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tutorial fixes

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http://go/go-review/1024014
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Russ Cox 2009-11-07 18:05:30 -08:00
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@ -38,9 +38,8 @@ our old, now capitalized and package-qualified friend, <code>fmt.Printf</code>.
<p> <p>
Function declarations are introduced with the <code>func</code> keyword. Function declarations are introduced with the <code>func</code> keyword.
<p> <p>
Notice that string constants can contain Unicode characters, encoded in UTF-8. 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 (In fact, Go source files are defined to be encoded in UTF-8.)
to store Unicode strings represented in UTF-8.
<p> <p>
The comment convention is the same as in C++: The comment convention is the same as in C++:
<p> <p>
@ -92,7 +91,7 @@ Next up, here's a version of the Unix utility <code>echo(1)</code>:
09 &quot;flag&quot;; // command line option parser 09 &quot;flag&quot;; // command line option parser
10 ) 10 )
<p> <p>
12 var n_flag = flag.Bool(&quot;n&quot;, false, &quot;don't print final newline&quot;) 12 var omitNewline = flag.Bool(&quot;n&quot;, false, &quot;don't print final newline&quot;)
<p> <p>
14 const ( 14 const (
15 Space = &quot; &quot;; 15 Space = &quot; &quot;;
@ -108,7 +107,7 @@ Next up, here's a version of the Unix utility <code>echo(1)</code>:
25 } 25 }
26 s += flag.Arg(i) 26 s += flag.Arg(i)
27 } 27 }
28 if !*n_flag { 28 if !*omitNewline {
29 s += Newline 29 s += Newline
30 } 30 }
31 os.Stdout.WriteString(s); 31 os.Stdout.WriteString(s);
@ -149,7 +148,7 @@ a naming conflict.
Given <code>os.Stdout</code> we can use its <code>WriteString</code> method to print the string. Given <code>os.Stdout</code> we can use its <code>WriteString</code> method to print the string.
<p> <p>
Having imported the <code>flag</code> package, line 12 creates a global variable to hold Having imported the <code>flag</code> package, line 12 creates a global variable to hold
the value of echo's <code>-n</code> flag. The variable <code>n_flag</code> has type <code>*bool</code>, pointer the value of echo's <code>-n</code> flag. The variable <code>omitNewline</code> has type <code>*bool</code>, pointer
to <code>bool</code>. to <code>bool</code>.
<p> <p>
In <code>main.main</code>, we parse the arguments (line 20) and then create a local In <code>main.main</code>, we parse the arguments (line 20) and then create a local
@ -179,10 +178,6 @@ or we could go even shorter and write the idiom
</pre> </pre>
<p> <p>
The <code>:=</code> operator is used a lot in Go to represent an initializing declaration. The <code>:=</code> operator is used a lot in Go to represent an initializing declaration.
(For those who know Limbo, its <code>:=</code> construct is the same, but notice
that Go has no colon after the name in a full <code>var</code> declaration.
Also, for simplicity of parsing, <code>:=</code> only works inside functions, not at
the top level.)
There's one in the <code>for</code> clause on the next line: There's one in the <code>for</code> clause on the next line:
<p> <p>
<pre> <!-- progs/echo.go /for/ --> <pre> <!-- progs/echo.go /for/ -->
@ -211,7 +206,7 @@ It's defined that way. Falling off the end of <code>main.main</code> means
</pre> </pre>
<p> <p>
The <code>os</code> package contains other essentials for getting The <code>os</code> package contains other essentials for getting
started; for instance, <code>os.Args</code> is an array used by the started; for instance, <code>os.Args</code> is a slice used by the
<code>flag</code> package to access the command-line arguments. <code>flag</code> package to access the command-line arguments.
<p> <p>
<h2>An Interlude about Types</h2> <h2>An Interlude about Types</h2>
@ -225,7 +220,7 @@ they are not the same type. There is also a <code>byte</code> synonym for
<code>uint8</code>, which is the element type for strings. <code>uint8</code>, which is the element type for strings.
<p> <p>
Speaking of <code>string</code>, that's a built-in type as well. Strings are Speaking of <code>string</code>, that's a built-in type as well. Strings are
<i>immutable values</i> -- they are not just arrays of <code>byte</code> values. <i>immutable values</i> - they are not just arrays of <code>byte</code> values.
Once you've built a string <i>value</i>, you can't change it, although 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 of course you can change a string <i>variable</i> simply by
reassigning it. This snippet from <code>strings.go</code> is legal code: reassigning it. This snippet from <code>strings.go</code> is legal code:
@ -255,11 +250,11 @@ read on.
Arrays are declared like this: Arrays are declared like this:
<p> <p>
<pre> <pre>
var array_of_int [10]int; var arrayOfInt [10]int;
</pre> </pre>
<p> <p>
Arrays, like strings, are values, but they are mutable. This differs Arrays, like strings, are values, but they are mutable. This differs
from C, in which <code>array_of_int</code> would be usable as a pointer to <code>int</code>. from C, in which <code>arrayOfInt</code> would be usable as a pointer to <code>int</code>.
In Go, since arrays are values, it's meaningful (and useful) to talk In Go, since arrays are values, it's meaningful (and useful) to talk
about pointers to arrays. about pointers to arrays.
<p> <p>
@ -275,7 +270,7 @@ an underlying, often anonymous, regular array. Multiple slices
can share data if they represent pieces of the same array; can share data if they represent pieces of the same array;
multiple arrays can never share data. multiple arrays can never share data.
<p> <p>
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, regular arrays; they're more flexible, have reference semantics,
and are efficient. What they lack is the precise control of storage 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 layout of a regular array; if you want to have a hundred elements
@ -284,7 +279,7 @@ array.
<p> <p>
When passing an array to a function, you almost always want When passing an array to a function, you almost always want
to declare the formal parameter to be a slice. When you call 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. create (efficiently) a slice reference and pass that.
<p> <p>
Using slices one can write this function (from <code>sum.go</code>): Using slices one can write this function (from <code>sum.go</code>):
@ -307,10 +302,10 @@ and invoke it like this:
<p> <p>
Note how the return type (<code>int</code>) is defined for <code>sum()</code> by stating it Note how the return type (<code>int</code>) is defined for <code>sum()</code> by stating it
after the parameter list. after the parameter list.
The expression <code>[3]int{1,2,3}</code> -- a type followed by a brace-bounded expression The expression <code>[3]int{1,2,3}</code> - a type followed by a brace-bounded expression
-- is a constructor for a value, in this case an array of 3 <code>ints</code>. Putting an <code>&amp;</code> - is a constructor for a value, in this case an array of 3 <code>ints</code>. Putting an <code>&amp;</code>
in front gives us the address of a unique instance of the value. We pass the in front gives us the address of a unique instance of the value. We pass the
pointer to <code>sum()</code> by (automatically) promoting it to a slice. pointer to <code>sum()</code> by (implicitly) promoting it to a slice.
<p> <p>
If you are creating a regular array but want the compiler to count the If you are creating a regular array but want the compiler to count the
elements for you, use <code>...</code> as the array size: elements for you, use <code>...</code> as the array size:
@ -334,15 +329,15 @@ There are also maps, which you can initialize like this:
<p> <p>
The built-in function <code>len()</code>, which returns number of elements, The built-in function <code>len()</code>, which returns number of elements,
makes its first appearance in <code>sum</code>. It works on strings, arrays, makes its first appearance in <code>sum</code>. It works on strings, arrays,
slices, and maps. slices, maps, and channels.
<p> <p>
<p> <p>
<h2>An Interlude about Allocation</h2> <h2>An Interlude about Allocation</h2>
<p> <p>
Most types in Go are values. If you have an <code>int</code> or a <code>struct</code> Most types in Go are values. If you have an <code>int</code> or a <code>struct</code>
or an array, assignment or an array, assignment
copies the contents of the object. To allocate something on the stack, copies the contents of the object.
just declare a variable. To allocate it on the heap, use <code>new()</code>, which To allocate a new variable, use <code>new()</code>, which
returns a pointer to the allocated storage. returns a pointer to the allocated storage.
<p> <p>
<pre> <pre>
@ -373,7 +368,7 @@ If you just declare the map, as in
</pre> </pre>
<p> <p>
it creates a <code>nil</code> reference that cannot hold anything. To use the map, it creates a <code>nil</code> reference that cannot hold anything. To use the map,
you must first initialize the reference using <code>make()</code> or by assignment to an you must first initialize the reference using <code>make()</code> or by assignment of an
existing map. existing map.
<p> <p>
Note that <code>new(T)</code> returns type <code>*T</code> while <code>make(T)</code> returns type Note that <code>new(T)</code> returns type <code>*T</code> while <code>make(T)</code> returns type
@ -390,7 +385,7 @@ can overflow only when they are assigned to an integer variable with
too little precision to represent the value. too little precision to represent the value.
<p> <p>
<pre> <pre>
const hard_eight = (1 &lt;&lt; 100) &gt;&gt; 97 // legal const hardEight = (1 &lt;&lt; 100) &gt;&gt; 97 // legal
</pre> </pre>
<p> <p>
There are nuances that deserve redirection to the legalese of the There are nuances that deserve redirection to the legalese of the
@ -431,10 +426,10 @@ sort of open/close/read/write interface. Here's the start of <code>file.go</cod
15 } 15 }
</pre> </pre>
<p> <p>
The first line declares the name of the package -- <code>file</code> -- The first few lines declare the name of the package - <code>file</code> -
and then we import two packages. The <code>os</code> package hides the differences 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 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. and reproduce the rudiments of its file I/O.
<p> <p>
The other item is the low-level, external <code>syscall</code> package, which provides The other item is the low-level, external <code>syscall</code> package, which provides
@ -456,7 +451,7 @@ In Go, the term for publicly visible names is ''exported''.
In the case of <code>File</code>, all its fields are lower case and so invisible to users, but we 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. will soon give it some exported, upper-case methods.
<p> <p>
First, though, here is a factory to create them: First, though, here is a factory to create a <code>File</code>:
<p> <p>
<pre> <!-- progs/file.go /newFile/ /^}/ --> <pre> <!-- progs/file.go /newFile/ /^}/ -->
17 func newFile(fd int, name string) *File { 17 func newFile(fd int, name string) *File {
@ -513,17 +508,16 @@ they look just like a second parameter list. The function
<code>syscall.Open</code> <code>syscall.Open</code>
also has a multi-value return, which we can grab with the multi-variable also has a multi-value return, which we can grab with the multi-variable
declaration on line 31; it declares <code>r</code> and <code>e</code> to hold the two values, declaration on line 31; it declares <code>r</code> and <code>e</code> to hold the two values,
both of type <code>int64</code> (although you'd have to look at the <code>syscall</code> package both of type <code>int</code> (although you'd have to look at the <code>syscall</code> package
to see that). Finally, line 35 returns two values: a pointer to the new <code>File</code> to see that). Finally, line 35 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 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>. be negative and <code>NewFile</code> will return <code>nil</code>.
<p> <p>
About those errors: The <code>os</code> library includes a general notion of an error About those errors: The <code>os</code> library includes a general notion of an error.
string, maintaining a unique set of errors throughout the program. It's a It's a good idea to use its facility in your own interfaces, as we do here, for
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 consistent error handling throughout Go code. In <code>Open</code> we use a
conversion to <code>os.Errno</code> to translate Unix's integer <code>errno</code> value into conversion to translate Unix's integer <code>errno</code> value into the integer type
an error value, which will be stored in a unique instance of type <code>os.Error</code>. <code>os.Errno</code>, which implements <code>os.Error</code>.
<p> <p>
Now that we can build <code>Files</code>, we can write methods for them. To declare 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 a method of a type, we define a function to have an explicit receiver
@ -574,7 +568,7 @@ each of which declares a receiver variable <code>file</code>.
There is no implicit <code>this</code> and the receiver variable must be used to access 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 members of the structure. Methods are not declared within
the <code>struct</code> declaration itself. The <code>struct</code> declaration defines only data members. the <code>struct</code> declaration itself. The <code>struct</code> 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 <code>structs</code>. We'll see an example with arrays later. array, not just for <code>structs</code>. We'll see an example with arrays later.
<p> <p>
The <code>String</code> method is so called because of a printing convention we'll The <code>String</code> method is so called because of a printing convention we'll
@ -606,7 +600,7 @@ We can now use our new package:
21 } 21 }
</pre> </pre>
<p> <p>
The import of ''<code>./file</code>'' tells the compiler to use our own package rather than 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. something from the directory of installed packages.
<p> <p>
Finally we can run the program: Finally we can run the program:
@ -677,7 +671,7 @@ 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 case expressions don't need to be constants or even integers, as long as
they all have the same type. they all have the same type.
<p> <p>
Since the <code>switch</code> value is just <code>true</code>, we could leave it off -- as is also Since the <code>switch</code> value is just <code>true</code>, we could leave it off - as is also
the situation the situation
in a <code>for</code> statement, a missing value means <code>true</code>. In fact, such a <code>switch</code> 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 is a form of <code>if-else</code> chain. While we're here, it should be mentioned that in
@ -701,8 +695,8 @@ Here is code from <code>progs/cat_rot13.go</code>:
29 } 29 }
</pre> </pre>
<p> <p>
Any type that implements the two methods of <code>reader</code> -- regardless of whatever Any type that has the two methods of <code>reader</code> - regardless of whatever
other methods the type may also contain -- is said to <i>implement</i> the 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 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> <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 instead of a <code>*file.File</code> and it would work just fine, but let's embellish a little
@ -738,8 +732,8 @@ we have a second implementation of the <code>reader</code> interface.
<p> <p>
To use the new feature, we define a flag: To use the new feature, we define a flag:
<p> <p>
<pre> <!-- progs/cat_rot13.go /rot13_flag/ --> <pre> <!-- progs/cat_rot13.go /rot13Flag/ -->
14 var rot13_flag = flag.Bool(&quot;rot13&quot;, false, &quot;rot13 the input&quot;) 14 var rot13Flag = flag.Bool(&quot;rot13&quot;, false, &quot;rot13 the input&quot;)
</pre> </pre>
<p> <p>
and use it from within a mostly unchanged <code>cat()</code> function: and use it from within a mostly unchanged <code>cat()</code> function:
@ -749,7 +743,7 @@ and use it from within a mostly unchanged <code>cat()</code> function:
53 const NBUF = 512; 53 const NBUF = 512;
54 var buf [NBUF]byte; 54 var buf [NBUF]byte;
<p> <p>
56 if *rot13_flag { 56 if *rot13Flag {
57 r = newRotate13(r) 57 r = newRotate13(r)
58 } 58 }
59 for { 59 for {
@ -789,7 +783,7 @@ Here it is in action:
Fans of dependency injection may take cheer from how easily interfaces Fans of dependency injection may take cheer from how easily interfaces
allow us to substitute the implementation of a file descriptor. allow us to substitute the implementation of a file descriptor.
<p> <p>
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. type if the type implements all the methods declared in the interface.
This means This means
that a type may implement an arbitrary number of different interfaces. that a type may implement an arbitrary number of different interfaces.
@ -807,7 +801,7 @@ useful for things like containers.
<p> <p>
<h2>Sorting</h2> <h2>Sorting</h2>
<p> <p>
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 separate the definition of what an object does from how it does it, allowing
distinct implementations to be represented at different times by the distinct implementations to be represented at different times by the
same interface variable. same interface variable.
@ -898,8 +892,8 @@ Within the <code>fmt</code> package, <code>Printf</code> is declared with this s
</pre> </pre>
<p> <p>
That <code>...</code> represents the variadic argument list that in C would That <code>...</code> represents the variadic argument list that in C would
be handled using the <code>stdarg.h</code> macros, but in Go is passed using be handled using the <code>stdarg.h</code> macros but in Go is passed using
an empty interface variable (<code>interface {}</code>) that is then unpacked an empty interface variable (<code>interface {}</code>) and then unpacked
using the reflection library. It's off topic here but the use of using the reflection library. It's off topic here but the use of
reflection helps explain some of the nice properties of Go's <code>Printf</code>, reflection 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 due to the ability of <code>Printf</code> to discover the type of its arguments
@ -940,7 +934,7 @@ is
You can drop the formatting altogether if you use <code>Print</code> or <code>Println</code> 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. instead of <code>Printf</code>. Those routines do fully automatic formatting.
The <code>Print</code> function just prints its elements out using the equivalent The <code>Print</code> function just prints its elements out using the equivalent
of <code>%v</code> while <code>Println</code> automatically inserts spaces between arguments 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 and adds a newline. The output of each of these two lines is identical
to that of the <code>Printf</code> call above. to that of the <code>Printf</code> call above.
<p> <p>
@ -968,7 +962,7 @@ Here's a simple example.
18 } 18 }
</pre> </pre>
<p> <p>
Since <code>*T</code> has a <code>String()</code> method, the Since <code>*testType</code> has a <code>String()</code> method, the
default formatter for that type will use it and produce the output default formatter for that type will use it and produce the output
<p> <p>
<pre> <pre>
@ -1039,7 +1033,7 @@ you want.
<p> <p>
<h2>Prime numbers</h2> <h2>Prime numbers</h2>
<p> <p>
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. It's a big subject so to be brief we assume some familiarity with the topic.
<p> <p>
A classic program in the style is a prime sieve. A classic program in the style is a prime sieve.
@ -1112,7 +1106,7 @@ this starts the function running in parallel with the current
computation but in the same address space: computation but in the same address space:
<p> <p>
<pre> <pre>
go sum(huge_array); // calculate sum in the background go sum(hugeArray); // calculate sum in the background
</pre> </pre>
<p> <p>
If you want to know when the calculation is done, pass a channel If you want to know when the calculation is done, pass a channel
@ -1120,7 +1114,7 @@ on which it can report back:
<p> <p>
<pre> <pre>
ch := make(chan int); ch := make(chan int);
go sum(huge_array, ch); go sum(hugeArray, ch);
// ... do something else for a while // ... do something else for a while
result := &lt;-ch; // wait for, and retrieve, result result := &lt;-ch; // wait for, and retrieve, result
</pre> </pre>
@ -1164,7 +1158,7 @@ of <code>generate</code>, from <code>progs/sieve1.go</code>:
</pre> </pre>
<p> <p>
This version does all the setup internally. It creates the output 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 returns the channel to the caller. It is a factory for concurrent
execution, starting the goroutine and returning its connection. execution, starting the goroutine and returning its connection.
<p> <p>
@ -1221,7 +1215,7 @@ Now <code>main</code>'s interface to the prime sieve is a channel of primes:
<h2>Multiplexing</h2> <h2>Multiplexing</h2>
<p> <p>
With channels, it's possible to serve multiple independent client goroutines without 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. 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 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 to illustrate the idea. It starts by defining a <code>request</code> type, which embeds a channel
@ -1261,8 +1255,8 @@ a long-running operation, starting a goroutine to do the actual work.
26 } 26 }
</pre> </pre>
<p> <p>
We construct a server in a familiar way, starting it up and returning a channel to We construct a server in a familiar way, starting it and returning a channel
connect to it: connected to it:
<p> <p>
<pre> <!-- progs/server.go /func.startServer/ /^}/ --> <pre> <!-- progs/server.go /func.startServer/ /^}/ -->
28 func startServer(op binOp) chan *request { 28 func startServer(op binOp) chan *request {
@ -1272,8 +1266,8 @@ connect to it:
32 } 32 }
</pre> </pre>
<p> <p>
Here's a simple test. It starts a server with an addition operator, and sends out 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 <code>N</code> requests without waiting for the replies. Only after all the requests are sent
does it check the results. does it check the results.
<p> <p>
<pre> <!-- progs/server.go /func.main/ /^}/ --> <pre> <!-- progs/server.go /func.main/ /^}/ -->
@ -1297,7 +1291,7 @@ does it check the results.
51 } 51 }
</pre> </pre>
<p> <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, 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: we can provide a second, <code>quit</code> channel to the server:
<p> <p>
@ -1325,7 +1319,7 @@ It passes the quit channel to the <code>server</code> function, which uses it li
30 } 30 }
</pre> </pre>
<p> <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 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 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 the server to honor requests until it receives a quit message, at which point it

104
doc/go_tutorial.txt
View File

@ -32,9 +32,8 @@ our old, now capitalized and package-qualified friend, "fmt.Printf".
Function declarations are introduced with the "func" keyword. Function declarations are introduced with the "func" keyword.
Notice that string constants can contain Unicode characters, encoded in UTF-8. 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 (In fact, Go source files are defined to be encoded in UTF-8.)
to store Unicode strings represented in UTF-8.
The comment convention is the same as in C++: 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. 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 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". to "bool".
In "main.main", we parse the arguments (line 20) and then create a local 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 := ""; s := "";
The ":=" operator is used a lot in Go to represent an initializing declaration. 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: There's one in the "for" clause on the next line:
--PROG progs/echo.go /for/ --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) os.Exit(1)
The "os" package contains other essentials for getting 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. "flag" package to access the command-line arguments.
An Interlude about Types 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. "uint8", which is the element type for strings.
Speaking of "string", that's a built-in type as well. Strings are 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 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 of course you can change a string <i>variable</i> simply by
reassigning it. This snippet from "strings.go" is legal code: reassigning it. This snippet from "strings.go" is legal code:
@ -196,10 +191,10 @@ read on.
Arrays are declared like this: 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 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 In Go, since arrays are values, it's meaningful (and useful) to talk
about pointers to arrays. 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; can share data if they represent pieces of the same array;
multiple arrays can never share data. 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, regular arrays; they're more flexible, have reference semantics,
and are efficient. What they lack is the precise control of storage 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 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 When passing an array to a function, you almost always want
to declare the formal parameter to be a slice. When you call 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. create (efficiently) a slice reference and pass that.
Using slices one can write this function (from "sum.go"): 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 Note how the return type ("int") is defined for "sum()" by stating it
after the parameter list. after the parameter list.
The expression "[3]int{1,2,3}" -- a type followed by a brace-bounded expression 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 "&amp;" - is a constructor for a value, in this case an array of 3 "ints". Putting an "&amp;"
in front gives us the address of a unique instance of the value. We pass the 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 If you are creating a regular array but want the compiler to count the
elements for you, use "..." as the array size: 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, The built-in function "len()", which returns number of elements,
makes its first appearance in "sum". It works on strings, arrays, makes its first appearance in "sum". It works on strings, arrays,
slices, and maps. slices, maps, and channels.
An Interlude about Allocation 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" Most types in Go are values. If you have an "int" or a "struct"
or an array, assignment or an array, assignment
copies the contents of the object. To allocate something on the stack, copies the contents of the object.
just declare a variable. To allocate it on the heap, use "new()", which To allocate a new variable, use "new()", which
returns a pointer to the allocated storage. returns a pointer to the allocated storage.
type T struct { a, b int } type T struct { a, b int }
@ -290,7 +285,7 @@ If you just declare the map, as in
var m map[string]int; var m map[string]int;
it creates a "nil" reference that cannot hold anything. To use the map, 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. existing map.
Note that "new(T)" returns type "*T" while "make(T)" returns type 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 can overflow only when they are assigned to an integer variable with
too little precision to represent the value. too little precision to represent the value.
const hard_eight = (1 &lt;&lt; 100) &gt;&gt; 97 // legal const hardEight = (1 &lt;&lt; 100) &gt;&gt; 97 // legal
There are nuances that deserve redirection to the legalese of the There are nuances that deserve redirection to the legalese of the
language specification but here are some illustrative examples: 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/ /^}/ --PROG progs/file.go /package/ /^}/
The first line declares the name of the package -- "file" -- The first few lines declare the name of the package - "file" -
and then we import two packages. The "os" package hides the differences and then import two packages. The "os" package hides the differences
between various operating systems to give a consistent view of files and 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. and reproduce the rudiments of its file I/O.
The other item is the low-level, external "syscall" package, which provides 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 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. 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/ /^}/ --PROG progs/file.go /newFile/ /^}/
@ -393,17 +388,16 @@ they look just like a second parameter list. The function
"syscall.Open" "syscall.Open"
also has a multi-value return, which we can grab with the multi-variable 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, 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" 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 and the error. If "syscall.Open" fails, the file descriptor "r" will
be negative and "NewFile" will return "nil". be negative and "NewFile" will return "nil".
About those errors: The "os" library includes a general notion of an error 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 It's a good idea to use its facility in your own interfaces, as we do here, for
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 consistent error handling throughout Go code. In "Open" we use a
conversion to "os.Errno" to translate Unix's integer "errno" value into conversion to translate Unix's integer "errno" value into the integer type
an error value, which will be stored in a unique instance of type "os.Error". "os.Errno", which implements "os.Error".
Now that we can build "Files", we can write methods for them. To declare 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 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 There is no implicit "this" and the receiver variable must be used to access
members of the structure. Methods are not declared within members of the structure. Methods are not declared within
the "struct" declaration itself. The "struct" declaration defines only data members. 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. 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 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 --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. something from the directory of installed packages.
Finally we can run the program: 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 case expressions don't need to be constants or even integers, as long as
they all have the same type. 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 the situation
in a "for" statement, a missing value means "true". In fact, such a "switch" 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 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/ /^}/ --PROG progs/cat_rot13.go /type.reader/ /^}/
Any type that implements the two methods of "reader" -- regardless of whatever Any type that has the two methods of "reader" - regardless of whatever
other methods the type may also contain -- is said to <i>implement</i> the other methods the type may also have - is said to <i>implement</i> the
interface. Since "file.File" implements these methods, it implements the interface. Since "file.File" implements these methods, it implements the
"reader" interface. We could tweak the "cat" subroutine to accept a "reader" "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 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: 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: 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 Fans of dependency injection may take cheer from how easily interfaces
allow us to substitute the implementation of a file descriptor. 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. type if the type implements all the methods declared in the interface.
This means This means
that a type may implement an arbitrary number of different interfaces. that a type may implement an arbitrary number of different interfaces.
@ -537,7 +531,7 @@ useful for things like containers.
Sorting 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 separate the definition of what an object does from how it does it, allowing
distinct implementations to be represented at different times by the distinct implementations to be represented at different times by the
same interface variable. 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) Printf(format string, v ...) (n int, errno os.Error)
That "..." represents the variadic argument list that in C would That "..." represents the variadic argument list that in C would
be handled using the "stdarg.h" macros, but in Go is passed using be handled using the "stdarg.h" macros but in Go is passed using
an empty interface variable ("interface {}") that is then unpacked an empty interface variable ("interface {}") and then unpacked
using the reflection library. It's off topic here but the use of 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", 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 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" You can drop the formatting altogether if you use "Print" or "Println"
instead of "Printf". Those routines do fully automatic formatting. instead of "Printf". Those routines do fully automatic formatting.
The "Print" function just prints its elements out using the equivalent 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 and adds a newline. The output of each of these two lines is identical
to that of the "Printf" call above. to that of the "Printf" call above.
@ -628,7 +622,7 @@ Here's a simple example.
--PROG progs/print_string.go 'NR==9' END --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 default formatter for that type will use it and produce the output
77 Sunset Strip 77 Sunset Strip
@ -692,7 +686,7 @@ you want.
Prime numbers 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. 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. 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 this starts the function running in parallel with the current
computation but in the same address space: 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 If you want to know when the calculation is done, pass a channel
on which it can report back: on which it can report back:
ch := make(chan int); ch := make(chan int);
go sum(huge_array, ch); go sum(hugeArray, ch);
// ... do something else for a while // ... do something else for a while
result := &lt;-ch; // wait for, and retrieve, result result := &lt;-ch; // wait for, and retrieve, result
@ -773,7 +767,7 @@ of "generate", from "progs/sieve1.go":
--PROG progs/sieve1.go /func.generate/ /^}/ --PROG progs/sieve1.go /func.generate/ /^}/
This version does all the setup internally. It creates the output 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 returns the channel to the caller. It is a factory for concurrent
execution, starting the goroutine and returning its connection. 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 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. 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 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 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/ /^}/ --PROG progs/server.go /func.server/ /^}/
We construct a server in a familiar way, starting it up and returning a channel to We construct a server in a familiar way, starting it and returning a channel
connect to it: connected to it:
--PROG progs/server.go /func.startServer/ /^}/ --PROG progs/server.go /func.startServer/ /^}/
Here's a simple test. It starts a server with an addition operator, and sends out 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 "N" requests without waiting for the replies. Only after all the requests are sent
does it check the results. does it check the results.
--PROG progs/server.go /func.main/ /^}/ --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, there are a number of lingering goroutines blocked on communication. To solve this,
we can provide a second, "quit" channel to the server: 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/ /^}/ --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 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 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 the server to honor requests until it receives a quit message, at which point it

4
doc/progs/cat_rot13.go
View File

@ -11,7 +11,7 @@ import (
"os"; "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 { func rot13(b byte) byte {
if 'a' <= b && b <= 'z' { if 'a' <= b && b <= 'z' {
@ -53,7 +53,7 @@ func cat(r reader) {
const NBUF = 512; const NBUF = 512;
var buf [NBUF]byte; var buf [NBUF]byte;
if *rot13_flag { if *rot13Flag {
r = newRotate13(r) r = newRotate13(r)
} }
for { for {

4
doc/progs/echo.go
View File

@ -9,7 +9,7 @@ import (
"flag"; // command line option parser "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 ( const (
Space = " "; Space = " ";
@ -25,7 +25,7 @@ func main() {
} }
s += flag.Arg(i) s += flag.Arg(i)
} }
if !*n_flag { if !*omitNewline {
s += Newline s += Newline
} }
os.Stdout.WriteString(s); os.Stdout.WriteString(s);