summaryrefslogtreecommitdiffhomepage
path: root/go.html.markdown
diff options
context:
space:
mode:
authorSonia Keys <soniakeys@gmail.com>2013-08-13 13:52:13 -0400
committerSonia Keys <soniakeys@gmail.com>2013-08-13 13:52:13 -0400
commitde6069d3d60eb2da7ee945c38fbe8d7249c66a6a (patch)
treebcbedfe4b520fe67a56d2eeb67b815da0a0056d9 /go.html.markdown
parentb86da2e2082e7fd1438d48e12125aeaa6a187b92 (diff)
Go first draft
Diffstat (limited to 'go.html.markdown')
-rw-r--r--go.html.markdown423
1 files changed, 423 insertions, 0 deletions
diff --git a/go.html.markdown b/go.html.markdown
new file mode 100644
index 00000000..9888b37d
--- /dev/null
+++ b/go.html.markdown
@@ -0,0 +1,423 @@
+---
+name: Go
+category: language
+language: Go
+filename: learngo.go
+contributors:
+ - ["Sonia Keys", "https://github.com/soniakeys"]
+---
+
+Go was created out of the need to get work done. It's not the latest trend
+in computer science, but it is the newest fastest way to solve real-world
+problems.
+
+It has familiar concepts of imperative languages with static typing.
+It's fast to compile and fast to execute, it adds easy-to-understand
+concurrency to leverage today's multi-core CPUs, and has features to
+help with large-scale programming.
+
+Go comes with a great standard library and an enthusiastic community.
+
+```Go
+// Single line comment
+/* Multi-
+ line comment */
+
+// A package clause starts every source file.
+// Main is a special name declaring an executable rather than a library.
+package main
+
+// An import declaration comes next. It declares library packages referenced
+// in this file. The list must be exactly correct! Missing or unused packages
+// are errors, not warnings.
+import (
+ "fmt" // A package in the Go standard library
+ "net/http" // Yes, a web server!
+ "strconv" // String conversions
+)
+
+// A function definition. Main is special. It is the entry point for the
+// executable program. Love it or hate it, Go uses brace brackets.
+func main() {
+ // Println is a function that outputs a line to stdout. It can be
+ // called here because fmt has been imported and the function name
+ // "Println" is upper case. Symbols starting with an upper case letter
+ // are publicly visible. No other special syntax is needed to export
+ // something from a package.
+ // To call Println, qualify it with the package name, fmt.
+ fmt.Println("Hello world!")
+
+ // Call another function within this package.
+ beyondHello()
+}
+
+// Idiomatic Go uses camel case. Functions have parameters in parentheses.
+// If there are no parameters, empty parens are still required.
+func beyondHello() {
+ var x int // Variable declaration. Variables must be declared before use.
+ x = 3 // Variable assignment.
+ // "Short" declarations use := syntax to declare and assign, infering the
+ // type from the right hand side as much as possible and using some
+ // defaults where the rhs could be interpreted different ways.
+ // Idiomatic Go uses short declarations in preference to var keyword.
+ y := 4
+ sum, prod := learnMultiple(x, y) // function returns two values
+ fmt.Println("sum:", sum, "prod:", prod) // simple output
+ learnTypes() // < y minutes, learn more!
+}
+
+// Functions can have parameters and (multiple!) return values.
+// In declarations, the symbol precedes the type, and the type does not have
+// to be repeated if it is the same for multiple symbols in a row.
+func learnMultiple(x, y int) (sum, prod int) {
+ return x + y, x * y // return two values
+}
+
+// Some built-in types and literals.
+func learnTypes() {
+ // Short declaration usually gives you what you want.
+ s := "Learn Go!" // string type
+
+ s2 := `A "raw" string literal
+can include line breaks.` // same string type
+
+ // non-ASCII literal. Go source is UTF-8.
+ g := 'Σ' // rune type, an alias for uint32, holds a UTF-8 code point
+
+ f := 3.14195 // float64, an IEEE-754 64-bit floating point number
+ c := 3 + 4i // complex128, represented internally with two float64s
+
+ // You can use var syntax with an initializer if you want
+ // something other than the default that a short declaration gives you.
+ var u uint = 7 // unsigned, but implementation dependent size as with int
+ var pi float32 = 22. / 7
+
+ // Or more idiomatically, use conversion syntax with a short declaration.
+ n := byte('\n') // byte is an alias for uint8
+
+ // Arrays have size fixed at compile time.
+ var a4 [4]int // an array of 4 ints, initialized to all 0
+ a3 := [...]int{3, 1, 5} // an array of 3 ints, initialized as shown
+
+ // Slices have dynamic size. Arrays and slices each have advantages
+ // but use cases for slices are much more common.
+ s3 := []int{4, 5, 9} // compare to a3. no ellipsis here
+ s4 := make([]int, 4) // allocates slice of 4 ints, initialized to all 0
+ var d2 [][]float64 // declaration only, nothing allocated here
+ bs := []byte("a slice") // type conversion syntax
+
+ p, q := learnMemory() // A little side bar.
+ // Did you read it? This short declaration declares p and q to be of
+ // type pointer to int. P is now pointing into a block of of 20 ints, but
+ // the only one accessible is the one that p is pointing at. There is
+ // no p++ to get at the next one.
+ fmt.Println(*p, *q) // * follows a pointer. This prints two ints.
+
+ // Maps are a dynamically growable associative array type, like the
+ // hash or dictionary types of some other languages.
+ m := map[string]int{"three": 3, "four": 4}
+ m["one"] = 1
+
+ // Unused variables are an error in Go.
+ // The underbar lets you "use" a variable but discard its value.
+ _, _, _, _, _, _, _, _, _ = s2, g, f, u, pi, n, a3, s4, bs
+ // Output of course counts as using a variable.
+ fmt.Println(s, c, a4, s3, d2, m)
+
+ learnFlowControl() // back in the flow
+}
+
+// Go is fully garbage collected. It has pointers but no pointer arithmetic.
+// You can make a mistake with a nil pointer, but not by incrementing a pointer.
+func learnMemory() (p, q *int) {
+ // Named return values p and q have type pointer to int. They are
+ // initialized to nil at this point. Evaluating *p or *q here would cause
+ // a panic--a run time error.
+ p = new(int) // built-in function new allocates memory.
+ // The allocated int is initialized to 0, p is no longer nil.
+ s := make([]int, 20) // allocate 20 ints as a single block of memory
+ s[3] = 7 // assign one of them
+ r := -2 // declare another local variable
+ return &s[3], &r // Oh my.
+ // The line above returns two values, yes, and both of the expressions
+ // are valid. & takes the address of an object. Elements of a slice are
+ // addressable, and so are local variables. Built-in functions new and
+ // make explicitly allocate memory, but local objects can be allocated
+ // as needed. Here memory for r will be still be referenced after the
+ // function returns so it will be allocated as well. The int allocated
+ // with new on the other hand will no longer be referenced and can be
+ // garbage collected as needed by the Go runtime. The memory allocated
+ // with make will still be referenced at that one element, and so it
+ // cannot be garbage collected. All 20 ints remain in memory because
+ // one of them is still referenced.
+}
+
+func expensiveComputation() int {
+ return 1e6
+}
+
+func learnFlowControl() {
+ // If statements require brace brackets, and do not require parens.
+ if true {
+ fmt.Println("told ya")
+ }
+ // This is how we format the brace brackets. Formatting is standardized
+ // by the command line command "go fmt." Everybody does it. You will
+ // suffer endless disparaging remarks until you conform as well.
+ if false {
+ // pout
+ } else {
+ // gloat
+ }
+ // If statements can be chained of course, but it's idiomatic to use
+ // the handy switch statement instead.
+ x := 1
+ switch x {
+ case 0:
+ case 1:
+ // cases don't "fall through"
+ case 2:
+ // unreached
+ }
+ // Like if, for doesn't use parens either. The scope of a variable
+ // declared in the first clause of the for statement is the statement
+ // and block. This x shadows the x declared above, but goes out of
+ // scope after the for block.
+ for x := 0; x < 3; x++ { // ++ is a statement
+ fmt.Println("iteration", x)
+ }
+ // x == 1 here.
+
+ // For is the only loop statement in Go, but it has alternate forms.
+ for { // infinite loop
+ break // just kidding
+ continue // unreached
+ }
+ // The initial assignment of the for statement is handy enough that Go
+ // if statements can have one as well. Just like in the for statement,
+ // the := here means to declare and assign y first, then test y > x.
+ // The scope of y is limited to the if statement and block.
+ if y := expensiveComputation(); y > x {
+ x = y
+ }
+ // Functions are first class objects and function literals are handy.
+ // Function literals are closures.
+ xBig := func() bool {
+ return x > 100 // references x declared above switch statement.
+ }
+ fmt.Println("xBig:", xBig()) // true (we last assigned 1e6 to x)
+ x /= 1e5 // this makes it == 10
+ fmt.Println("xBig:", xBig()) // false now
+
+ // When you need it, you'll love it. Actually Go's goto has been reformed
+ // a bit to avoid indeterminate states. You can't jump around variable
+ // declarations and you can't jump into blocks.
+ goto love
+love:
+
+ learnInterfaces() // Good stuff coming up!
+}
+
+// An interface is a list of functionality that a type supports. Notably
+// missing from an interface definition is any declaration of which types
+// implement the interface. Types simply implement an interface or they don't.
+//
+// An interface can have any number of methods, but it's actually common
+// for an interface to have only single method. It is idiomatic in this
+// case for the single method to be named with some action, and for the
+// interface name to end in "er."
+//
+// An interface definition is one kind of a type definition. Interface is
+// a built in type. Stringer is defined here as an interface type with one
+// method, String.
+type Stringer interface {
+ String() string
+}
+
+// Struct is another built in type. A struct aggregates "fields."
+// Pair here has two fields, ints named x and y.
+type pair struct {
+ x, y int
+}
+
+// User defined types can have "methods." These are functions that operate
+// in the context of an instance of the user defined type. The instance
+// is called the "receiver" and is identified with a declaration just in front
+// of the method name. The receiver here is "p." In most ways the receiver
+// works just like a function parameter.
+//
+// This String method has the same name and return value as the String method
+// of the Stringer interface. Further, String is the only method of Stringer.
+// The pair type thus implements all methods of the Stringer interface and
+// we say simply that pair implements Stringer. No other syntax is needed.
+func (p pair) String() string {
+ // Sprintf is another public function in package fmt.
+ // Dot syntax references fields of p.
+ return fmt.Sprintf("(%d, %d)", p.x, p.y)
+}
+
+func learnInterfaces() {
+ // Brace syntax is a "struct literal." It evaluates to an initialized
+ // struct. The := syntax declares and initializes p to this struct.
+ p := pair{3, 4}
+ fmt.Println(p.String()) // call String method of p, of type pair.
+ var i Stringer // declare i of type Stringer.
+ i = p // valid because pair implements Stringer
+ // Call String method of i, of type Stringer. Output same as above.
+ fmt.Println(i.String())
+ // It gets more interesting now. We defined Stringer in this file,
+ // but the same interface happens to be defined in package fmt.
+ // Pair thus implements fmt.Stringer as well, and does so with no
+ // declaration of the fact. The definition of pair doesn't mention
+ // any interfaces at all, and of course the authors of fmt.Stringer
+ // had no idea that we were going to define pair.
+ //
+ // Functions in the fmt package know how to print some standard built in
+ // types, and beyond that, they see if a type implements fmt.Stringer.
+ // If so, they simply call the String method to ask an object for a
+ // printable representation of itself.
+ fmt.Println(p) // output same as above. Println calls String method.
+ fmt.Println(i) // output same as above
+
+ learnErrorHandling()
+}
+
+func learnErrorHandling() {
+ // Sometimes you just need to know if something worked or not. Go has
+ // a ", ok" idiom for that. Something, a map expression here, but commonly
+ // a function, can return a boolean value of ok or not ok as a second
+ // return value.
+ m := map[int]string{3: "three", 4: "four"}
+ if x, ok := m[1]; !ok { // , ok is optional but see how useful it is.
+ fmt.Println("no one there")
+ } else {
+ fmt.Print(x)
+ }
+ // An error value communicates not just "ok" but more about the problem.
+ if _, err := strconv.Atoi("non-int"); err != nil { // _ discards value
+ // prints "strconv.ParseInt: parsing "non-int": invalid syntax"
+ fmt.Println(err)
+ }
+ // error is a built in type. It is an interface with a single method,
+ // defined internally as,
+ //
+ // type error interface {
+ // Error() string
+ // }
+ //
+ // The string returned by the Error method is conventionally a printable
+ // error message. You can define your own error types by simply adding
+ // an Error method. Your type then automatically implements the error
+ // interface. We've seen two interfaces now, fmt.Stringer and error.
+
+ // We'll revisit interfaces a little later. Meanwhile,
+ learnConcurrency()
+}
+
+// Go has concurrency support in the language definition. The element of
+// concurrent execution is called a "goroutine" and is similar to a thread
+// but "lighter." Goroutines are multiplexed to operating system threads
+// and a running Go program can have far more goroutines than available OS
+// threads. If a machine has multiple CPU cores, goroutines can run in
+// parallel.
+//
+// Go "Channels" allow communication between goroutines in a way that is
+// both powerful and easy to understand. Channel is a type in Go and objects
+// of type channel are first class objects--they can be assigned to variables,
+// passed around to functions, and so on. A channel works conceptually much
+// like a Unix pipe. You put data in at one end and it comes out the other.
+// Channel "send" and "receive" operations are goroutine-safe. No locks
+// or additional synchronization is needed.
+
+// Inc increments a number, and sends the result on a channel. The channel
+// operation makes this function useful to run concurrently with other
+// goroutines. There is no special declaration though that says this function
+// is concurrent. It is an ordinary function that happens to have a
+// parameter of channel type.
+func inc(i int, c chan int) {
+ c <- i + 1 // <- is the "send" operator when a channel appears on the left.
+}
+
+// We'll use inc to increment some numbers concurrently.
+func learnConcurrency() {
+ // Same make function used earlier to make a slice. Make allocates and
+ // initializes slices, maps, and channels.
+ c := make(chan int)
+ // Start three concurrent goroutines. Numbers will be incremented
+ // concurrently, perhaps in parallel if the machine is capable and
+ // properly configured. All three send to the same channel.
+ go inc(0, c) // go is a statement that starts a new goroutine.
+ go inc(10, c)
+ go inc(-805, c)
+ // Read three results from the channel and print them out.
+ // There is no telling in what order the results will arrive!
+ fmt.Println(<-c, <-c, <-c) // channel on right, <- is "receive" operator.
+
+ cs := make(chan string) // another channel, this one handles strings.
+ cc := make(chan chan string) // a channel of channels.
+ go func() { c <- 84 }() // start a new goroutine just to send a value
+ go func() { cs <- "wordy" }() // again, for cs this time
+ // Select has syntax like a switch statement but is doing something
+ // pretty different. Each case involves a channel operation. In rough
+ // terms, a case is selected at random out of the cases that are ready to
+ // communicate. If none are ready, select waits for one to become ready.
+ select {
+ case i := <-c: // the value received can be assigned to a variable
+ fmt.Println("it's a", i)
+ case <-cs: // or the value received can be discarded
+ fmt.Println("it's a string")
+ case <-cc: // empty channel, not ready for communication.
+ fmt.Println("didn't happen.")
+ }
+ // At this point a value was taken from either c or cs. One of the two
+ // goroutines started above has completed, the other will remain blocked.
+
+ learnWebProgramming() // Go does it. You want to do it too.
+}
+
+// A simple web server can be created with a single function from the standard
+// library. ListenAndServe, in package net/http, listens at the specified
+// TCP address and uses an object that knows how to serve data. "Knows how"
+// means "satisfies an interface." The second parameter is of type interface,
+// specifically http.Handler. http.Handler has a single method, ServeHTTP.
+func learnWebProgramming() {
+ err := http.ListenAndServe(":8080", pair{})
+ // Error returns are ubiquitous in Go. Always check error returns and
+ // do something with them. Often it's enough to print it out as an
+ // indication of what failed. Of course there are better things to do
+ // in production code: log it, try something else, shut everything down,
+ // and so on.
+ fmt.Println(err)
+}
+
+// You can make any type into an http.Hander by implementing ServeHTTP.
+// Lets use the pair type we defined earlier, just because we have it
+// sitting around. ServeHTTP has two parameters. The request parameter
+// is a struct that we'll ignore here. http.ResponseWriter is yet another
+// interface! Here it is an object supplied to us with the guarantee that
+// it implements its interface, which includes a method Write.
+// We call this Write method to serve data.
+func (p pair) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+ w.Write([]byte("You learned Go in Y minutes!"))
+}
+
+// And that's it for a proof-of-concept web server! If you run this program
+// it will print out all the lines from the earlier parts of the lesson, then
+// start this web server. To hit the web server, just point a browser at
+// localhost:8080 and you'll see the message. (Then you can probably press
+// ctrl-C to kill it.)
+```
+
+## Further Reading
+
+The root of all things Go is the [official Go web site](http://golang.org/).
+There you can follow the tutorial, play interactively, and read lots.
+
+The language definition itself is highly recommended. It's easy to read
+and amazingly short (as language definitions go these days.)
+
+On the reading list for students of Go is the source code to the standard
+library. Comprehensively documented, it demonstrates the best of readable
+and understandable Go, Go style, and Go idioms. Click on a function name
+in the documentation and the source code comes up!
+