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go
golang
Commits
898714a9
Commit
898714a9
authored
Nov 08, 2009
by
Russ Cox
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tutorial fixes
R=r CC=go-dev
http://go/go-review/1024014
parent
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go_tutorial.html
doc/go_tutorial.html
+53
-59
go_tutorial.txt
doc/go_tutorial.txt
+49
-55
cat_rot13.go
doc/progs/cat_rot13.go
+2
-2
echo.go
doc/progs/echo.go
+2
-2
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doc/go_tutorial.html
View file @
898714a9
...
...
@@ -38,9 +38,8 @@ our old, now capitalized and package-qualified friend, <code>fmt.Printf</code>.
<p>
Function declarations are introduced with the
<code>
func
</code>
keyword.
<p>
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.)
<p>
The comment convention is the same as in C++:
<p>
...
...
@@ -92,7 +91,7 @@ Next up, here's a version of the Unix utility <code>echo(1)</code>:
09
"
flag
"
; // command line option parser
10 )
<p>
12 var
n_flag
= flag.Bool(
"
n
"
, false,
"
don't print final newline
"
)
12 var
omitNewline
= flag.Bool(
"
n
"
, false,
"
don't print final newline
"
)
<p>
14 const (
15 Space =
"
"
;
...
...
@@ -108,7 +107,7 @@ Next up, here's a version of the Unix utility <code>echo(1)</code>:
25 }
26 s += flag.Arg(i)
27 }
28 if !*
n_flag
{
28 if !*
omitNewline
{
29 s += Newline
30 }
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.
<p>
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>
.
<p>
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>
<p>
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:
<p>
<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>
<p>
The
<code>
os
</code>
package contains other essentials for getting
started; for instance,
<code>
os.Args
</code>
is a
n 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.
<p>
<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.
<p>
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
of course you can change a string
<i>
variable
</i>
simply by
reassigning it. This snippet from
<code>
strings.go
</code>
is legal code:
...
...
@@ -255,11 +250,11 @@ read on.
Arrays are declared like this:
<p>
<pre>
var array
_of_i
nt [10]int;
var array
OfI
nt [10]int;
</pre>
<p>
Arrays, like strings, are values, but they are mutable. This differs
from C, in which
<code>
array
_of_i
nt
</code>
would be usable as a pointer to
<code>
int
</code>
.
from C, in which
<code>
array
OfI
nt
</code>
would be usable as a pointer to
<code>
int
</code>
.
In Go, since arrays are values, it's meaningful (and useful) to talk
about pointers to arrays.
<p>
...
...
@@ -275,7 +270,7 @@ 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.
<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,
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
...
...
@@ -284,7 +279,7 @@ array.
<p>
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.
<p>
Using slices one can write this function (from
<code>
sum.go
</code>
):
...
...
@@ -307,10 +302,10 @@ and invoke it like this:
<p>
Note how the return type (
<code>
int
</code>
) is defined for
<code>
sum()
</code>
by stating it
after the parameter list.
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>
&
</code>
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>
&
</code>
in front gives us the address of a unique instance of the value. We pass the
pointer to
<code>
sum()
</code>
by (
automatical
ly) promoting it to a slice.
pointer to
<code>
sum()
</code>
by (
implicit
ly) promoting it to a slice.
<p>
If you are creating a regular array but want the compiler to count the
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>
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,
slices,
and map
s.
slices,
maps, and channel
s.
<p>
<p>
<h2>
An Interlude about Allocation
</h2>
<p>
Most types in Go are values. If you have an
<code>
int
</code>
or a
<code>
struct
</code>
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
<code>
new()
</code>
, which
copies the contents of the object.
To allocate a new variable
, use
<code>
new()
</code>
, which
returns a pointer to the allocated storage.
<p>
<pre>
...
...
@@ -373,7 +368,7 @@ If you just declare the map, as in
</pre>
<p>
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.
<p>
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.
<p>
<pre>
const hard
_e
ight = (1
<<
100)
>>
97 // legal
const hard
E
ight = (1
<<
100)
>>
97 // legal
</pre>
<p>
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 }
</pre>
<p>
The first
line declares the name of the package --
<code>
file
</code>
-
-
and then
we
import two packages. The
<code>
os
</code>
package hides the differences
The first
few lines declare the name of the package -
<code>
file
</code>
-
and then import two packages. The
<code>
os
</code>
package hides the differences
between various operating systems to give a consistent view of files and
so on; here we're
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.
<p>
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
will soon give it some exported, upper-case methods.
<p>
First, though, here is a factory to create
them
:
First, though, here is a factory to create
a
<code>
File
</code>
:
<p>
<pre>
<!-- progs/file.go /newFile/ /^}/ -->
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>
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,
both of type
<code>
int
64
</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>
and the error. If
<code>
syscall.Open
</code>
fails, the file descriptor
<code>
r
</code>
will
be negative and
<code>
NewFile
</code>
will return
<code>
nil
</code>
.
<p>
About those errors: The
<code>
os
</code>
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
<code>
os
</code>
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
<code>
Open
</code>
we use a
conversion to
<code>
os.Errno
</code>
to translate Unix's integer
<code>
errno
</code>
value into
an error value, which will be stored in a unique instance of type
<code>
os.Error
</code>
.
conversion to
translate Unix's integer
<code>
errno
</code>
value into the integer type
<code>
os.Errno
</code>
, which implements
<code>
os.Error
</code>
.
<p>
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
...
...
@@ -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
members of the structure. Methods are not declared within
the
<code>
struct
</code>
declaration itself. The
<code>
struct
</code>
declaration defines only data members.
In fact, methods can be created for any type you name, such as an integer or
In fact, methods can be created for a
lmost a
ny type you name, such as an integer or
array, not just for
<code>
structs
</code>
. We'll see an example with arrays later.
<p>
The
<code>
String
</code>
method is so called because of a printing convention we'll
...
...
@@ -606,7 +600,7 @@ We can now use our new package:
21 }
</pre>
<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.
<p>
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
they all have the same type.
<p>
Since the
<code>
switch
</code>
value is just
<code>
true
</code>
, we could leave it off -
-
as is also
Since the
<code>
switch
</code>
value is just
<code>
true
</code>
, we could leave it off - as is also
the situation
in a
<code>
for
</code>
statement, a missing value means
<code>
true
</code>
. In fact, such a
<code>
switch
</code>
is a form of
<code>
if-else
</code>
chain. While we're here, it should be mentioned that in
...
...
@@ -701,8 +695,8 @@ Here is code from <code>progs/cat_rot13.go</code>:
29 }
</pre>
<p>
Any type that
implements the two methods of
<code>
reader
</code>
-
- 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
<code>
reader
</code>
- regardless of whatever
other methods the type may also
have
- is said to
<i>
implement
</i>
the
interface. Since
<code>
file.File
</code>
implements these methods, it implements the
<code>
reader
</code>
interface. We could tweak the
<code>
cat
</code>
subroutine to accept a
<code>
reader
</code>
instead of a
<code>
*file.File
</code>
and it would work just fine, but let's embellish a little
...
...
@@ -738,8 +732,8 @@ we have a second implementation of the <code>reader</code> interface.
<p>
To use the new feature, we define a flag:
<p>
<pre>
<!-- progs/cat_rot13.go /rot13
_f
lag/ -->
14 var rot13
_f
lag = flag.Bool(
"
rot13
"
, false,
"
rot13 the input
"
)
<pre>
<!-- progs/cat_rot13.go /rot13
F
lag/ -->
14 var rot13
F
lag = flag.Bool(
"
rot13
"
, false,
"
rot13 the input
"
)
</pre>
<p>
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;
54 var buf [NBUF]byte;
<p>
56 if *rot13
_f
lag {
56 if *rot13
F
lag {
57 r = newRotate13(r)
58 }
59 for {
...
...
@@ -789,7 +783,7 @@ 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.
<p>
Interfaces are a distinct feature of Go. An interface is implemented by a
Interfaces are a distinct
ive
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.
...
...
@@ -807,7 +801,7 @@ useful for things like containers.
<p>
<h2>
Sorting
</h2>
<p>
Interfaces provide a simple form of polymorphism
since t
hey completely
Interfaces provide a simple form of polymorphism
. T
hey 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.
...
...
@@ -898,8 +892,8 @@ Within the <code>fmt</code> package, <code>Printf</code> is declared with this s
</pre>
<p>
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
an empty interface variable (
<code>
interface {}
</code>
)
that is
then unpacked
be handled using the
<code>
stdarg.h
</code>
macros but in Go is passed using
an empty interface variable (
<code>
interface {}
</code>
)
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
<code>
Printf
</code>
,
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>
instead of
<code>
Printf
</code>
. Those routines do fully automatic formatting.
The
<code>
Print
</code>
function just prints its elements out using the equivalent
of
<code>
%v
</code>
while
<code>
Println
</code>
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
to that of the
<code>
Printf
</code>
call above.
<p>
...
...
@@ -968,7 +962,7 @@ Here's a simple example.
18 }
</pre>
<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
<p>
<pre>
...
...
@@ -1039,7 +1033,7 @@ you want.
<p>
<h2>
Prime numbers
</h2>
<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.
<p>
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:
<p>
<pre>
go sum(huge
_a
rray); // calculate sum in the background
go sum(huge
A
rray); // calculate sum in the background
</pre>
<p>
If you want to know when the calculation is done, pass a channel
...
...
@@ -1120,7 +1114,7 @@ on which it can report back:
<p>
<pre>
ch := make(chan int);
go sum(huge
_a
rray, ch);
go sum(huge
A
rray, ch);
// ... do something else for a while
result :=
<
-ch; // wait for, and retrieve, result
</pre>
...
...
@@ -1164,7 +1158,7 @@ of <code>generate</code>, from <code>progs/sieve1.go</code>:
</pre>
<p>
This version does all the setup internally. It creates the output
channel, launches a goroutine
internally us
ing a function literal, and
channel, launches a goroutine
runn
ing a function literal, and
returns the channel to the caller. It is a factory for concurrent
execution, starting the goroutine and returning its connection.
<p>
...
...
@@ -1221,7 +1215,7 @@ Now <code>main</code>'s interface to the prime sieve is a channel of primes:
<h2>
Multiplexing
</h2>
<p>
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
<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 }
</pre>
<p>
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
connect
ed
to it:
<p>
<pre>
<!-- progs/server.go /func.startServer/ /^}/ -->
28 func startServer(op binOp) chan *request {
...
...
@@ -1272,8 +1266,8 @@ connect to it:
32 }
</pre>
<p>
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
...
...
doc/go_tutorial.txt
View file @
898714a9
...
...
@@ -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 a
n 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_i
nt [10]int;
var array
OfI
nt [10]int;
Arrays, like strings, are values, but they are mutable. This differs
from C, in which "array
_of_i
nt" would be usable as a pointer to "int".
from C, in which "array
OfI
nt" 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 (
automatical
ly) promoting it to a slice.
pointer to "sum()" by (
implicit
ly) 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 map
s.
slices,
maps, and channel
s.
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
_e
ight = (1 << 100) >> 97 // legal
const hard
E
ight = (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 "int
64
" (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 a
lmost a
ny 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
_f
lag/
--PROG progs/cat_rot13.go /rot13
F
lag/
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 distinct
ive
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 t
hey completely
Interfaces provide a simple form of polymorphism
. T
hey 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
_a
rray); // calculate sum in the background
go sum(huge
A
rray); // 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
_a
rray, ch);
go sum(huge
A
rray, 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 us
ing a function literal, and
channel, launches a goroutine
runn
ing 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
connect
ed
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
...
...
doc/progs/cat_rot13.go
View file @
898714a9
...
...
@@ -11,7 +11,7 @@ import (
"os"
;
)
var
rot13
_f
lag
=
flag
.
Bool
(
"rot13"
,
false
,
"rot13 the input"
)
var
rot13
F
lag
=
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
_f
lag
{
if
*
rot13
F
lag
{
r
=
newRotate13
(
r
)
}
for
{
...
...
doc/progs/echo.go
View file @
898714a9
...
...
@@ -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
);
...
...
Write
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