so on; here we're going to use its error handling utilities
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and reproduce the rudiments of its file I/O.
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<p>
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The other item is the low-level, external <code>syscall</code> package, which provides
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@ -456,7 +451,7 @@ In Go, the term for publicly visible names is ''exported''.
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In the case of <code>File</code>, all its fields are lower case and so invisible to users, but we
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will soon give it some exported, upper-case methods.
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<p>
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First, though, here is a factory to create them:
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First, though, here is a factory to create a <code>File</code>:
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<p>
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<pre> <!-- progs/file.go /newFile/ /^}/ -->
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17 func newFile(fd int, name string) *File {
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@ -513,17 +508,16 @@ they look just like a second parameter list. The function
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<code>syscall.Open</code>
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also has a multi-value return, which we can grab with the multi-variable
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declaration on line 31; it declares <code>r</code> and <code>e</code> to hold the two values,
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both of type <code>int64</code> (although you'd have to look at the <code>syscall</code> package
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both of type <code>int</code> (although you'd have to look at the <code>syscall</code> package
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to see that). Finally, line 35 returns two values: a pointer to the new <code>File</code>
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and the error. If <code>syscall.Open</code> fails, the file descriptor <code>r</code> will
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be negative and <code>NewFile</code> will return <code>nil</code>.
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<p>
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About those errors: The <code>os</code> library includes a general notion of an error
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string, maintaining a unique set of errors throughout the program. It's a
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good idea to use its facility in your own interfaces, as we do here, for
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About those errors: The <code>os</code> library includes a general notion of an error.
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It's a good idea to use its facility in your own interfaces, as we do here, for
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consistent error handling throughout Go code. In <code>Open</code> we use a
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conversion to <code>os.Errno</code> to translate Unix's integer <code>errno</code> value into
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an error value, which will be stored in a unique instance of type <code>os.Error</code>.
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conversion to translate Unix's integer <code>errno</code> value into the integer type
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<code>os.Errno</code>, which implements <code>os.Error</code>.
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<p>
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Now that we can build <code>Files</code>, we can write methods for them. To declare
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a method of a type, we define a function to have an explicit receiver
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@ -574,7 +568,7 @@ each of which declares a receiver variable <code>file</code>.
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There is no implicit <code>this</code> and the receiver variable must be used to access
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members of the structure. Methods are not declared within
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the <code>struct</code> declaration itself. The <code>struct</code> declaration defines only data members.
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In fact, methods can be created for any type you name, such as an integer or
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In fact, methods can be created for almost any type you name, such as an integer or
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array, not just for <code>structs</code>. We'll see an example with arrays later.
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<p>
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The <code>String</code> method is so called because of a printing convention we'll
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@ -606,7 +600,7 @@ We can now use our new package:
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21 }
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</pre>
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<p>
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The import of ''<code>./file</code>'' tells the compiler to use our own package rather than
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The ''<code>./</code>'' in the import of ''<code>./file</code>'' tells the compiler to use our own package rather than
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something from the directory of installed packages.
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<p>
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Finally we can run the program:
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@ -677,7 +671,7 @@ from top to bottom looking for the first case that matches the value; the
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case expressions don't need to be constants or even integers, as long as
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they all have the same type.
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<p>
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Since the <code>switch</code> value is just <code>true</code>, we could leave it off -- as is also
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Since the <code>switch</code> value is just <code>true</code>, we could leave it off - as is also
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the situation
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in a <code>for</code> statement, a missing value means <code>true</code>. In fact, such a <code>switch</code>
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is a form of <code>if-else</code> chain. While we're here, it should be mentioned that in
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@ -701,8 +695,8 @@ Here is code from <code>progs/cat_rot13.go</code>:
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29 }
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</pre>
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<p>
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Any type that implements the two methods of <code>reader</code> -- regardless of whatever
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other methods the type may also contain -- is said to <i>implement</i> the
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Any type that has the two methods of <code>reader</code> - regardless of whatever
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other methods the type may also have - is said to <i>implement</i> the
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interface. Since <code>file.File</code> implements these methods, it implements the
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<code>reader</code> interface. We could tweak the <code>cat</code> subroutine to accept a <code>reader</code>
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instead of a <code>*file.File</code> and it would work just fine, but let's embellish a little
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@ -738,8 +732,8 @@ we have a second implementation of the <code>reader</code> interface.
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<p>
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To use the new feature, we define a flag:
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<p>
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<pre> <!-- progs/cat_rot13.go /rot13_flag/ -->
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14 var rot13_flag = flag.Bool("rot13", false, "rot13 the input")
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<pre> <!-- progs/cat_rot13.go /rot13Flag/ -->
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14 var rot13Flag = flag.Bool("rot13", false, "rot13 the input")
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</pre>
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<p>
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and use it from within a mostly unchanged <code>cat()</code> function:
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@ -749,7 +743,7 @@ and use it from within a mostly unchanged <code>cat()</code> function:
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53 const NBUF = 512;
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54 var buf [NBUF]byte;
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<p>
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56 if *rot13_flag {
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56 if *rot13Flag {
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57 r = newRotate13(r)
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58 }
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59 for {
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@ -789,7 +783,7 @@ Here it is in action:
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Fans of dependency injection may take cheer from how easily interfaces
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allow us to substitute the implementation of a file descriptor.
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<p>
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Interfaces are a distinct feature of Go. An interface is implemented by a
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Interfaces are a distinctive feature of Go. An interface is implemented by a
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type if the type implements all the methods declared in the interface.
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This means
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that a type may implement an arbitrary number of different interfaces.
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@ -807,7 +801,7 @@ useful for things like containers.
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<p>
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<h2>Sorting</h2>
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<p>
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Interfaces provide a simple form of polymorphism since they completely
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Interfaces provide a simple form of polymorphism. They completely
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separate the definition of what an object does from how it does it, allowing
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distinct implementations to be represented at different times by the
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same interface variable.
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@ -898,8 +892,8 @@ Within the <code>fmt</code> package, <code>Printf</code> is declared with this s
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</pre>
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<p>
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That <code>...</code> represents the variadic argument list that in C would
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be handled using the <code>stdarg.h</code> macros, but in Go is passed using
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an empty interface variable (<code>interface {}</code>) that is then unpacked
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be handled using the <code>stdarg.h</code> macros but in Go is passed using
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an empty interface variable (<code>interface {}</code>) and then unpacked
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using the reflection library. It's off topic here but the use of
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reflection helps explain some of the nice properties of Go's <code>Printf</code>,
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due to the ability of <code>Printf</code> to discover the type of its arguments
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@ -940,7 +934,7 @@ is
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You can drop the formatting altogether if you use <code>Print</code> or <code>Println</code>
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instead of <code>Printf</code>. Those routines do fully automatic formatting.
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The <code>Print</code> function just prints its elements out using the equivalent
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of <code>%v</code> while <code>Println</code> automatically inserts spaces between arguments
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of <code>%v</code> while <code>Println</code> inserts spaces between arguments
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and adds a newline. The output of each of these two lines is identical
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to that of the <code>Printf</code> call above.
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<p>
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@ -968,7 +962,7 @@ Here's a simple example.
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18 }
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</pre>
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<p>
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Since <code>*T</code> has a <code>String()</code> method, the
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Since <code>*testType</code> has a <code>String()</code> method, the
|
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default formatter for that type will use it and produce the output
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<p>
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<pre>
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@ -1039,7 +1033,7 @@ you want.
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<p>
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<h2>Prime numbers</h2>
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<p>
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Now we come to processes and communication -- concurrent programming.
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Now we come to processes and communication - concurrent programming.
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It's a big subject so to be brief we assume some familiarity with the topic.
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<p>
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A classic program in the style is a prime sieve.
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@ -1112,7 +1106,7 @@ this starts the function running in parallel with the current
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computation but in the same address space:
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<p>
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<pre>
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go sum(huge_array); // calculate sum in the background
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go sum(hugeArray); // calculate sum in the background
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</pre>
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<p>
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If you want to know when the calculation is done, pass a channel
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@ -1120,7 +1114,7 @@ on which it can report back:
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<p>
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<pre>
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ch := make(chan int);
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go sum(huge_array, ch);
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go sum(hugeArray, ch);
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// ... do something else for a while
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result := <-ch; // wait for, and retrieve, result
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</pre>
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@ -1164,7 +1158,7 @@ of <code>generate</code>, from <code>progs/sieve1.go</code>:
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</pre>
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<p>
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This version does all the setup internally. It creates the output
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channel, launches a goroutine internally using a function literal, and
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channel, launches a goroutine running a function literal, and
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returns the channel to the caller. It is a factory for concurrent
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execution, starting the goroutine and returning its connection.
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<p>
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@ -1221,7 +1215,7 @@ Now <code>main</code>'s interface to the prime sieve is a channel of primes:
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<h2>Multiplexing</h2>
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<p>
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With channels, it's possible to serve multiple independent client goroutines without
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writing an actual multiplexer. The trick is to send the server a channel in the message,
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writing an explicit multiplexer. The trick is to send the server a channel in the message,
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which it will then use to reply to the original sender.
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A realistic client-server program is a lot of code, so here is a very simple substitute
|
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to illustrate the idea. It starts by defining a <code>request</code> type, which embeds a channel
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@ -1261,8 +1255,8 @@ a long-running operation, starting a goroutine to do the actual work.
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26 }
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</pre>
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<p>
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We construct a server in a familiar way, starting it up and returning a channel to
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connect to it:
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We construct a server in a familiar way, starting it and returning a channel
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connected to it:
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<p>
|
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<pre> <!-- progs/server.go /func.startServer/ /^}/ -->
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28 func startServer(op binOp) chan *request {
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@ -1272,8 +1266,8 @@ connect to it:
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32 }
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</pre>
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<p>
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Here's a simple test. It starts a server with an addition operator, and sends out
|
||||
lots of requests but doesn't wait for the reply. Only after all the requests are sent
|
||||
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>
|
||||
<pre> <!-- progs/server.go /func.main/ /^}/ -->
|
||||
@ -1297,7 +1291,7 @@ does it check the results.
|
||||
51 }
|
||||
</pre>
|
||||
<p>
|
||||
One annoyance with this program is that it doesn't exit cleanly; when <code>main</code> returns
|
||||
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>
|
||||
@ -1325,7 +1319,7 @@ It passes the quit channel to the <code>server</code> function, which uses it li
|
||||
30 }
|
||||
</pre>
|
||||
<p>
|
||||
Inside <code>server</code>, a <code>select</code> statement chooses which of the multiple communications
|
||||
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
|
||||
|
||||
@ -32,9 +32,8 @@ our old, now capitalized and package-qualified friend, "fmt.Printf".
|
||||
|
||||
Function declarations are introduced with the "func" keyword.
|
||||
|
||||
Notice that string constants can contain Unicode characters, encoded in UTF-8.
|
||||
Go is defined to accept UTF-8 input. Strings are arrays of bytes, usually used
|
||||
to store Unicode strings represented in UTF-8.
|
||||
String constants can contain Unicode characters, encoded in UTF-8.
|
||||
(In fact, Go source files are defined to be encoded in UTF-8.)
|
||||
|
||||
The comment convention is the same as in C++:
|
||||
|
||||
@ -108,7 +107,7 @@ a naming conflict.
|
||||
Given "os.Stdout" we can use its "WriteString" method to print the string.
|
||||
|
||||
Having imported the "flag" package, line 12 creates a global variable to hold
|
||||
the value of echo's "-n" flag. The variable "n_flag" has type "*bool", pointer
|
||||
the value of echo's "-n" flag. The variable "omitNewline" has type "*bool", pointer
|
||||
to "bool".
|
||||
|
||||
In "main.main", we parse the arguments (line 20) and then create a local
|
||||
@ -132,10 +131,6 @@ or we could go even shorter and write the idiom
|
||||
s := "";
|
||||
|
||||
The ":=" operator is used a lot in Go to represent an initializing declaration.
|
||||
(For those who know Limbo, its ":=" construct is the same, but notice
|
||||
that Go has no colon after the name in a full "var" declaration.
|
||||
Also, for simplicity of parsing, ":=" only works inside functions, not at
|
||||
the top level.)
|
||||
There's one in the "for" clause on the next line:
|
||||
|
||||
--PROG progs/echo.go /for/
|
||||
@ -160,7 +155,7 @@ It's defined that way. Falling off the end of "main.main" means
|
||||
os.Exit(1)
|
||||
|
||||
The "os" package contains other essentials for getting
|
||||
started; for instance, "os.Args" is an array used by the
|
||||
started; for instance, "os.Args" is a slice used by the
|
||||
"flag" package to access the command-line arguments.
|
||||
|
||||
An Interlude about Types
|
||||
@ -175,7 +170,7 @@ they are not the same type. There is also a "byte" synonym for
|
||||
"uint8", which is the element type for strings.
|
||||
|
||||
Speaking of "string", that's a built-in type as well. Strings are
|
||||
<i>immutable values</i> -- they are not just arrays of "byte" values.
|
||||
<i>immutable values</i> - they are not just arrays of "byte" values.
|
||||
Once you've built a string <i>value</i>, you can't change it, although
|
||||
of course you can change a string <i>variable</i> simply by
|
||||
reassigning it. This snippet from "strings.go" is legal code:
|
||||
@ -196,10 +191,10 @@ read on.
|
||||
|
||||
Arrays are declared like this:
|
||||
|
||||
var array_of_int [10]int;
|
||||
var arrayOfInt [10]int;
|
||||
|
||||
Arrays, like strings, are values, but they are mutable. This differs
|
||||
from C, in which "array_of_int" would be usable as a pointer to "int".
|
||||
from C, in which "arrayOfInt" would be usable as a pointer to "int".
|
||||
In Go, since arrays are values, it's meaningful (and useful) to talk
|
||||
about pointers to arrays.
|
||||
|
||||
@ -215,7 +210,7 @@ an underlying, often anonymous, regular array. Multiple slices
|
||||
can share data if they represent pieces of the same array;
|
||||
multiple arrays can never share data.
|
||||
|
||||
Slices are actually much more common in Go programs than
|
||||
Slices are much more common in Go programs than
|
||||
regular arrays; they're more flexible, have reference semantics,
|
||||
and are efficient. What they lack is the precise control of storage
|
||||
layout of a regular array; if you want to have a hundred elements
|
||||
@ -224,7 +219,7 @@ array.
|
||||
|
||||
When passing an array to a function, you almost always want
|
||||
to declare the formal parameter to be a slice. When you call
|
||||
the function, take the address of the array and Go will automatically
|
||||
the function, take the address of the array and Go will
|
||||
create (efficiently) a slice reference and pass that.
|
||||
|
||||
Using slices one can write this function (from "sum.go"):
|
||||
@ -237,10 +232,10 @@ and invoke it like this:
|
||||
|
||||
Note how the return type ("int") is defined for "sum()" by stating it
|
||||
after the parameter list.
|
||||
The expression "[3]int{1,2,3}" -- a type followed by a brace-bounded expression
|
||||
-- is a constructor for a value, in this case an array of 3 "ints". Putting an "&"
|
||||
The expression "[3]int{1,2,3}" - a type followed by a brace-bounded expression
|
||||
- is a constructor for a value, in this case an array of 3 "ints". Putting an "&"
|
||||
in front gives us the address of a unique instance of the value. We pass the
|
||||
pointer to "sum()" by (automatically) promoting it to a slice.
|
||||
pointer to "sum()" by (implicitly) promoting it to a slice.
|
||||
|
||||
If you are creating a regular array but want the compiler to count the
|
||||
elements for you, use "..." as the array size:
|
||||
@ -258,7 +253,7 @@ There are also maps, which you can initialize like this:
|
||||
|
||||
The built-in function "len()", which returns number of elements,
|
||||
makes its first appearance in "sum". It works on strings, arrays,
|
||||
slices, and maps.
|
||||
slices, maps, and channels.
|
||||
|
||||
|
||||
An Interlude about Allocation
|
||||
@ -266,8 +261,8 @@ An Interlude about Allocation
|
||||
|
||||
Most types in Go are values. If you have an "int" or a "struct"
|
||||
or an array, assignment
|
||||
copies the contents of the object. To allocate something on the stack,
|
||||
just declare a variable. To allocate it on the heap, use "new()", which
|
||||
copies the contents of the object.
|
||||
To allocate a new variable, use "new()", which
|
||||
returns a pointer to the allocated storage.
|
||||
|
||||
type T struct { a, b int }
|
||||
@ -290,7 +285,7 @@ If you just declare the map, as in
|
||||
var m map[string]int;
|
||||
|
||||
it creates a "nil" reference that cannot hold anything. To use the map,
|
||||
you must first initialize the reference using "make()" or by assignment to an
|
||||
you must first initialize the reference using "make()" or by assignment of an
|
||||
existing map.
|
||||
|
||||
Note that "new(T)" returns type "*T" while "make(T)" returns type
|
||||
@ -307,7 +302,7 @@ 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.
|
||||
|
||||
const hard_eight = (1 << 100) >> 97 // legal
|
||||
const hardEight = (1 << 100) >> 97 // legal
|
||||
|
||||
There are nuances that deserve redirection to the legalese of the
|
||||
language specification but here are some illustrative examples:
|
||||
@ -334,10 +329,10 @@ sort of open/close/read/write interface. Here's the start of "file.go":
|
||||
|
||||
--PROG progs/file.go /package/ /^}/
|
||||
|
||||
The first line declares the name of the package -- "file" --
|
||||
and then we import two packages. The "os" package hides the differences
|
||||
The first few lines declare the name of the package - "file" -
|
||||
and then import two packages. The "os" package hides the differences
|
||||
between various operating systems to give a consistent view of files and
|
||||
so on; here we're only going to use its error handling utilities
|
||||
so on; here we're going to use its error handling utilities
|
||||
and reproduce the rudiments of its file I/O.
|
||||
|
||||
The other item is the low-level, external "syscall" package, which provides
|
||||
@ -359,7 +354,7 @@ In Go, the term for publicly visible names is ''exported''.
|
||||
In the case of "File", all its fields are lower case and so invisible to users, but we
|
||||
will soon give it some exported, upper-case methods.
|
||||
|
||||
First, though, here is a factory to create them:
|
||||
First, though, here is a factory to create a "File":
|
||||
|
||||
--PROG progs/file.go /newFile/ /^}/
|
||||
|
||||
@ -393,17 +388,16 @@ they look just like a second parameter list. The function
|
||||
"syscall.Open"
|
||||
also has a multi-value return, which we can grab with the multi-variable
|
||||
declaration on line 31; it declares "r" and "e" to hold the two values,
|
||||
both of type "int64" (although you'd have to look at the "syscall" package
|
||||
both of type "int" (although you'd have to look at the "syscall" package
|
||||
to see that). Finally, line 35 returns two values: a pointer to the new "File"
|
||||
and the error. If "syscall.Open" fails, the file descriptor "r" will
|
||||
be negative and "NewFile" will return "nil".
|
||||
|
||||
About those errors: The "os" library includes a general notion of an error
|
||||
string, maintaining a unique set of errors throughout the program. It's a
|
||||
good idea to use its facility in your own interfaces, as we do here, for
|
||||
About those errors: The "os" library includes a general notion of an error.
|
||||
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 "Open" we use a
|
||||
conversion to "os.Errno" to translate Unix's integer "errno" value into
|
||||
an error value, which will be stored in a unique instance of type "os.Error".
|
||||
conversion to translate Unix's integer "errno" value into the integer type
|
||||
"os.Errno", which implements "os.Error".
|
||||
|
||||
Now that we can build "Files", we can write methods for them. To declare
|
||||
a method of a type, we define a function to have an explicit receiver
|
||||
@ -416,7 +410,7 @@ each of which declares a receiver variable "file".
|
||||
There is no implicit "this" and the receiver variable must be used to access
|
||||
members of the structure. Methods are not declared within
|
||||
the "struct" declaration itself. The "struct" declaration defines only data members.
|
||||
In fact, methods can be created for any type you name, such as an integer or
|
||||
In fact, methods can be created for almost any type you name, such as an integer or
|
||||
array, not just for "structs". We'll see an example with arrays later.
|
||||
|
||||
The "String" method is so called because of a printing convention we'll
|
||||
@ -430,7 +424,7 @@ We can now use our new package:
|
||||
|
||||
--PROG progs/helloworld3.go /package/ END
|
||||
|
||||
The import of ''"./file"'' tells the compiler to use our own package rather than
|
||||
The ''"./"'' in the import of ''"./file"'' tells the compiler to use our own package rather than
|
||||
something from the directory of installed packages.
|
||||
|
||||
Finally we can run the program:
|
||||
@ -457,7 +451,7 @@ 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.
|
||||
|
||||
Since the "switch" value is just "true", we could leave it off -- as is also
|
||||
Since the "switch" value is just "true", we could leave it off - as is also
|
||||
the situation
|
||||
in a "for" statement, a missing value means "true". In fact, such a "switch"
|
||||
is a form of "if-else" chain. While we're here, it should be mentioned that in
|
||||
@ -476,8 +470,8 @@ Here is code from "progs/cat_rot13.go":
|
||||
|
||||
--PROG progs/cat_rot13.go /type.reader/ /^}/
|
||||
|
||||
Any type that implements the two methods of "reader" -- regardless of whatever
|
||||
other methods the type may also contain -- is said to <i>implement</i> the
|
||||
Any type that has the two methods of "reader" - regardless of whatever
|
||||
other methods the type may also have - is said to <i>implement</i> the
|
||||
interface. Since "file.File" implements these methods, it implements the
|
||||
"reader" interface. We could tweak the "cat" subroutine to accept a "reader"
|
||||
instead of a "*file.File" and it would work just fine, but let's embellish a little
|
||||
@ -492,7 +486,7 @@ we have a second implementation of the "reader" interface.
|
||||
|
||||
To use the new feature, we define a flag:
|
||||
|
||||
--PROG progs/cat_rot13.go /rot13_flag/
|
||||
--PROG progs/cat_rot13.go /rot13Flag/
|
||||
|
||||
and use it from within a mostly unchanged "cat()" function:
|
||||
|
||||
@ -518,7 +512,7 @@ Here it is in action:
|
||||
Fans of dependency injection may take cheer from how easily interfaces
|
||||
allow us to substitute the implementation of a file descriptor.
|
||||
|
||||
Interfaces are a distinct feature of Go. An interface is implemented by a
|
||||
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.
|
||||
@ -537,7 +531,7 @@ useful for things like containers.
|
||||
Sorting
|
||||
----
|
||||
|
||||
Interfaces provide a simple form of polymorphism since they completely
|
||||
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.
|
||||
@ -584,8 +578,8 @@ Within the "fmt" package, "Printf" is declared with this signature:
|
||||
Printf(format string, v ...) (n int, errno os.Error)
|
||||
|
||||
That "..." represents the variadic argument list that in C would
|
||||
be handled using the "stdarg.h" macros, but in Go is passed using
|
||||
an empty interface variable ("interface {}") that is then unpacked
|
||||
be handled using the "stdarg.h" macros but in Go is passed using
|
||||
an empty interface variable ("interface {}") and then unpacked
|
||||
using the reflection library. It's off topic here but the use of
|
||||
reflection helps explain some of the nice properties of Go's "Printf",
|
||||
due to the ability of "Printf" to discover the type of its arguments
|
||||
@ -614,7 +608,7 @@ is
|
||||
You can drop the formatting altogether if you use "Print" or "Println"
|
||||
instead of "Printf". Those routines do fully automatic formatting.
|
||||
The "Print" function just prints its elements out using the equivalent
|
||||
of "%v" while "Println" automatically inserts spaces between arguments
|
||||
of "%v" while "Println" inserts spaces between arguments
|
||||
and adds a newline. The output of each of these two lines is identical
|
||||
to that of the "Printf" call above.
|
||||
|
||||
@ -628,7 +622,7 @@ Here's a simple example.
|
||||
|
||||
--PROG progs/print_string.go 'NR==9' END
|
||||
|
||||
Since "*T" has a "String()" method, the
|
||||
Since "*testType" has a "String()" method, the
|
||||
default formatter for that type will use it and produce the output
|
||||
|
||||
77 Sunset Strip
|
||||
@ -692,7 +686,7 @@ you want.
|
||||
Prime numbers
|
||||
----
|
||||
|
||||
Now we come to processes and communication -- concurrent programming.
|
||||
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.
|
||||
|
||||
A classic program in the style is a prime sieve.
|
||||
@ -746,13 +740,13 @@ invoke the function, prefixing the call with the keyword "go";
|
||||
this starts the function running in parallel with the current
|
||||
computation but in the same address space:
|
||||
|
||||
go sum(huge_array); // calculate sum in the background
|
||||
go sum(hugeArray); // calculate sum in the background
|
||||
|
||||
If you want to know when the calculation is done, pass a channel
|
||||
on which it can report back:
|
||||
|
||||
ch := make(chan int);
|
||||
go sum(huge_array, ch);
|
||||
go sum(hugeArray, ch);
|
||||
// ... do something else for a while
|
||||
result := <-ch; // wait for, and retrieve, result
|
||||
|
||||
@ -773,7 +767,7 @@ of "generate", from "progs/sieve1.go":
|
||||
--PROG progs/sieve1.go /func.generate/ /^}/
|
||||
|
||||
This version does all the setup internally. It creates the output
|
||||
channel, launches a goroutine internally using a function literal, and
|
||||
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.
|
||||
|
||||
@ -799,7 +793,7 @@ Multiplexing
|
||||
----
|
||||
|
||||
With channels, it's possible to serve multiple independent client goroutines without
|
||||
writing an actual multiplexer. The trick is to send the server a channel in the message,
|
||||
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 "request" type, which embeds a channel
|
||||
@ -820,18 +814,18 @@ a long-running operation, starting a goroutine to do the actual work.
|
||||
|
||||
--PROG progs/server.go /func.server/ /^}/
|
||||
|
||||
We construct a server in a familiar way, starting it up and returning a channel to
|
||||
connect to it:
|
||||
We construct a server in a familiar way, starting it and returning a channel
|
||||
connected to it:
|
||||
|
||||
--PROG progs/server.go /func.startServer/ /^}/
|
||||
|
||||
Here's a simple test. It starts a server with an addition operator, and sends out
|
||||
lots of requests but doesn't wait for the reply. Only after all the requests are sent
|
||||
Here's a simple test. It starts a server with an addition operator and sends out
|
||||
"N" requests without waiting for the replies. Only after all the requests are sent
|
||||
does it check the results.
|
||||
|
||||
--PROG progs/server.go /func.main/ /^}/
|
||||
|
||||
One annoyance with this program is that it doesn't exit cleanly; when "main" returns
|
||||
One annoyance with this program is that it doesn't shut down the server cleanly; when "main" returns
|
||||
there are a number of lingering goroutines blocked on communication. To solve this,
|
||||
we can provide a second, "quit" channel to the server:
|
||||
|
||||
@ -841,7 +835,7 @@ It passes the quit channel to the "server" function, which uses it like this:
|
||||
|
||||
--PROG progs/server1.go /func.server/ /^}/
|
||||
|
||||
Inside "server", a "select" statement chooses which of the multiple communications
|
||||
Inside "server", the "select" 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 "select" allows
|
||||
the server to honor requests until it receives a quit message, at which point it
|
||||
|
||||
@ -11,7 +11,7 @@ import (
|
||||
"os";
|
||||
)
|
||||
|
||||
var rot13_flag = flag.Bool("rot13", false, "rot13 the input")
|
||||
var rot13Flag = flag.Bool("rot13", false, "rot13 the input")
|
||||
|
||||
func rot13(b byte) byte {
|
||||
if 'a' <= b && b <= 'z' {
|
||||
@ -53,7 +53,7 @@ func cat(r reader) {
|
||||
const NBUF = 512;
|
||||
var buf [NBUF]byte;
|
||||
|
||||
if *rot13_flag {
|
||||
if *rot13Flag {
|
||||
r = newRotate13(r)
|
||||
}
|
||||
for {
|
||||
|
||||
@ -9,7 +9,7 @@ import (
|
||||
"flag"; // command line option parser
|
||||
)
|
||||
|
||||
var n_flag = flag.Bool("n", false, "don't print final newline")
|
||||
var omitNewline = flag.Bool("n", false, "don't print final newline")
|
||||
|
||||
const (
|
||||
Space = " ";
|
||||
@ -25,7 +25,7 @@ func main() {
|
||||
}
|
||||
s += flag.Arg(i)
|
||||
}
|
||||
if !*n_flag {
|
||||
if !*omitNewline {
|
||||
s += Newline
|
||||
}
|
||||
os.Stdout.WriteString(s);
|
||||
|
||||
Loading…
Reference in New Issue
Block a user