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+<!-- A Tutorial for the Go Programming Language -->
+<h2>Introduction</h2>
+<p>
+This document is a tutorial introduction to the basics of the Go programming
+language, intended for programmers familiar with C or C++. It is not a comprehensive
+guide to the language; at the moment the document closest to that is the
+<a href='/doc/go_spec.html'>language specification</a>.
+After you've read this tutorial, you should look at
+<a href='/doc/effective_go.html'>Effective Go</a>,
+which digs deeper into how the language is used and
+talks about the style and idioms of programming in Go.
+Also, slides from a 3-day course about Go are available.
+They provide some background and a lot of examples:
+<a href='/doc/GoCourseDay1.pdf'>Day 1</a>,
+<a href='/doc/GoCourseDay2.pdf'>Day 2</a>,
+<a href='/doc/GoCourseDay3.pdf'>Day 3</a>.
+<p>
+The presentation here proceeds through a series of modest programs to illustrate
+key features of the language.  All the programs work (at time of writing) and are
+checked into the repository in the directory <a href='/doc/progs'><code>/doc/progs/</code></a>.
+<p>
+<h2>Hello, World</h2>
+<p>
+Let's start in the usual way:
+<p>
+{{code "progs/helloworld.go" `/package/` "$"}}
+<p>
+Every Go source file declares, using a <code>package</code> statement, which package it's part of.
+It may also import other packages to use their facilities.
+This program imports the package <code>fmt</code> to gain access to
+our old, now capitalized and package-qualified, friend, <code>fmt.Printf</code>.
+<p>
+Functions are introduced with the <code>func</code> keyword.
+The <code>main</code> package's <code>main</code> function is where the program starts running (after
+any initialization).
+<p>
+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>
+<pre>
+/* ... */
+// ...
+</pre>
+<p>
+Later we'll have much more to say about printing.
+<p>
+<h2>Semicolons</h2>
+<p>
+You might have noticed that our program has no semicolons.  In Go
+code, the only place you typically see semicolons is separating the
+clauses of <code>for</code> loops and the like; they are not necessary after
+every statement.
+<p>
+In fact, what happens is that the formal language uses semicolons,
+much as in C or Java, but they are inserted automatically
+at the end of every line that looks like the end of a statement. You
+don't need to type them yourself.
+<p>
+For details about how this is done you can see the language
+specification, but in practice all you need to know is that you
+never need to put a semicolon at the end of a line.  (You can put
+them in if you want to write multiple statements per line.) As an
+extra help, you can also leave out a semicolon immediately before
+a closing brace.
+<p>
+This approach makes for clean-looking, semicolon-free code.  The
+one surprise is that it's important to put the opening
+brace of a construct such as an <code>if</code> statement on the same line as
+the <code>if</code>; if you don't, there are situations that may not compile
+or may give the wrong result.  The language forces the brace style
+to some extent.
+<p>
+<h2>Compiling</h2>
+<p>
+Go is a compiled language.  At the moment there are two compilers.
+<code>Gccgo</code> is a Go compiler that uses the GCC back end.  There is also a
+suite of compilers with different (and odd) names for each architecture:
+<code>6g</code> for the 64-bit x86, <code>8g</code> for the 32-bit x86, and more.  These
+compilers run significantly faster but generate less efficient code
+than <code>gccgo</code>.  At the time of writing (late 2009), they also have
+a more robust run-time system although <code>gccgo</code> is catching up.
+<p>
+Here's how to compile and run our program.  With <code>6g</code>, say,
+<p>
+<pre>
+$ 6g helloworld.go  # compile; object goes into helloworld.6
+$ 6l helloworld.6   # link; output goes into 6.out
+$ 6.out
+Hello, world; or Καλημέρα κόσμε; or こんにちは 世界
+$
+</pre>
+<p>
+With <code>gccgo</code> it looks a little more traditional.
+<p>
+<pre>
+$ gccgo helloworld.go
+$ a.out
+Hello, world; or Καλημέρα κόσμε; or こんにちは 世界
+$
+</pre>
+<p>
+<h2>Echo</h2>
+<p>
+Next up, here's a version of the Unix utility <code>echo(1)</code>:
+<p>
+{{code "progs/echo.go" `/package/` "$"}}
+<p>
+This program is small but it's doing a number of new things.  In the last example,
+we saw <code>func</code> introduce a function.  The keywords <code>var</code>, <code>const</code>, and <code>type</code>
+(not used yet) also introduce declarations, as does <code>import</code>.
+Notice that we can group declarations of the same sort into
+parenthesized lists, one item per line, as in the <code>import</code> and <code>const</code> clauses here.
+But it's not necessary to do so; we could have said
+<p>
+<pre>
+const Space = " "
+const Newline = "\n"
+</pre>
+<p>
+This program imports the <code>&quot;os&quot;</code> package to access its <code>Stdout</code> variable, of type
+<code>*os.File</code>.  The <code>import</code> statement is actually a declaration: in its general form,
+as used in our ``hello world'' program,
+it names the identifier (<code>fmt</code>)
+that will be used to access members of the package imported from the file (<code>&quot;fmt&quot;</code>),
+found in the current directory or in a standard location.
+In this program, though, we've dropped the explicit name from the imports; by default,
+packages are imported using the name defined by the imported package,
+which by convention is of course the file name itself.  Our ``hello world'' program
+could have said just <code>import &quot;fmt&quot;</code>.
+<p>
+You can specify your
+own import names if you want but it's only necessary if you need to resolve
+a naming conflict.
+<p>
+Given <code>os.Stdout</code> we can use its <code>WriteString</code> method to print the string.
+<p>
+After importing the <code>flag</code> package, we use a <code>var</code> declaration
+to create and initialize a global variable, called <code>omitNewline</code>,
+to hold the value of echo's <code>-n</code> flag. 
+The variable  has type <code>*bool</code>, pointer to <code>bool</code>.
+<p>
+In <code>main.main</code>, we parse the arguments (the call to <code>flag.Parse</code>) and then create a local
+string variable with which to build the output.
+<p>
+The declaration statement has the form
+<p>
+<pre>
+var s string = ""
+</pre>
+<p>
+This is the <code>var</code> keyword, followed by the name of the variable, followed by
+its type, followed by an equals sign and an initial value for the variable.
+<p>
+Go tries to be terse, and this declaration could be shortened.  Since the
+string constant is of type string, we don't have to tell the compiler that.
+We could write
+<p>
+<pre>
+var s = ""
+</pre>
+<p>
+or we could go even shorter and write the idiom
+<p>
+<pre>
+s := ""
+</pre>
+<p>
+The <code>:=</code> operator is used a lot in Go to represent an initializing declaration.
+There's one in the <code>for</code> clause on the next line:
+<p>
+{{code  "progs/echo.go" `/for/`}}
+<p>
+The <code>flag</code> package has parsed the arguments and left the non-flag arguments
+in a list that can be iterated over in the obvious way.
+<p>
+The Go <code>for</code> statement differs from that of C in a number of ways.  First,
+it's the only looping construct; there is no <code>while</code> or <code>do</code>.  Second,
+there are no parentheses on the clause, but the braces on the body
+are mandatory.  The same applies to the <code>if</code> and <code>switch</code> statements.
+Later examples will show some other ways <code>for</code> can be written.
+<p>
+The body of the loop builds up the string <code>s</code> by appending (using <code>+=</code>)
+the arguments and separating spaces. After the loop, if the <code>-n</code> flag is not
+set, the program appends a newline. Finally, it writes the result.
+<p>
+Notice that <code>main.main</code> is a niladic function with no return type.
+It's defined that way.  Falling off the end of <code>main.main</code> means
+''success''; if you want to signal an erroneous return, call
+<p>
+<pre>
+os.Exit(1)
+</pre>
+<p>
+The <code>os</code> package contains other essentials for getting
+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>
+<p>
+Go has some familiar types such as <code>int</code> and <code>uint</code> (unsigned <code>int</code>), which represent
+values of the ''appropriate'' size for the machine. It also defines
+explicitly-sized types such as <code>int8</code>, <code>float64</code>, and so on, plus
+unsigned integer types such as <code>uint</code>, <code>uint32</code>, etc.
+These are distinct types; even if <code>int</code> and <code>int32</code> are both 32 bits in size,
+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>
+Floating-point types are always sized: <code>float32</code> and <code>float64</code>,
+plus <code>complex64</code> (two <code>float32s</code>) and <code>complex128</code>
+(two <code>float64s</code>).  Complex numbers are outside the
+scope of this tutorial.
+<p>
+Speaking of <code>string</code>, that's a built-in type as well.  Strings are
+<i>immutable values</i>&mdash;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:
+<p>
+{{code "progs/strings.go" `/hello/` `/ciao/`}}
+<p>
+However the following statements are illegal because they would modify
+a <code>string</code> value:
+<p>
+<pre>
+s[0] = 'x'
+(*p)[1] = 'y'
+</pre>
+<p>
+In C++ terms, Go strings are a bit like <code>const strings</code>, while pointers
+to strings are analogous to <code>const string</code> references.
+<p>
+Yes, there are pointers.  However, Go simplifies their use a little;
+read on.
+<p>
+Arrays are declared like this:
+<p>
+<pre>
+var arrayOfInt [10]int
+</pre>
+<p>
+Arrays, like strings, are values, but they are mutable. This differs
+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
+about pointers to arrays.
+<p>
+The size of the array is part of its type; however, one can declare
+a <i>slice</i> variable to hold a reference to any array, of any size,
+with the same element type.
+A <i>slice
+expression</i> has the form <code>a[low : high]</code>, representing
+the internal array indexed from <code>low</code> through <code>high-1</code>; the resulting
+slice is indexed from <code>0</code> through <code>high-low-1</code>.
+In short, slices look a lot like arrays but with
+no explicit size (<code>[]</code> vs. <code>[10]</code>) and they reference a segment of
+an underlying, usually 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 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
+of an array stored within your structure, you should use a regular
+array. To create one, use a compound value <i>constructor</i>&mdash;an
+expression formed
+from a type followed by a brace-bounded expression like this:
+<p>
+<pre>
+[3]int{1,2,3}
+</pre>
+<p>
+In this case the constructor builds an array of 3 <code>ints</code>.
+<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, slice the array to create
+(efficiently) a slice reference and pass that.
+By default, the lower and upper bounds of a slice match the
+ends of the existing object, so the concise notation <code>[:]</code>
+will slice the whole array.
+<p>
+Using slices one can write this function (from <code>sum.go</code>):
+<p>
+{{code "progs/sum.go" `/sum/` `/^}/`}}
+<p>
+Note how the return type (<code>int</code>) is defined for <code>sum</code> by stating it
+after the parameter list.
+<p>
+To call the function, we slice the array.  This intricate call (we'll show
+a simpler way in a moment) constructs
+an array and slices it:
+<p>
+<pre>
+s := sum([3]int{1,2,3}[:])
+</pre>
+<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:
+<p>
+<pre>
+s := sum([...]int{1,2,3}[:])
+</pre>
+<p>
+That's fussier than necessary, though.
+In practice, unless you're meticulous about storage layout within a
+data structure, a slice itself&mdash;using empty brackets with no size&mdash;is all you need:
+<p>
+<pre>
+s := sum([]int{1,2,3})
+</pre>
+<p>
+There are also maps, which you can initialize like this:
+<p>
+<pre>
+m := map[string]int{"one":1 , "two":2}
+</pre>
+<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, maps, and channels.
+<p>
+By the way, another thing that works on strings, arrays, slices, maps
+and channels is the <code>range</code> clause on <code>for</code> loops.  Instead of writing
+<p>
+<pre>
+for i := 0; i &lt; len(a); i++ { ... }
+</pre>
+<p>
+to loop over the elements of a slice (or map or ...) , we could write
+<p>
+<pre>
+for i, v := range a { ... }
+</pre>
+<p>
+This assigns <code>i</code> to the index and <code>v</code> to the value of the successive
+elements of the target of the range.   See
+<a href='/doc/effective_go.html'>Effective Go</a>
+for more examples of its use.
+<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 a new variable, use the built-in function <code>new</code>, which
+returns a pointer to the allocated storage.
+<p>
+<pre>
+type T struct { a, b int }
+var t *T = new(T)
+</pre>
+<p>
+or the more idiomatic
+<p>
+<pre>
+t := new(T)
+</pre>
+<p>
+Some types&mdash;maps, slices, and channels (see below)&mdash;have reference semantics.
+If you're holding a slice or a map and you modify its contents, other variables
+referencing the same underlying data will see the modification.  For these three
+types you want to use the built-in function <code>make</code>:
+<p>
+<pre>
+m := make(map[string]int)
+</pre>
+<p>
+This statement initializes a new map ready to store entries.
+If you just declare the map, as in
+<p>
+<pre>
+var m map[string]int
+</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 from an
+existing map.
+<p>
+Note that <code>new(T)</code> returns type <code>*T</code> while <code>make(T)</code> returns type
+<code>T</code>.  If you (mistakenly) allocate a reference object with <code>new</code> rather than <code>make</code>,
+you receive a pointer to a nil reference, equivalent to
+declaring an uninitialized variable and taking its address.
+<p>
+<h2>An Interlude about Constants</h2>
+<p>
+Although integers come in lots of sizes in Go, integer constants do not.
+There are no constants like <code>0LL</code> or <code>0x0UL</code>.   Instead, integer
+constants are evaluated as large-precision values that
+can overflow only when they are assigned to an integer variable with
+too little precision to represent the value.
+<p>
+<pre>
+const hardEight = (1 &lt;&lt; 100) &gt;&gt; 97  // legal
+</pre>
+<p>
+There are nuances that deserve redirection to the legalese of the
+language specification but here are some illustrative examples:
+<p>
+<pre>
+var a uint64 = 0  // a has type uint64, value 0
+a := uint64(0)    // equivalent; uses a "conversion"
+i := 0x1234       // i gets default type: int
+var j int = 1e6   // legal - 1000000 is representable in an int
+x := 1.5          // a float64, the default type for floating constants
+i3div2 := 3/2     // integer division - result is 1
+f3div2 := 3./2.   // floating-point division - result is 1.5
+</pre>
+<p>
+Conversions only work for simple cases such as converting <code>ints</code> of one
+sign or size to another and between integers and floating-point numbers,
+plus a couple of other instances outside the scope of a tutorial.
+There are no automatic numeric conversions of any kind in Go,
+other than that of making constants have concrete size and type when
+assigned to a variable.
+<p>
+<h2>An I/O Package</h2>
+<p>
+Next we'll look at a simple package for doing file I/O with an
+open/close/read/write interface.  Here's the start of <code>file.go</code>:
+<p>
+{{code "progs/file.go" `/package/` `/^}/`}}
+<p>
+The first few lines declare the name of the
+package&mdash;<code>file</code>&mdash;and then import two packages.  The <code>os</code>
+package hides the differences
+between various operating systems to give a consistent view of files and
+so on; here we're going to use its error handling utilities
+and reproduce the rudiments of its file I/O.
+<p>
+The other item is the low-level, external <code>syscall</code> package, which provides
+a primitive interface to the underlying operating system's calls.
+<p>
+Next is a type definition: the <code>type</code> keyword introduces a type declaration,
+in this case a data structure called <code>File</code>.
+To make things a little more interesting, our <code>File</code> includes the name of the file
+that the file descriptor refers to.
+<p>
+Because <code>File</code> starts with a capital letter, the type is available outside the package,
+that is, by users of the package.   In Go the rule about visibility of information is
+simple: if a name (of a top-level type, function, method, constant or variable, or of
+a structure field or method) is capitalized, users of the package may see it. Otherwise, the
+name and hence the thing being named is visible only inside the package in which
+it is declared.  This is more than a convention; the rule is enforced by the compiler.
+In Go, the term for publicly visible names is ''exported''.
+<p>
+In the case of <code>File</code>, all its fields are lower case and so invisible to users, but we
+will soon give it some exported, upper-case methods.
+<p>
+First, though, here is a factory to create a <code>File</code>:
+<p>
+{{code "progs/file.go" `/newFile/` `/^}/`}}
+<p>
+This returns a pointer to a new <code>File</code> structure with the file descriptor and name
+filled in.  This code uses Go's notion of a ''composite literal'', analogous to
+the ones used to build maps and arrays, to construct a new heap-allocated
+object.  We could write
+<p>
+<pre>
+n := new(File)
+n.fd = fd
+n.name = name
+return n
+</pre>
+<p>
+but for simple structures like <code>File</code> it's easier to return the address of a 
+composite literal, as is done here in the <code>return</code> statement from <code>newFile</code>.
+<p>
+We can use the factory to construct some familiar, exported variables of type <code>*File</code>:
+<p>
+{{code "progs/file.go" `/var/` `/^.$/`}}
+<p>
+The <code>newFile</code> function was not exported because it's internal. The proper,
+exported factory to use is <code>OpenFile</code> (we'll explain that name in a moment):
+<p>
+{{code "progs/file.go" `/func.OpenFile/` `/^}/`}}
+<p>
+There are a number of new things in these few lines.  First, <code>OpenFile</code> returns
+multiple values, a <code>File</code> and an error (more about errors in a moment).
+We declare the
+multi-value return as a parenthesized list of declarations; syntactically
+they look just like a second parameter list.  The function
+<code>syscall.Open</code>
+also has a multi-value return, which we can grab with the multi-variable
+declaration on the first line; it declares <code>r</code> and <code>e</code> to hold the two values,
+both of type <code>int</code> (although you'd have to look at the <code>syscall</code> package
+to see that).  Finally, <code>OpenFile</code> returns two values: a pointer to the new <code>File</code>
+and the error.  If <code>syscall.Open</code> fails, the file descriptor <code>r</code> will
+be negative and <code>newFile</code> will return <code>nil</code>.
+<p>
+About those errors:  The <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 translate Unix's integer <code>errno</code> value into the integer type
+<code>os.Errno</code>, which implements <code>os.Error</code>.
+<p>
+Why <code>OpenFile</code> and not <code>Open</code>? To mimic Go's <code>os</code> package, which
+our exercise is emulating. The <code>os</code> package takes the opportunity
+to make the two commonest cases - open for read and create for
+write - the simplest, just <code>Open</code> and <code>Create</code>.  <code>OpenFile</code> is the
+general case, analogous to the Unix system call <code>Open</code>.  Here is
+the implementation of our <code>Open</code> and <code>Create</code>; they're trivial
+wrappers that eliminate common errors by capturing
+the tricky standard arguments to open and, especially, to create a file:
+<p>
+{{code "progs/file.go" `/^const/` `/^}/`}}
+<p>
+{{code "progs/file.go" `/func.Create/` `/^}/`}}
+<p>
+Back to our main story.
+Now that we can build <code>Files</code>, we can write methods for them. To declare
+a method of a type, we define a function to have an explicit receiver
+of that type, placed
+in parentheses before the function name. Here are some methods for <code>*File</code>,
+each of which declares a receiver variable <code>file</code>.
+<p>
+{{code "progs/file.go" `/Close/` "$"}}
+<p>
+There is no implicit <code>this</code> and the receiver variable must be used to access
+members of the structure.  Methods are not declared within
+the <code>struct</code> declaration itself.  The <code>struct</code> declaration defines only data members.
+In fact, methods can be created for almost any type you name, such as an integer or
+array, not just for <code>structs</code>.   We'll see an example with arrays later.
+<p>
+The <code>String</code> method is so called because of a printing convention we'll
+describe later.
+<p>
+The methods use the public variable <code>os.EINVAL</code> to return the (<code>os.Error</code>
+version of the) Unix error code <code>EINVAL</code>.  The <code>os</code> library defines a standard
+set of such error values.
+<p>
+We can now use our new package:
+<p>
+{{code "progs/helloworld3.go" `/package/` "$"}}
+<p>
+The ''<code>./</code>'' in the import of ''<code>./file</code>'' tells the compiler
+to use our own package rather than
+something from the directory of installed packages.
+(Also, ''<code>file.go</code>'' must be compiled before we can import the
+package.)
+<p>
+Now we can compile and run the program. On Unix, this would be the result:
+<p>
+<pre>
+$ 6g file.go                       # compile file package
+$ 6g helloworld3.go                # compile main package
+$ 6l -o helloworld3 helloworld3.6  # link - no need to mention "file"
+$ helloworld3
+hello, world
+can't open file; err=No such file or directory
+$
+</pre>
+<p>
+<h2>Rotting cats</h2>
+<p>
+Building on the <code>file</code> package, here's a simple version of the Unix utility <code>cat(1)</code>,
+<code>progs/cat.go</code>:
+<p>
+{{code "progs/cat.go" `/package/` "$"}}
+<p>
+By now this should be easy to follow, but the <code>switch</code> statement introduces some
+new features.  Like a <code>for</code> loop, an <code>if</code> or <code>switch</code> can include an
+initialization statement.  The <code>switch</code> statement in <code>cat</code> uses one to create variables
+<code>nr</code> and <code>er</code> to hold the return values from the call to <code>f.Read</code>.  (The <code>if</code> a few lines later
+has the same idea.)  The <code>switch</code> statement is general: it evaluates the cases
+from  top to bottom looking for the first case that matches the value; the
+case expressions don't need to be constants or even integers, as long as
+they all have the same type.
+<p>
+Since the <code>switch</code> value is just <code>true</code>, we could leave it off&mdash;as is also
+the situation
+in a <code>for</code> statement, a missing value means <code>true</code>.  In fact, such a <code>switch</code>
+is a form of <code>if-else</code> chain. While we're here, it should be mentioned that in
+<code>switch</code> statements each <code>case</code> has an implicit <code>break</code>.
+<p>
+The argument to <code>file.Stdout.Write</code> is created by slicing the array <code>buf</code>.
+Slices provide the standard Go way to handle I/O buffers.
+<p>
+Now let's make a variant of <code>cat</code> that optionally does <code>rot13</code> on its input.
+It's easy to do by just processing the bytes, but instead we will exploit
+Go's notion of an <i>interface</i>.
+<p>
+The <code>cat</code> subroutine uses only two methods of <code>f</code>: <code>Read</code> and <code>String</code>,
+so let's start by defining an interface that has exactly those two methods.
+Here is code from <code>progs/cat_rot13.go</code>:
+<p>
+{{code "progs/cat_rot13.go" `/type.reader/` `/^}/`}}
+<p>
+Any type that has the two methods of <code>reader</code>&mdash;regardless of whatever
+other methods the type may also have&mdash;is said to <i>implement</i> the
+interface.  Since <code>file.File</code> implements these methods, it implements the
+<code>reader</code> interface.  We could tweak the <code>cat</code> subroutine to accept a <code>reader</code>
+instead of a <code>*file.File</code> and it would work just fine, but let's embellish a little
+first by writing a second type that implements <code>reader</code>, one that wraps an
+existing <code>reader</code> and does <code>rot13</code> on the data. To do this, we just define
+the type and implement the methods and with no other bookkeeping,
+we have a second implementation of the <code>reader</code> interface.
+<p>
+{{code "progs/cat_rot13.go" `/type.rotate13/` `/end.of.rotate13/`}}
+<p>
+(The <code>rot13</code> function called in <code>Read</code> is trivial and not worth reproducing here.)
+<p>
+To use the new feature, we define a flag:
+<p>
+{{code "progs/cat_rot13.go" `/rot13Flag/`}}
+<p>
+and use it from within a mostly unchanged <code>cat</code> function:
+<p>
+{{code "progs/cat_rot13.go" `/func.cat/` `/^}/`}}
+<p>
+(We could also do the wrapping in <code>main</code> and leave <code>cat</code> mostly alone, except
+for changing the type of the argument; consider that an exercise.)
+The <code>if</code> at the top of <code>cat</code> sets it all up: If the <code>rot13</code> flag is true, wrap the <code>reader</code>
+we received into a <code>rotate13</code> and proceed.  Note that the interface variables
+are values, not pointers: the argument is of type <code>reader</code>, not <code>*reader</code>,
+even though under the covers it holds a pointer to a <code>struct</code>.
+<p>
+Here it is in action:
+<p>
+<pre>
+$ echo abcdefghijklmnopqrstuvwxyz | ./cat
+abcdefghijklmnopqrstuvwxyz
+$ echo abcdefghijklmnopqrstuvwxyz | ./cat --rot13
+nopqrstuvwxyzabcdefghijklm
+$
+</pre>
+<p>
+Fans of dependency injection may take cheer from how easily interfaces
+allow us to substitute the implementation of a file descriptor.
+<p>
+Interfaces are a distinctive feature of Go.  An interface is implemented by a
+type if the type implements all the methods declared in the interface.
+This means
+that a type may implement an arbitrary number of different interfaces.
+There is no type hierarchy; things can be much more <i>ad hoc</i>,
+as we saw with <code>rot13</code>.  The type <code>file.File</code> implements <code>reader</code>; it could also
+implement a <code>writer</code>, or any other interface built from its methods that
+fits the current situation. Consider the <i>empty interface</i>
+<p>
+<pre>
+type Empty interface {}
+</pre>
+<p>
+<i>Every</i> type implements the empty interface, which makes it
+useful for things like containers.
+<p>
+<h2>Sorting</h2>
+<p>
+Interfaces provide a simple form of polymorphism.  They completely
+separate the definition of what an object does from how it does it, allowing
+distinct implementations to be represented at different times by the
+same interface variable.
+<p>
+As an example, consider this simple sort algorithm taken from <code>progs/sort.go</code>:
+<p>
+{{code "progs/sort.go" `/func.Sort/` `/^}/`}}
+<p>
+The code needs only three methods, which we wrap into sort's <code>Interface</code>:
+<p>
+{{code "progs/sort.go" `/interface/` `/^}/`}}
+<p>
+We can apply <code>Sort</code> to any type that implements <code>Len</code>, <code>Less</code>, and <code>Swap</code>.
+The <code>sort</code> package includes the necessary methods to allow sorting of
+arrays of integers, strings, etc.; here's the code for arrays of <code>int</code>
+<p>
+{{code "progs/sort.go" `/type.*IntSlice/` `/Swap/`}}
+<p>
+Here we see methods defined for non-<code>struct</code> types.  You can define methods
+for any type you define and name in your package.
+<p>
+And now a routine to test it out, from <code>progs/sortmain.go</code>.  This
+uses a function in the <code>sort</code> package, omitted here for brevity,
+to test that the result is sorted.
+<p>
+{{code "progs/sortmain.go" `/func.ints/` `/^}/`}}
+<p>
+If we have a new type we want to be able to sort, all we need to do is
+to implement the three methods for that type, like this:
+<p>
+{{code "progs/sortmain.go" `/type.day/` `/Swap/`}}
+<p>
+<p>
+<h2>Printing</h2>
+<p>
+The examples of formatted printing so far have been modest.  In this section
+we'll talk about how formatted I/O can be done well in Go.
+<p>
+We've seen simple uses of the package <code>fmt</code>, which
+implements <code>Printf</code>, <code>Fprintf</code>, and so on.
+Within the <code>fmt</code> package, <code>Printf</code> is declared with this signature:
+<p>
+<pre>
+Printf(format string, v ...interface{}) (n int, errno os.Error)
+</pre>
+<p>
+The token <code>...</code> introduces a variable-length argument list that in C would
+be handled using the <code>stdarg.h</code> macros.
+In Go, variadic functions are passed a slice of the arguments of the
+specified type.  In <code>Printf</code>'s case, the declaration says <code>...interface{}</code>
+so the actual type is a slice of empty interface values, <code>[]interface{}</code>.
+<code>Printf</code> can examine the arguments by iterating over the slice
+and, for each element, using a type switch or the reflection library
+to interpret the value.
+It's off topic here but such run-time type analysis
+helps explain some of the nice properties of Go's <code>Printf</code>,
+due to the ability of <code>Printf</code> to discover the type of its arguments
+dynamically.
+<p>
+For example, in C each format must correspond to the type of its
+argument.  It's easier in many cases in Go.  Instead of <code>%llud</code> you
+can just say <code>%d</code>; <code>Printf</code> knows the size and signedness of the
+integer and can do the right thing for you.  The snippet
+<p>
+{{code "progs/print.go" 10 11}}
+<p>
+prints
+<p>
+<pre>
+18446744073709551615 -1
+</pre>
+<p>
+In fact, if you're lazy the format <code>%v</code> will print, in a simple
+appropriate style, any value, even an array or structure.  The output of
+<p>
+{{code "progs/print.go" 14 20}}
+<p>
+is
+<p>
+<pre>
+18446744073709551615 {77 Sunset Strip} [1 2 3 4]
+</pre>
+<p>
+You can drop the formatting altogether if you use <code>Print</code> or <code>Println</code>
+instead of <code>Printf</code>.  Those routines do fully automatic formatting.
+The <code>Print</code> function just prints its elements out using the equivalent
+of <code>%v</code> while <code>Println</code> inserts spaces between arguments
+and adds a newline.  The output of each of these two lines is identical
+to that of the <code>Printf</code> call above.
+<p>
+{{code "progs/print.go" 21 22}}
+<p>
+If you have your own type you'd like <code>Printf</code> or <code>Print</code> to format,
+just give it a <code>String</code> method that returns a string.  The print
+routines will examine the value to inquire whether it implements
+the method and if so, use it rather than some other formatting.
+Here's a simple example.
+<p>
+{{code "progs/print_string.go" 9 "$"}}
+<p>
+Since <code>*testType</code> has a <code>String</code> method, the
+default formatter for that type will use it and produce the output
+<p>
+<pre>
+77 Sunset Strip
+</pre>
+<p>
+Observe that the <code>String</code> method calls <code>Sprint</code> (the obvious Go
+variant that returns a string) to do its formatting; special formatters
+can use the <code>fmt</code> library recursively.
+<p>
+Another feature of <code>Printf</code> is that the format <code>%T</code> will print a string
+representation of the type of a value, which can be handy when debugging
+polymorphic code.
+<p>
+It's possible to write full custom print formats with flags and precisions
+and such, but that's getting a little off the main thread so we'll leave it
+as an exploration exercise.
+<p>
+You might ask, though, how <code>Printf</code> can tell whether a type implements
+the <code>String</code> method.  Actually what it does is ask if the value can
+be converted to an interface variable that implements the method.
+Schematically, given a value <code>v</code>, it does this:
+<p>
+<p>
+<pre>
+type Stringer interface {
+    String() string
+}
+</pre>
+<p>
+<pre>
+s, ok := v.(Stringer)  // Test whether v implements "String()"
+if ok {
+    result = s.String()
+} else {
+    result = defaultOutput(v)
+}
+</pre>
+<p>
+The code uses a ``type assertion'' (<code>v.(Stringer)</code>) to test if the value stored in
+<code>v</code> satisfies the <code>Stringer</code> interface; if it does, <code>s</code>
+will become an interface variable implementing the method and <code>ok</code> will
+be <code>true</code>.  We then use the interface variable to call the method.
+(The ''comma, ok'' pattern is a Go idiom used to test the success of
+operations such as type conversion, map update, communications, and so on,
+although this is the only appearance in this tutorial.)
+If the value does not satisfy the interface, <code>ok</code> will be false.
+<p>
+In this snippet the name <code>Stringer</code> follows the convention that we add ''[e]r''
+to interfaces describing simple method sets like this.
+<p>
+One last wrinkle.  To complete the suite, besides <code>Printf</code> etc. and <code>Sprintf</code>
+etc., there are also <code>Fprintf</code> etc.  Unlike in C, <code>Fprintf</code>'s first argument is
+not a file.  Instead, it is a variable of type <code>io.Writer</code>, which is an
+interface type defined in the <code>io</code> library:
+<p>
+<pre>
+type Writer interface {
+    Write(p []byte) (n int, err os.Error)
+}
+</pre>
+<p>
+(This interface is another conventional name, this time for <code>Write</code>; there are also
+<code>io.Reader</code>, <code>io.ReadWriter</code>, and so on.)
+Thus you can call <code>Fprintf</code> on any type that implements a standard <code>Write</code>
+method, not just files but also network channels, buffers, whatever
+you want.
+<p>
+<h2>Prime numbers</h2>
+<p>
+Now we come to processes and communication&mdash;concurrent programming.
+It's a big subject so to be brief we assume some familiarity with the topic.
+<p>
+A classic program in the style is a prime sieve.
+(The sieve of Eratosthenes is computationally more efficient than
+the algorithm presented here, but we are more interested in concurrency than
+algorithmics at the moment.)
+It works by taking a stream of all the natural numbers and introducing
+a sequence of filters, one for each prime, to winnow the multiples of
+that prime.  At each step we have a sequence of filters of the primes
+so far, and the next number to pop out is the next prime, which triggers
+the creation of the next filter in the chain.
+<p>
+Here's a flow diagram; each box represents a filter element whose
+creation is triggered by the first number that flowed from the
+elements before it.
+<p>
+<br>
+<p>
+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<img src='sieve.gif'>
+<p>
+<br>
+<p>
+To create a stream of integers, we use a Go <i>channel</i>, which,
+borrowing from CSP's descendants, represents a communications
+channel that can connect two concurrent computations.
+In Go, channel variables are references to a run-time object that
+coordinates the communication; as with maps and slices, use
+<code>make</code> to create a new channel.
+<p>
+Here is the first function in <code>progs/sieve.go</code>:
+<p>
+{{code "progs/sieve.go" `/Send/` `/^}/`}}
+<p>
+The <code>generate</code> function sends the sequence 2, 3, 4, 5, ... to its
+argument channel, <code>ch</code>, using the binary communications operator <code>&lt;-</code>.
+Channel operations block, so if there's no recipient for the value on <code>ch</code>,
+the send operation will wait until one becomes available.
+<p>
+The <code>filter</code> function has three arguments: an input channel, an output
+channel, and a prime number.  It copies values from the input to the
+output, discarding anything divisible by the prime.  The unary communications
+operator <code>&lt;-</code> (receive) retrieves the next value on the channel.
+<p>
+{{code "progs/sieve.go" `/Copy.the/` `/^}/`}}
+<p>
+The generator and filters execute concurrently.  Go has
+its own model of process/threads/light-weight processes/coroutines,
+so to avoid notational confusion we call concurrently executing
+computations in Go <i>goroutines</i>.  To start a goroutine,
+invoke the function, prefixing the call with the keyword <code>go</code>;
+this starts the function running in parallel with the current
+computation but in the same address space:
+<p>
+<pre>
+go sum(hugeArray) // calculate sum in the background
+</pre>
+<p>
+If you want to know when the calculation is done, pass a channel
+on which it can report back:
+<p>
+<pre>
+ch := make(chan int)
+go sum(hugeArray, ch)
+// ... do something else for a while
+result := &lt;-ch  // wait for, and retrieve, result
+</pre>
+<p>
+Back to our prime sieve.  Here's how the sieve pipeline is stitched
+together:
+<p>
+{{code "progs/sieve.go" `/func.main/` `/^}/`}}
+<p>
+The first line of <code>main</code> creates the initial channel to pass to <code>generate</code>, which it
+then starts up.  As each prime pops out of the channel, a new <code>filter</code>
+is added to the pipeline and <i>its</i> output becomes the new value
+of <code>ch</code>.
+<p>
+The sieve program can be tweaked to use a pattern common
+in this style of programming.  Here is a variant version
+of <code>generate</code>, from <code>progs/sieve1.go</code>:
+<p>
+{{code "progs/sieve1.go" `/func.generate/` `/^}/`}}
+<p>
+This version does all the setup internally. It creates the output
+channel, launches a goroutine running a function literal, and
+returns the channel to the caller.  It is a factory for concurrent
+execution, starting the goroutine and returning its connection.
+<p>
+The function literal notation used in the <code>go</code> statement allows us to construct an
+anonymous function and invoke it on the spot. Notice that the local
+variable <code>ch</code> is available to the function literal and lives on even
+after <code>generate</code> returns.
+<p>
+The same change can be made to <code>filter</code>:
+<p>
+{{code "progs/sieve1.go" `/func.filter/` `/^}/`}}
+<p>
+The <code>sieve</code> function's main loop becomes simpler and clearer as a
+result, and while we're at it let's turn it into a factory too:
+<p>
+{{code "progs/sieve1.go" `/func.sieve/` `/^}/`}}
+<p>
+Now <code>main</code>'s interface to the prime sieve is a channel of primes:
+<p>
+{{code "progs/sieve1.go" `/func.main/` `/^}/`}}
+<p>
+<h2>Multiplexing</h2>
+<p>
+With channels, it's possible to serve multiple independent client goroutines without
+writing an explicit multiplexer.  The trick is to send the server a channel in the message,
+which it will then use to reply to the original sender.
+A realistic client-server program is a lot of code, so here is a very simple substitute
+to illustrate the idea.  It starts by defining a <code>request</code> type, which embeds a channel
+that will be used for the reply.
+<p>
+{{code "progs/server.go" `/type.request/` `/^}/`}}
+<p>
+The server will be trivial: it will do simple binary operations on integers.  Here's the
+code that invokes the operation and responds to the request:
+<p>
+{{code "progs/server.go" `/type.binOp/` `/^}/`}}
+<p>
+The type declaration makes <code>binOp</code> represent a function taking two integers and
+returning a third.
+<p>
+The <code>server</code> routine loops forever, receiving requests and, to avoid blocking due to
+a long-running operation, starting a goroutine to do the actual work.
+<p>
+{{code "progs/server.go" `/func.server/` `/^}/`}}
+<p>
+We construct a server in a familiar way, starting it and returning a channel
+connected to it:
+<p>
+{{code "progs/server.go" `/func.startServer/` `/^}/`}}
+<p>
+Here's a simple test.  It starts a server with an addition operator and sends out
+<code>N</code> requests without waiting for the replies.  Only after all the requests are sent
+does it check the results.
+<p>
+{{code "progs/server.go" `/func.main/` `/^}/`}}
+<p>
+One annoyance with this program is that it doesn't shut down the server cleanly; when <code>main</code> returns
+there are a number of lingering goroutines blocked on communication.  To solve this,
+we can provide a second, <code>quit</code> channel to the server:
+<p>
+{{code "progs/server1.go" `/func.startServer/` `/^}/`}}
+<p>
+It passes the quit channel to the <code>server</code> function, which uses it like this:
+<p>
+{{code "progs/server1.go" `/func.server/` `/^}/`}}
+<p>
+Inside <code>server</code>, the <code>select</code> statement chooses which of the multiple communications
+listed by its cases can proceed.  If all are blocked, it waits until one can proceed; if
+multiple can proceed, it chooses one at random.  In this instance, the <code>select</code> allows
+the server to honor requests until it receives a quit message, at which point it
+returns, terminating its execution.
+<p>
+<p>
+All that's left is to strobe the <code>quit</code> channel
+at the end of main:
+<p>
+{{code "progs/server1.go" `/adder,.quit/`}}
+...
+{{code "progs/server1.go" `/quit....true/`}}
+<p>
+There's a lot more to Go programming and concurrent programming in general but this
+quick tour should give you some of the basics.