go/doc/go_tutorial.txt

Let's Go
----

Rob Pike

----
(September 10, 2008)


This document is a tutorial introduction to the basics of the Go systems 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 draft
specification:

	/doc/go_lang.html

To check out the compiler and tools and be ready to run Go programs, see

	/doc/go_setup.html

The presentation 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 in at

	/doc/progs

Program snippets are annotated with the line number in the original file; for
cleanliness, blank lines remain blank.

Hello, World
----

Let's start in the usual way:

--PROG progs/helloworld.go

Every Go source file declares which package it's part of using a "package" statement.
The "main" package's "main" function is where the program starts running (after
any initialization).

Function declarations are introduced with the "func" keyword.

Notice that string constants can contain Unicode characters, encoded in UTF-8.
Go is defined to accept UTF-8 input.  Strings are arrays of bytes, usually used
to store Unicode strings represented in UTF-8.

The built-in function "print()" has been used during the early stages of
development of the language but is not guaranteed to last.  Here's a better version of the
program that doesn't depend on "print()":

--PROG progs/helloworld2.go

This version imports the ''os'' package to acess its "Stdout" variable, of type
"*OS.FD"; given "OS.Stdout" we can use its "WriteString" method to print the string.

The comment convention is the same as in C++:

	/* ... */
	// ...

Echo
----

Next up, here's a version of the Unix utility "echo(1)":

--PROG progs/echo.go

It's still fairly small but it's doing a number of new things.  In the last example,
we saw "func" introducing a function.  The keywords "var", "const", and "type"
(not used yet) also introduce declarations, as does "import".
Notice that we can group declarations of the same sort into
parenthesized, semicolon-separated lists if we want, as on lines 3-6 and 10-13.
But it's not necessary to do so; we could have said

	const Space = " "
	const Newline = "\n"

Semicolons aren't needed here; in fact, semicolons are unnecessary after any
top-level declaration, even though they are needed as separators <i>within</i>
a parenthesized list of declarations.

Having imported the "Flag" package, line 8 creates a global variable to hold
the value of echo's -n flag.  (The nil hides a nice feature not needed here;
see the source in "src/lib/flag.go" for details).

In "main.main", we parse the arguments (line 16) and then create a local
string variable we will use to build the output.

The declaration statement has the form

	var s string = "";

This is the "var" keyword, followed by the name of the variable, followed by
its type, followed by an equals sign and an initial value for the variable.

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

	var s = "";

or we could go even shorter and write the idiom

	s := "";

The := operator is used a lot in Go to represent an initializing declaration.
(For those who know Limbo, its := construct is the same, but notice
that Go has no colon after the name in a full "var" declaration.)
And there's one in the "for" clause on the next line:

--PROG  progs/echo.go /for/

The "Flag" package has parsed the arguments and left the non-flag arguments
in a list that can be iterated over in the obvious way.

The Go "for" statement differs from that of C in a number of ways.  First,
it's the only looping construct; there is no "while" or "do".  Second,
there are no parentheses on the clause, but the braces on the body
are mandatory.  (The same applies to the "if" statement.) Later examples
will show some other ways "for" can be written.

The body of the loop builds up the string "s" by appending (using +=)
the flags and separating spaces. After the loop, if the "-n" flag is not
set, it appends a newline, and then writes the result.

Notice that "main.main" is a niladic function with no return type.
It's defined that way.  Falling off the end of "main.main" means
''success''; if you want to signal erroneous return, use

	sys.exit(1)

The "sys" package is built in and contains some essentials for getting
started; for instance, "sys.argc()" and "sys.argv(int)" are used by the
"Flag" package to access the arguments.

An Interlude about Types
----

Go has some familiar types such as "int" and "float", which represent
values of the ''appropriate'' size for the machine. It also defines
specifically-sized types such as "int8", "float64", and so on, plus
unsigned integer types such as "uint", "uint32", etc.  And then there
is a "byte" synonym for "uint8", which is the element type for
strings.

Speaking of "string", that's a built-in type as well.  Strings are
<i>immutable values</i> -- they are not just arrays of "byte" values.
Once you've built a string <i>value</i>, you can't change it, although
of course you can change a string <i>variable</i> simply by
reassigning it.  This snippet from "strings.go" is legal code:

--PROG progs/strings.go /hello/ /ciao/

However the following statements are illegal because they would modify
a "string" value:

	s[0] = 'x';
	(*p)[1] = 'y';

In C++ terms, Go strings are a bit like "const strings", while pointers
to strings are analogous to "const string" references.

Yes, there are pointers.  However, Go simplifies their use a little;
read on.

Arrays are declared like this:

	var array_of_int [10]int;

Arrays, like strings, are values, but they are mutable. This differs
from C, in which "array_of_int" would be usable as a pointer to "int".
In Go, since arrays are values, it's meaningful (and useful) to talk
about pointers to arrays.

The size of the array is part of its type; however, one can declare
an <i>open array</i> variable, to which one can assign any array value
with the same element type.
(At the moment, only <i>pointers</i> to open arrays are implemented.)
Thus one can write this function (from "sum.go"):

--PROG progs/sum.go /sum/ /^}/

and invoke it like this:

--PROG progs/sum.go /1,2,3/

Note how the return type ("int") is defined for "sum()" by stating it
after the parameter list.  Also observe that although the argument
is a pointer to an array, we can index it directly ("a[i]" not "(*a)[i]").
The expression "[]int{1,2,3}" -- a type followed by a brace-bounded expression
-- is a constructor for a value, in this case an array of "int". We pass it
to "sum()" by taking its address.

The built-in function "len()" appeared there too - it works on strings,
arrays, and maps, which can be built like this:

	m := map[string] int {"one":1 , "two":2}

At least for now, maps are <i>always</i> pointers, so in this example
"m" has type "*map[string]int".  This may change.

You can also create a map (or anything else) with the built-in "new()"
function:

	m := new(map[string] int)

The "new()" function always returns a pointer, an address for the object
it creates.

An Interlude about Constants
----

Although integers come in lots of sizes in Go, integer constants do not.
There are no constants like "0ll" or "0x0UL".   Instead, integer
constants are evaluated as ideal, arbitrary precision values that
can overflow only when they are assigned to an integer variable of
some specific size.

	const hard_eight = (1 << 100) >> 97  // legal

There are nuances that deserve redirection to the legalese of the
language specification but here are some illustrative examples:

	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 float
	i3div2 = 3/2      // integer division - result is 1
	f3div2 = 3./2.    // floating point division - result is 1.5

Conversions only work for simple cases such as converting ints of one
sign or size to another, and between ints and floats, plus a few other
simple cases.  There are no automatic conversions of any kind in Go,
other than that of making constants have concrete size and type when
assigned to a variable.

An I/O Package
----

Next we'll look at a simple package for doing file I/O with the usual
sort of open/close/read/write interface.  Here's the start of "fd.go":

--PROG progs/fd.go /package/ /^}/

The first line declares the name of the package -- "fd" for ''file descriptor'' --
and then we import the low-level, external "syscall" package, which provides
a primitive interface to the underlying operating system's calls.

Next is a type definition: the "type" keyword introduces a type declaration,
in this case a data structure called "FD".
To make things a little more interesting, our "FD" includes the name of the file
that the file descriptor refers to.  The "export" keyword makes the declared
structure visible to users of the package.

Now we can write what is often called a factory:

--PROG progs/fd.go /NewFD/ /^}/

This returns a pointer to a new "FD" structure with the file descriptor and name
filled in.  We can use it to construct some familiar, exported variables of type "*FD":

--PROG progs/fd.go /export.var/ /^.$/

The "NewFD" function was not exported because it's internal. The proper factory
to use is "Open":

--PROG progs/fd.go /func.Open/ /^}/

There are a number of new things in these few lines.  First, "Open" returns
multiple values, an "FD" and an "errno" (Unix error number).  We declare the
multi-value return as a parenthesized list of declarations.  "Syscall.open"
also has a multi-value return, which we can grab with the multi-variable
declaration on line 27; it declares "r" and "e" to hold the two values,
both of type "int64" (although you'd have to look at the "syscall" package
to see that).  Finally, line 28 returns two values: a pointer to the new "FD"
and the return code.  If "Syscall.open" failed, the file descriptor "r" will
be negative and "NewFD" will return "nil".

Now that we can build "FDs", we can write methods to use 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 "FD",
each of which declares a receiver variable "fd".

--PROG progs/fd.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.

Finally, we can use our new package:

--PROG progs/helloworld3.go

and run the program:

	% helloworld3
	hello, world
	can't open file; errno=2
	%