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136 lines
4.5 KiB
Plaintext
136 lines
4.5 KiB
Plaintext
Get Going
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----
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Rob Pike
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----
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(September 10, 2008)
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This document is a tutorial introduction to the basics of the Go systems 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 closest to that is the draft specification:
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/doc/go_lang.html
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To check out the compiler and tools and be ready to run Go programs, see
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/doc/go_setup.html
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The presentation 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 in at
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/doc/progs
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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
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Every Go source file declares which package it's part of using a "package" statement.
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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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Function declarations are introduced with the "func" keyword.
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Notice that string constants can contain Unicode characters, encoded in UTF-8.
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Go is defined to accept UTF-8 input. Strings are arrays of bytes, usually used
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to store Unicode strings represented in UTF-8.
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The built-in function "print()" has been used during the early stages of
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development of the language but is not guaranteed to last. Here's a better version of the
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program that doesn't depend on this "print()":
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--PROG progs/helloworld2.go
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This version imports the ''os'' package to acess its "Stdout" variable, of type
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"*OS.FD"; given "OS.Stdout" we can use its "WriteString" method to print the string.
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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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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
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It's still fairly small but it's doing a number of new things. In the last example,
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we saw "func" introducing 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, semicolon-separated lists if we want, as on lines 3-6 and 10-13.
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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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Semicolons aren't needed here; in fact, semicolons are unnecessary after any
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top-level declaration, even though they are needed as separators <i>within</i>
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a parenthesized list of declarations.
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Having imported the "Flag" package, line 8 creates a global variable to hold
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the value of echo's -n flag. (The nil indicates a nice feature not needed here;
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see the source in "src/lib/flag.go" for details).
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In "main.main", we parse the arguments (line 16) 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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(For those who know Limbo, it's the same, except notice that there is no
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colon after the name in a full "var" declaration.)
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And here'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-flags in
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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. Later examples will show some other ways "for"
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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 flags and separating spaces. After the loop, if the "-n" flag is not
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set, it appends a newline, and then 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 erroneous return, use
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sys.exit(1)
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The "sys" package is built in and contains some essentials for getting
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started; for instance, "sys.argc()" and "sys.argv(int)" are used by the
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"Flag" package to access the arguments.
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More to come.
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