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939 lines
36 KiB
Plaintext
939 lines
36 KiB
Plaintext
<!-- A Tutorial for the Go Programming Language -->
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Introduction
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----
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This document is a tutorial introduction to the basics of the Go programming
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language, intended for programmers familiar with C or C++. It is not a comprehensive
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guide to the language; at the moment the document closest to that is the
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<a href='/doc/go_spec.html'>language specification</a>.
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After you've read this tutorial, you should look at
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<a href='/doc/effective_go.html'>Effective Go</a>,
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which digs deeper into how the language is used and
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talks about the style and idioms of programming in Go.
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Also, slides from a 3-day course about Go are available.
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They provide some background and a lot of examples:
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<a href='/doc/GoCourseDay1.pdf'>Day 1</a>,
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<a href='/doc/GoCourseDay2.pdf'>Day 2</a>,
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<a href='/doc/GoCourseDay3.pdf'>Day 3</a>.
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The presentation here proceeds through a series of modest programs to illustrate
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key features of the language. All the programs work (at time of writing) and are
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checked into the repository in the directory <a href='/doc/progs'>"/doc/progs/"</a>.
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Program snippets are annotated with the line number in the original file; for
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cleanliness, blank lines remain blank.
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Hello, World
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----
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Let's start in the usual way:
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--PROG progs/helloworld.go /package/ END
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Every Go source file declares, using a "package" statement, which package it's part of.
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It may also import other packages to use their facilities.
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This program imports the package "fmt" to gain access to
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our old, now capitalized and package-qualified, friend, "fmt.Printf".
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Functions are introduced with the "func" keyword.
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The "main" package's "main" function is where the program starts running (after
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any initialization).
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String constants can contain Unicode characters, encoded in UTF-8.
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(In fact, Go source files are defined to be encoded in UTF-8.)
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The comment convention is the same as in C++:
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/* ... */
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// ...
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Later we'll have much more to say about printing.
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Semicolons
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----
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You might have noticed that our program has no semicolons. In Go
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code, the only place you typically see semicolons is separating the
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clauses of "for" loops and the like; they are not necessary after
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every statement.
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In fact, what happens is that the formal language uses semicolons,
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much as in C or Java, but they are inserted automatically
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at the end of every line that looks like the end of a statement. You
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don't need to type them yourself.
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For details about how this is done you can see the language
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specification, but in practice all you need to know is that you
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never need to put a semicolon at the end of a line. (You can put
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them in if you want to write multiple statements per line.) As an
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extra help, you can also leave out a semicolon immediately before
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a closing brace.
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This approach makes for clean-looking, semicolon-free code. The
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one surprise is that it's important to put the opening
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brace of a construct such as an "if" statement on the same line as
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the "if"; if you don't, there are situations that may not compile
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or may give the wrong result. The language forces the brace style
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to some extent.
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Compiling
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----
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Go is a compiled language. At the moment there are two compilers.
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"Gccgo" is a Go compiler that uses the GCC back end. There is also a
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suite of compilers with different (and odd) names for each architecture:
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"6g" for the 64-bit x86, "8g" for the 32-bit x86, and more. These
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compilers run significantly faster but generate less efficient code
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than "gccgo". At the time of writing (late 2009), they also have
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a more robust run-time system although "gccgo" is catching up.
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Here's how to compile and run our program. With "6g", say,
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$ 6g helloworld.go # compile; object goes into helloworld.6
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$ 6l helloworld.6 # link; output goes into 6.out
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$ 6.out
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Hello, world; or Καλημέρα κόσμε; or こんにちは 世界
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$
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With "gccgo" it looks a little more traditional.
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$ gccgo helloworld.go
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$ a.out
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Hello, world; or Καλημέρα κόσμε; or こんにちは 世界
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$
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Echo
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----
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Next up, here's a version of the Unix utility "echo(1)":
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--PROG progs/echo.go /package/ END
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This program is small but it's doing a number of new things. In the last example,
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we saw "func" introduce a function. The keywords "var", "const", and "type"
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(not used yet) also introduce declarations, as does "import".
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Notice that we can group declarations of the same sort into
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parenthesized lists, one item per line, as on lines 7-10 and 14-17.
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But it's not necessary to do so; we could have said
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const Space = " "
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const Newline = "\n"
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This program imports the ""os"" package to access its "Stdout" variable, of type
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"*os.File". The "import" statement is actually a declaration: in its general form,
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as used in our ``hello world'' program,
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it names the identifier ("fmt")
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that will be used to access members of the package imported from the file (""fmt""),
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found in the current directory or in a standard location.
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In this program, though, we've dropped the explicit name from the imports; by default,
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packages are imported using the name defined by the imported package,
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which by convention is of course the file name itself. Our ``hello world'' program
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could have said just "import "fmt"".
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You can specify your
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own import names if you want but it's only necessary if you need to resolve
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a naming conflict.
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Given "os.Stdout" we can use its "WriteString" method to print the string.
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Having imported the "flag" package, line 12 creates a global variable to hold
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the value of echo's "-n" flag. The variable "omitNewline" has type "*bool", pointer
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to "bool".
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In "main.main", we parse the arguments (line 20) and then create a local
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string variable we will use to build the output.
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The declaration statement has the form
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var s string = ""
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This is the "var" keyword, followed by the name of the variable, followed by
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its type, followed by an equals sign and an initial value for the variable.
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Go tries to be terse, and this declaration could be shortened. Since the
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string constant is of type string, we don't have to tell the compiler that.
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We could write
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var s = ""
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or we could go even shorter and write the idiom
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s := ""
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The ":=" operator is used a lot in Go to represent an initializing declaration.
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There's one in the "for" clause on the next line:
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--PROG progs/echo.go /for/
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The "flag" package has parsed the arguments and left the non-flag arguments
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in a list that can be iterated over in the obvious way.
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The Go "for" statement differs from that of C in a number of ways. First,
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it's the only looping construct; there is no "while" or "do". Second,
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there are no parentheses on the clause, but the braces on the body
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are mandatory. The same applies to the "if" and "switch" statements.
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Later examples will show some other ways "for" can be written.
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The body of the loop builds up the string "s" by appending (using "+=")
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the arguments and separating spaces. After the loop, if the "-n" flag is not
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set, the program appends a newline. Finally, it writes the result.
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Notice that "main.main" is a niladic function with no return type.
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It's defined that way. Falling off the end of "main.main" means
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''success''; if you want to signal an erroneous return, call
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os.Exit(1)
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The "os" package contains other essentials for getting
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started; for instance, "os.Args" is a slice used by the
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"flag" package to access the command-line arguments.
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An Interlude about Types
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----
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Go has some familiar types such as "int" and "uint" (unsigned "int"), which represent
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values of the ''appropriate'' size for the machine. It also defines
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explicitly-sized types such as "int8", "float64", and so on, plus
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unsigned integer types such as "uint", "uint32", etc.
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These are distinct types; even if "int" and "int32" are both 32 bits in size,
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they are not the same type. There is also a "byte" synonym for
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"uint8", which is the element type for strings.
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Floating-point types are always sized: "float32" and "float64",
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plus "complex64" (two "float32s") and "complex128"
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(two "float64s"). Complex numbers are outside the
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scope of this tutorial.
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Speaking of "string", that's a built-in type as well. Strings are
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<i>immutable values</i>—they are not just arrays of "byte" values.
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Once you've built a string <i>value</i>, you can't change it, although
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of course you can change a string <i>variable</i> simply by
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reassigning it. This snippet from "strings.go" is legal code:
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--PROG progs/strings.go /hello/ /ciao/
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However the following statements are illegal because they would modify
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a "string" value:
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s[0] = 'x'
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(*p)[1] = 'y'
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In C++ terms, Go strings are a bit like "const strings", while pointers
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to strings are analogous to "const string" references.
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Yes, there are pointers. However, Go simplifies their use a little;
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read on.
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Arrays are declared like this:
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var arrayOfInt [10]int
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Arrays, like strings, are values, but they are mutable. This differs
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from C, in which "arrayOfInt" would be usable as a pointer to "int".
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In Go, since arrays are values, it's meaningful (and useful) to talk
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about pointers to arrays.
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The size of the array is part of its type; however, one can declare
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a <i>slice</i> variable to hold a reference to any array, of any size,
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with the same element type.
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A <i>slice
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expression</i> has the form "a[low : high]", representing
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the internal array indexed from "low" through "high-1"; the resulting
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slice is indexed from "0" through "high-low-1".
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In short, slices look a lot like arrays but with
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no explicit size ("[]" vs. "[10]") and they reference a segment of
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an underlying, usually anonymous, regular array. Multiple slices
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can share data if they represent pieces of the same array;
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multiple arrays can never share data.
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Slices are much more common in Go programs than
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regular arrays; they're more flexible, have reference semantics,
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and are efficient. What they lack is the precise control of storage
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layout of a regular array; if you want to have a hundred elements
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of an array stored within your structure, you should use a regular
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array. To create one, use a compound value <i>constructor</i>—an
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expression formed
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from a type followed by a brace-bounded expression like this:
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[3]int{1,2,3}
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In this case the constructor builds an array of 3 "ints".
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When passing an array to a function, you almost always want
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to declare the formal parameter to be a slice. When you call
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the function, slice the array to create
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(efficiently) a slice reference and pass that.
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By default, the lower and upper bounds of a slice match the
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ends of the existing object, so the concise notation "[:]"
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will slice the whole array.
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Using slices one can write this function (from "sum.go"):
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--PROG progs/sum.go /sum/ /^}/
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Note how the return type ("int") is defined for "sum" by stating it
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after the parameter list.
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To call the function, we slice the array. This intricate call (we'll show
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a simpler way in a moment) constructs
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an array and slices it:
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s := sum([3]int{1,2,3}[:])
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If you are creating a regular array but want the compiler to count the
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elements for you, use "..." as the array size:
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s := sum([...]int{1,2,3}[:])
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That's fussier than necessary, though.
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In practice, unless you're meticulous about storage layout within a
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data structure, a slice itself—using empty brackets with no size—is all you need:
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s := sum([]int{1,2,3})
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There are also maps, which you can initialize like this:
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m := map[string]int{"one":1 , "two":2}
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The built-in function "len", which returns number of elements,
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makes its first appearance in "sum". It works on strings, arrays,
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slices, maps, and channels.
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By the way, another thing that works on strings, arrays, slices, maps
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and channels is the "range" clause on "for" loops. Instead of writing
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for i := 0; i < len(a); i++ { ... }
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to loop over the elements of a slice (or map or ...) , we could write
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for i, v := range a { ... }
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This assigns "i" to the index and "v" to the value of the successive
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elements of the target of the range. See
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<a href='/doc/effective_go.html'>Effective Go</a>
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for more examples of its use.
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An Interlude about Allocation
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----
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Most types in Go are values. If you have an "int" or a "struct"
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or an array, assignment
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copies the contents of the object.
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To allocate a new variable, use the built-in function "new", which
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returns a pointer to the allocated storage.
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type T struct { a, b int }
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var t *T = new(T)
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or the more idiomatic
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t := new(T)
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Some types—maps, slices, and channels (see below)—have reference semantics.
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If you're holding a slice or a map and you modify its contents, other variables
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referencing the same underlying data will see the modification. For these three
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types you want to use the built-in function "make":
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m := make(map[string]int)
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This statement initializes a new map ready to store entries.
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If you just declare the map, as in
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var m map[string]int
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it creates a "nil" reference that cannot hold anything. To use the map,
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you must first initialize the reference using "make" or by assignment from an
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existing map.
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Note that "new(T)" returns type "*T" while "make(T)" returns type
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"T". If you (mistakenly) allocate a reference object with "new" rather than "make",
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you receive a pointer to a nil reference, equivalent to
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declaring an uninitialized variable and taking its address.
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An Interlude about Constants
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----
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Although integers come in lots of sizes in Go, integer constants do not.
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There are no constants like "0LL" or "0x0UL". Instead, integer
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constants are evaluated as large-precision values that
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can overflow only when they are assigned to an integer variable with
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too little precision to represent the value.
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const hardEight = (1 << 100) >> 97 // legal
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There are nuances that deserve redirection to the legalese of the
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language specification but here are some illustrative examples:
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var a uint64 = 0 // a has type uint64, value 0
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a := uint64(0) // equivalent; uses a "conversion"
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i := 0x1234 // i gets default type: int
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var j int = 1e6 // legal - 1000000 is representable in an int
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x := 1.5 // a float64, the default type for floating constants
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i3div2 := 3/2 // integer division - result is 1
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f3div2 := 3./2. // floating-point division - result is 1.5
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Conversions only work for simple cases such as converting "ints" of one
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sign or size to another and between integers and floating-point numbers,
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plus a couple of other instances outside the scope of a tutorial.
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There are no automatic numeric conversions of any kind in Go,
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other than that of making constants have concrete size and type when
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assigned to a variable.
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An I/O Package
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----
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Next we'll look at a simple package for doing file I/O with an
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open/close/read/write interface. Here's the start of "file.go":
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--PROG progs/file.go /package/ /^}/
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The first few lines declare the name of the
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package—"file"—and then import two packages. The "os"
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package hides the differences
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between various operating systems to give a consistent view of files and
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so on; here we're going to use its error handling utilities
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and reproduce the rudiments of its file I/O.
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The other item is the low-level, external "syscall" package, which provides
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a primitive interface to the underlying operating system's calls.
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Next is a type definition: the "type" keyword introduces a type declaration,
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in this case a data structure called "File".
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To make things a little more interesting, our "File" includes the name of the file
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that the file descriptor refers to.
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Because "File" starts with a capital letter, the type is available outside the package,
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that is, by users of the package. In Go the rule about visibility of information is
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simple: if a name (of a top-level type, function, method, constant or variable, or of
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a structure field or method) is capitalized, users of the package may see it. Otherwise, the
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name and hence the thing being named is visible only inside the package in which
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it is declared. This is more than a convention; the rule is enforced by the compiler.
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In Go, the term for publicly visible names is ''exported''.
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In the case of "File", all its fields are lower case and so invisible to users, but we
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will soon give it some exported, upper-case methods.
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First, though, here is a factory to create a "File":
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--PROG progs/file.go /newFile/ /^}/
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This returns a pointer to a new "File" structure with the file descriptor and name
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filled in. This code uses Go's notion of a ''composite literal'', analogous to
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the ones used to build maps and arrays, to construct a new heap-allocated
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object. We could write
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n := new(File)
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n.fd = fd
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n.name = name
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return n
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but for simple structures like "File" it's easier to return the address of a
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composite literal, as is done here on line 21.
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We can use the factory to construct some familiar, exported variables of type "*File":
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--PROG progs/file.go /var/ /^.$/
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The "newFile" function was not exported because it's internal. The proper,
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exported factory to use is "OpenFile" (we'll explain that name in a moment):
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--PROG progs/file.go /func.OpenFile/ /^}/
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There are a number of new things in these few lines. First, "OpenFile" returns
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multiple values, a "File" and an error (more about errors in a moment).
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We declare the
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multi-value return as a parenthesized list of declarations; syntactically
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they look just like a second parameter list. The function
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"syscall.Open"
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also has a multi-value return, which we can grab with the multi-variable
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declaration on line 31; it declares "r" and "e" to hold the two values,
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both of type "int" (although you'd have to look at the "syscall" package
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to see that). Finally, line 35 returns two values: a pointer to the new "File"
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and the error. If "syscall.Open" fails, the file descriptor "r" will
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be negative and "newFile" will return "nil".
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About those errors: The "os" library includes a general notion of an error.
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It's a good idea to use its facility in your own interfaces, as we do here, for
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consistent error handling throughout Go code. In "Open" we use a
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conversion to translate Unix's integer "errno" value into the integer type
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"os.Errno", which implements "os.Error".
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Why "OpenFile" and not "Open"? To mimic Go's "os" package, which
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our exercise is emulating. The "os" package takes the opportunity
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to make the two commonest cases - open for read and create for
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write - the simplest, just "Open" and "Create". "OpenFile" is the
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general case, analogous to the Unix system call "Open". Here is
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the implementation of our "Open" and "Create"; they're trivial
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wrappers that eliminate common errors by capturing
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the tricky standard arguments to open and, especially, to create a file:
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--PROG progs/file.go /^const/ /^}/
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--PROG progs/file.go /func.Create/ /^}/
|
|
|
|
Back to our main story.
|
|
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
|
|
of that type, placed
|
|
in parentheses before the function name. Here are some methods for "*File",
|
|
each of which declares a receiver variable "file".
|
|
|
|
--PROG progs/file.go /Close/ END
|
|
|
|
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 almost any type you name, such as an integer or
|
|
array, not just for "structs". We'll see an example with arrays later.
|
|
|
|
The "String" method is so called because of a printing convention we'll
|
|
describe later.
|
|
|
|
The methods use the public variable "os.EINVAL" to return the ("os.Error"
|
|
version of the) Unix error code "EINVAL". The "os" library defines a standard
|
|
set of such error values.
|
|
|
|
We can now use our new package:
|
|
|
|
--PROG progs/helloworld3.go /package/ END
|
|
|
|
The ''"./"'' in the import of ''"./file"'' tells the compiler
|
|
to use our own package rather than
|
|
something from the directory of installed packages.
|
|
(Also, ''"file.go"'' must be compiled before we can import the
|
|
package.)
|
|
|
|
Now we can compile and run the program. On Unix, this would be the result:
|
|
|
|
$ 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
|
|
$
|
|
|
|
Rotting cats
|
|
----
|
|
|
|
Building on the "file" package, here's a simple version of the Unix utility "cat(1)",
|
|
"progs/cat.go":
|
|
|
|
--PROG progs/cat.go /package/ END
|
|
|
|
By now this should be easy to follow, but the "switch" statement introduces some
|
|
new features. Like a "for" loop, an "if" or "switch" can include an
|
|
initialization statement. The "switch" on line 18 uses one to create variables
|
|
"nr" and "er" to hold the return values from the call to "f.Read". (The "if" on line 25
|
|
has the same idea.) The "switch" 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.
|
|
|
|
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
|
|
"switch" statements each "case" has an implicit "break".
|
|
|
|
Line 25 calls "Write" by slicing the incoming buffer, which is itself a slice.
|
|
Slices provide the standard Go way to handle I/O buffers.
|
|
|
|
Now let's make a variant of "cat" that optionally does "rot13" 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>.
|
|
|
|
The "cat" subroutine uses only two methods of "f": "Read" and "String",
|
|
so let's start by defining an interface that has exactly those two methods.
|
|
Here is code from "progs/cat_rot13.go":
|
|
|
|
--PROG progs/cat_rot13.go /type.reader/ /^}/
|
|
|
|
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
|
|
first by writing a second type that implements "reader", one that wraps an
|
|
existing "reader" and does "rot13" 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 "reader" interface.
|
|
|
|
--PROG progs/cat_rot13.go /type.rotate13/ /end.of.rotate13/
|
|
|
|
(The "rot13" function called on line 42 is trivial and not worth reproducing here.)
|
|
|
|
To use the new feature, we define a flag:
|
|
|
|
--PROG progs/cat_rot13.go /rot13Flag/
|
|
|
|
and use it from within a mostly unchanged "cat" function:
|
|
|
|
--PROG progs/cat_rot13.go /func.cat/ /^}/
|
|
|
|
(We could also do the wrapping in "main" and leave "cat" mostly alone, except
|
|
for changing the type of the argument; consider that an exercise.)
|
|
Lines 56 through 58 set it all up: If the "rot13" flag is true, wrap the "reader"
|
|
we received into a "rotate13" and proceed. Note that the interface variables
|
|
are values, not pointers: the argument is of type "reader", not "*reader",
|
|
even though under the covers it holds a pointer to a "struct".
|
|
|
|
Here it is in action:
|
|
|
|
<pre>
|
|
$ echo abcdefghijklmnopqrstuvwxyz | ./cat
|
|
abcdefghijklmnopqrstuvwxyz
|
|
$ echo abcdefghijklmnopqrstuvwxyz | ./cat --rot13
|
|
nopqrstuvwxyzabcdefghijklm
|
|
$
|
|
</pre>
|
|
|
|
Fans of dependency injection may take cheer from how easily interfaces
|
|
allow us to substitute the implementation of a file descriptor.
|
|
|
|
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 "rot13". The type "file.File" implements "reader"; it could also
|
|
implement a "writer", or any other interface built from its methods that
|
|
fits the current situation. Consider the <i>empty interface</i>
|
|
|
|
<pre>
|
|
type Empty interface {}
|
|
</pre>
|
|
|
|
<i>Every</i> type implements the empty interface, which makes it
|
|
useful for things like containers.
|
|
|
|
Sorting
|
|
----
|
|
|
|
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.
|
|
|
|
As an example, consider this simple sort algorithm taken from "progs/sort.go":
|
|
|
|
--PROG progs/sort.go /func.Sort/ /^}/
|
|
|
|
The code needs only three methods, which we wrap into sort's "Interface":
|
|
|
|
--PROG progs/sort.go /interface/ /^}/
|
|
|
|
We can apply "Sort" to any type that implements "Len", "Less", and "Swap".
|
|
The "sort" package includes the necessary methods to allow sorting of
|
|
arrays of integers, strings, etc.; here's the code for arrays of "int"
|
|
|
|
--PROG progs/sort.go /type.*IntSlice/ /Swap/
|
|
|
|
Here we see methods defined for non-"struct" types. You can define methods
|
|
for any type you define and name in your package.
|
|
|
|
And now a routine to test it out, from "progs/sortmain.go". This
|
|
uses a function in the "sort" package, omitted here for brevity,
|
|
to test that the result is sorted.
|
|
|
|
--PROG progs/sortmain.go /func.ints/ /^}/
|
|
|
|
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:
|
|
|
|
--PROG progs/sortmain.go /type.day/ /Swap/
|
|
|
|
|
|
Printing
|
|
----
|
|
|
|
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.
|
|
|
|
We've seen simple uses of the package "fmt", which
|
|
implements "Printf", "Fprintf", and so on.
|
|
Within the "fmt" package, "Printf" is declared with this signature:
|
|
|
|
Printf(format string, v ...interface{}) (n int, errno os.Error)
|
|
|
|
The token "..." introduces a variable-length argument list that in C would
|
|
be handled using the "stdarg.h" macros.
|
|
In Go, variadic functions are passed a slice of the arguments of the
|
|
specified type. In "Printf"'s case, the declaration says "...interface{}"
|
|
so the actual type is a slice of empty interface values, "[]interface{}".
|
|
"Printf" 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 "Printf",
|
|
due to the ability of "Printf" to discover the type of its arguments
|
|
dynamically.
|
|
|
|
For example, in C each format must correspond to the type of its
|
|
argument. It's easier in many cases in Go. Instead of "%llud" you
|
|
can just say "%d"; "Printf" knows the size and signedness of the
|
|
integer and can do the right thing for you. The snippet
|
|
|
|
--PROG progs/print.go 'NR==10' 'NR==11'
|
|
|
|
prints
|
|
|
|
18446744073709551615 -1
|
|
|
|
In fact, if you're lazy the format "%v" will print, in a simple
|
|
appropriate style, any value, even an array or structure. The output of
|
|
|
|
--PROG progs/print.go 'NR==14' 'NR==20'
|
|
|
|
is
|
|
|
|
18446744073709551615 {77 Sunset Strip} [1 2 3 4]
|
|
|
|
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" 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.
|
|
|
|
--PROG progs/print.go 'NR==21' 'NR==22'
|
|
|
|
If you have your own type you'd like "Printf" or "Print" to format,
|
|
just give it a "String" 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.
|
|
|
|
--PROG progs/print_string.go 'NR==9' END
|
|
|
|
Since "*testType" has a "String" method, the
|
|
default formatter for that type will use it and produce the output
|
|
|
|
77 Sunset Strip
|
|
|
|
Observe that the "String" method calls "Sprint" (the obvious Go
|
|
variant that returns a string) to do its formatting; special formatters
|
|
can use the "fmt" library recursively.
|
|
|
|
Another feature of "Printf" is that the format "%T" will print a string
|
|
representation of the type of a value, which can be handy when debugging
|
|
polymorphic code.
|
|
|
|
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.
|
|
|
|
You might ask, though, how "Printf" can tell whether a type implements
|
|
the "String" 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 "v", it does this:
|
|
|
|
|
|
type Stringer interface {
|
|
String() string
|
|
}
|
|
|
|
s, ok := v.(Stringer) // Test whether v implements "String()"
|
|
if ok {
|
|
result = s.String()
|
|
} else {
|
|
result = defaultOutput(v)
|
|
}
|
|
|
|
The code uses a ``type assertion'' ("v.(Stringer)") to test if the value stored in
|
|
"v" satisfies the "Stringer" interface; if it does, "s"
|
|
will become an interface variable implementing the method and "ok" will
|
|
be "true". 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, "ok" will be false.
|
|
|
|
In this snippet the name "Stringer" follows the convention that we add ''[e]r''
|
|
to interfaces describing simple method sets like this.
|
|
|
|
One last wrinkle. To complete the suite, besides "Printf" etc. and "Sprintf"
|
|
etc., there are also "Fprintf" etc. Unlike in C, "Fprintf"'s first argument is
|
|
not a file. Instead, it is a variable of type "io.Writer", which is an
|
|
interface type defined in the "io" library:
|
|
|
|
type Writer interface {
|
|
Write(p []byte) (n int, err os.Error)
|
|
}
|
|
|
|
(This interface is another conventional name, this time for "Write"; there are also
|
|
"io.Reader", "io.ReadWriter", and so on.)
|
|
Thus you can call "Fprintf" on any type that implements a standard "Write"
|
|
method, not just files but also network channels, buffers, whatever
|
|
you want.
|
|
|
|
Prime numbers
|
|
----
|
|
|
|
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.
|
|
(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.
|
|
|
|
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.
|
|
|
|
<br>
|
|
|
|
<img src='sieve.gif'>
|
|
|
|
<br>
|
|
|
|
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
|
|
"make" to create a new channel.
|
|
|
|
Here is the first function in "progs/sieve.go":
|
|
|
|
--PROG progs/sieve.go /Send/ /^}/
|
|
|
|
The "generate" function sends the sequence 2, 3, 4, 5, ... to its
|
|
argument channel, "ch", using the binary communications operator "<-".
|
|
Channel operations block, so if there's no recipient for the value on "ch",
|
|
the send operation will wait until one becomes available.
|
|
|
|
The "filter" 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 "<-" (receive) retrieves the next value on the channel.
|
|
|
|
--PROG progs/sieve.go /Copy.the/ /^}/
|
|
|
|
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 "go";
|
|
this starts the function running in parallel with the current
|
|
computation but in the same address space:
|
|
|
|
go sum(hugeArray) // calculate sum in the background
|
|
|
|
If you want to know when the calculation is done, pass a channel
|
|
on which it can report back:
|
|
|
|
ch := make(chan int)
|
|
go sum(hugeArray, ch)
|
|
// ... do something else for a while
|
|
result := <-ch // wait for, and retrieve, result
|
|
|
|
Back to our prime sieve. Here's how the sieve pipeline is stitched
|
|
together:
|
|
|
|
--PROG progs/sieve.go /func.main/ /^}/
|
|
|
|
Line 29 creates the initial channel to pass to "generate", which it
|
|
then starts up. As each prime pops out of the channel, a new "filter"
|
|
is added to the pipeline and <i>its</i> output becomes the new value
|
|
of "ch".
|
|
|
|
The sieve program can be tweaked to use a pattern common
|
|
in this style of programming. Here is a variant version
|
|
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 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.
|
|
|
|
The function literal notation (lines 12-16) allows us to construct an
|
|
anonymous function and invoke it on the spot. Notice that the local
|
|
variable "ch" is available to the function literal and lives on even
|
|
after "generate" returns.
|
|
|
|
The same change can be made to "filter":
|
|
|
|
--PROG progs/sieve1.go /func.filter/ /^}/
|
|
|
|
The "sieve" 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:
|
|
|
|
--PROG progs/sieve1.go /func.sieve/ /^}/
|
|
|
|
Now "main"'s interface to the prime sieve is a channel of primes:
|
|
|
|
--PROG progs/sieve1.go /func.main/ /^}/
|
|
|
|
Multiplexing
|
|
----
|
|
|
|
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 "request" type, which embeds a channel
|
|
that will be used for the reply.
|
|
|
|
--PROG progs/server.go /type.request/ /^}/
|
|
|
|
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:
|
|
|
|
--PROG progs/server.go /type.binOp/ /^}/
|
|
|
|
Line 14 defines the name "binOp" to be a function taking two integers and
|
|
returning a third.
|
|
|
|
The "server" routine loops forever, receiving requests and, to avoid blocking due to
|
|
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 and returning a channel
|
|
connected to it:
|
|
|
|
--PROG progs/server.go /func.startServer/ /^}/
|
|
|
|
Here's a simple test. It starts a server with an addition operator and sends out
|
|
"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 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:
|
|
|
|
--PROG progs/server1.go /func.startServer/ /^}/
|
|
|
|
It passes the quit channel to the "server" function, which uses it like this:
|
|
|
|
--PROG progs/server1.go /func.server/ /^}/
|
|
|
|
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
|
|
returns, terminating its execution.
|
|
|
|
|
|
All that's left is to strobe the "quit" channel
|
|
at the end of main:
|
|
|
|
--PROG progs/server1.go /adder,.quit/
|
|
...
|
|
--PROG progs/server1.go /quit....true/
|
|
|
|
There's a lot more to Go programming and concurrent programming in general but this
|
|
quick tour should give you some of the basics.
|