| <!-- A Tutorial for the Go Programming Language --> |
| Introduction |
| ---- |
| |
| This document is a tutorial introduction to the basics of the Go programming |
| language, intended for programmers familiar with C or C++. It is not a comprehensive |
| guide to the language; at the moment the document closest to that is the |
| <a href='/doc/go_spec.html'>language specification</a>. |
| After you've read this tutorial, you should look at |
| <a href='/doc/effective_go.html'>Effective Go</a>, |
| which digs deeper into how the language is used and |
| talks about the style and idioms of programming in Go. |
| Also, slides from a 3-day course about Go are available. |
| Although they're badly out of date, they provide some |
| background and a lot of examples: |
| <a href='/doc/GoCourseDay1.pdf'>Day 1</a>, |
| <a href='/doc/GoCourseDay2.pdf'>Day 2</a>, |
| <a href='/doc/GoCourseDay3.pdf'>Day 3</a>. |
| |
| The presentation here proceeds through a series of modest programs to illustrate |
| key features of the language. All the programs work (at time of writing) and are |
| checked into the repository in the directory <a href='/doc/progs'>"/doc/progs/"</a>. |
| |
| 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 /package/ END |
| |
| Every Go source file declares, using a "package" statement, which package it's part of. |
| It may also import other packages to use their facilities. |
| This program imports the package "fmt" to gain access to |
| our old, now capitalized and package-qualified, friend, "fmt.Printf". |
| |
| Functions are introduced with the "func" keyword. |
| The "main" package's "main" function is where the program starts running (after |
| any initialization). |
| |
| String constants can contain Unicode characters, encoded in UTF-8. |
| (In fact, Go source files are defined to be encoded in UTF-8.) |
| |
| The comment convention is the same as in C++: |
| |
| /* ... */ |
| // ... |
| |
| Later we'll have much more to say about printing. |
| |
| Semicolons |
| ---- |
| |
| You might have noticed that our program has no semicolons. In Go |
| code, the only place you typically see semicolons is separating the |
| clauses of "for" loops and the like; they are not necessary after |
| every statement. |
| |
| In fact, what happens is that the formal language uses semicolons, |
| much as in C or Java, but they are inserted automatically |
| at the end of every line that looks like the end of a statement. You |
| don't need to type them yourself. |
| |
| For details about how this is done you can see the language |
| specification, but in practice all you need to know is that you |
| never need to put a semicolon at the end of a line. (You can put |
| them in if you want to write multiple statements per line.) As an |
| extra help, you can also leave out a semicolon immediately before |
| a closing brace. |
| |
| This approach makes for clean-looking, semicolon-free code. The |
| one surprise is that it's important to put the opening |
| brace of a construct such as an "if" statement on the same line as |
| the "if"; if you don't, there are situations that may not compile |
| or may give the wrong result. The language forces the brace style |
| to some extent. |
| |
| Compiling |
| ---- |
| |
| Go is a compiled language. At the moment there are two compilers. |
| "Gccgo" is a Go compiler that uses the GCC back end. There is also a |
| suite of compilers with different (and odd) names for each architecture: |
| "6g" for the 64-bit x86, "8g" for the 32-bit x86, and more. These |
| compilers run significantly faster but generate less efficient code |
| than "gccgo". At the time of writing (late 2009), they also have |
| a more robust run-time system although "gccgo" is catching up. |
| |
| Here's how to compile and run our program. With "6g", say, |
| |
| $ 6g helloworld.go # compile; object goes into helloworld.6 |
| $ 6l helloworld.6 # link; output goes into 6.out |
| $ 6.out |
| Hello, world; or Καλημέρα κόσμε; or こんにちは 世界 |
| $ |
| |
| With "gccgo" it looks a little more traditional. |
| |
| $ gccgo helloworld.go |
| $ a.out |
| Hello, world; or Καλημέρα κόσμε; or こんにちは 世界 |
| $ |
| |
| Echo |
| ---- |
| |
| Next up, here's a version of the Unix utility "echo(1)": |
| |
| --PROG progs/echo.go /package/ END |
| |
| This program is small but it's doing a number of new things. In the last example, |
| we saw "func" introduce 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 lists, one item per line, as on lines 7-10 and 14-17. |
| But it's not necessary to do so; we could have said |
| |
| const Space = " " |
| const Newline = "\n" |
| |
| This program imports the ""os"" package to access its "Stdout" variable, of type |
| "*os.File". The "import" statement is actually a declaration: in its general form, |
| as used in our ``hello world'' program, |
| it names the identifier ("fmt") |
| that will be used to access members of the package imported from the file (""fmt""), |
| found in the current directory or in a standard location. |
| In this program, though, we've dropped the explicit name from the imports; by default, |
| packages are imported using the name defined by the imported package, |
| which by convention is of course the file name itself. Our ``hello world'' program |
| could have said just "import "fmt"". |
| |
| You can specify your |
| own import names if you want but it's only necessary if you need to resolve |
| a naming conflict. |
| |
| Given "os.Stdout" we can use its "WriteString" method to print the string. |
| |
| Having imported the "flag" package, line 12 creates a global variable to hold |
| the value of echo's "-n" flag. The variable "omitNewline" has type "*bool", pointer |
| to "bool". |
| |
| In "main.main", we parse the arguments (line 20) 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. |
| 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" and "switch" statements. |
| 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 arguments and separating spaces. After the loop, if the "-n" flag is not |
| set, the program appends a newline. Finally, it 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 an erroneous return, call |
| |
| os.Exit(1) |
| |
| The "os" package contains other essentials for getting |
| started; for instance, "os.Args" is a slice used by the |
| "flag" package to access the command-line arguments. |
| |
| An Interlude about Types |
| ---- |
| |
| Go has some familiar types such as "int" and "uint" (unsigned "int"), which represent |
| values of the ''appropriate'' size for the machine. It also defines |
| explicitly-sized types such as "int8", "float64", and so on, plus |
| unsigned integer types such as "uint", "uint32", etc. |
| These are distinct types; even if "int" and "int32" are both 32 bits in size, |
| they are not the same type. There is also a "byte" synonym for |
| "uint8", which is the element type for strings. |
| |
| Floating-point types are always sized: "float32" and "float64", |
| plus "complex64" (two "float32s") and "complex128" |
| (two "float64s"). Complex numbers are outside the |
| scope of this tutorial. |
| |
| 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 arrayOfInt [10]int |
| |
| Arrays, like strings, are values, but they are mutable. This differs |
| from C, in which "arrayOfInt" 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 |
| a <i>slice</i> variable to hold a reference to any array, of any size, |
| with the same element type. |
| A <i>slice |
| expression</i> has the form "a[low : high]", representing |
| the internal array indexed from "low" through "high-1"; the resulting |
| slice is indexed from "0" through "high-low-1". |
| In short, slices look a lot like arrays but with |
| no explicit size ("[]" vs. "[10]") and they reference a segment of |
| an underlying, usually anonymous, regular array. Multiple slices |
| can share data if they represent pieces of the same array; |
| multiple arrays can never share data. |
| |
| Slices are much more common in Go programs than |
| regular arrays; they're more flexible, have reference semantics, |
| and are efficient. What they lack is the precise control of storage |
| layout of a regular array; if you want to have a hundred elements |
| of an array stored within your structure, you should use a regular |
| array. To create one, use a compound value <i>constructor</i>—an |
| expression formed |
| from a type followed by a brace-bounded expression like this: |
| |
| [3]int{1,2,3} |
| |
| In this case the constructor builds an array of 3 "ints". |
| |
| When passing an array to a function, you almost always want |
| to declare the formal parameter to be a slice. When you call |
| the function, slice the array to create |
| (efficiently) a slice reference and pass that. |
| By default, the lower and upper bounds of a slice match the |
| ends of the existing object, so the concise notation "[:]" |
| will slice the whole array. |
| |
| Using slices one can write this function (from "sum.go"): |
| |
| --PROG progs/sum.go /sum/ /^}/ |
| |
| Note how the return type ("int") is defined for "sum()" by stating it |
| after the parameter list. |
| |
| To call the function, we slice the array. This intricate call (we'll show |
| a simpler way in a moment) constructs |
| an array and slices it: |
| |
| s := sum([3]int{1,2,3}[:]) |
| |
| If you are creating a regular array but want the compiler to count the |
| elements for you, use "..." as the array size: |
| |
| s := sum([...]int{1,2,3}[:]) |
| |
| That's fussier than necessary, though. |
| In practice, unless you're meticulous about storage layout within a |
| data structure, a slice itself—using empty brackets with no size—is all you need: |
| |
| s := sum([]int{1,2,3}) |
| |
| There are also maps, which you can initialize like this: |
| |
| m := map[string]int{"one":1 , "two":2} |
| |
| The built-in function "len()", which returns number of elements, |
| makes its first appearance in "sum". It works on strings, arrays, |
| slices, maps, and channels. |
| |
| By the way, another thing that works on strings, arrays, slices, maps |
| and channels is the "range" clause on "for" loops. Instead of writing |
| |
| for i := 0; i < len(a); i++ { ... } |
| |
| to loop over the elements of a slice (or map or ...) , we could write |
| |
| for i, v := range a { ... } |
| |
| This assigns "i" to the index and "v" to the value of the successive |
| elements of the target of the range. See |
| <a href='/doc/effective_go.html'>Effective Go</a> |
| for more examples of its use. |
| |
| |
| An Interlude about Allocation |
| ---- |
| |
| Most types in Go are values. If you have an "int" or a "struct" |
| or an array, assignment |
| copies the contents of the object. |
| To allocate a new variable, use "new()", which |
| returns a pointer to the allocated storage. |
| |
| type T struct { a, b int } |
| var t *T = new(T) |
| |
| or the more idiomatic |
| |
| t := new(T) |
| |
| Some types—maps, slices, and channels (see below)—have reference semantics. |
| If you're holding a slice or a map and you modify its contents, other variables |
| referencing the same underlying data will see the modification. For these three |
| types you want to use the built-in function "make()": |
| |
| m := make(map[string]int) |
| |
| This statement initializes a new map ready to store entries. |
| If you just declare the map, as in |
| |
| var m map[string]int |
| |
| it creates a "nil" reference that cannot hold anything. To use the map, |
| you must first initialize the reference using "make()" or by assignment from an |
| existing map. |
| |
| Note that "new(T)" returns type "*T" while "make(T)" returns type |
| "T". If you (mistakenly) allocate a reference object with "new()", |
| you receive a pointer to a nil reference, equivalent to |
| declaring an uninitialized variable and taking its address. |
| |
| 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 large-precision values that |
| can overflow only when they are assigned to an integer variable with |
| too little precision to represent the value. |
| |
| const hardEight = (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 float64, the default type for floating constants |
| i3div2 := 3/2 // integer division - result is 1 |
| f3div2 := 3./2. // floating-point division - result is 1.5 |
| |
| Conversions only work for simple cases such as converting "ints" of one |
| sign or size to another and between integers and floating-point numbers, |
| plus a couple of other instances outside the scope of a tutorial. |
| There are no automatic numeric conversions of any kind in Go, |
| other than that of making constants have concrete size and type when |
| assigned to a variable. |
| |
| An I/O Package |
| ---- |
| |
| Next we'll look at a simple package for doing file I/O with an |
| open/close/read/write interface. Here's the start of "file.go": |
| |
| --PROG progs/file.go /package/ /^}/ |
| |
| The first few lines declare the name of the |
| package—"file"—and then import two packages. The "os" |
| package hides the differences |
| between various operating systems to give a consistent view of files and |
| so on; here we're going to use its error handling utilities |
| and reproduce the rudiments of its file I/O. |
| |
| The other item is 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 "File". |
| To make things a little more interesting, our "File" includes the name of the file |
| that the file descriptor refers to. |
| |
| Because "File" starts with a capital letter, the type is available outside the package, |
| that is, by users of the package. In Go the rule about visibility of information is |
| simple: if a name (of a top-level type, function, method, constant or variable, or of |
| a structure field or method) is capitalized, users of the package may see it. Otherwise, the |
| name and hence the thing being named is visible only inside the package in which |
| it is declared. This is more than a convention; the rule is enforced by the compiler. |
| In Go, the term for publicly visible names is ''exported''. |
| |
| In the case of "File", all its fields are lower case and so invisible to users, but we |
| will soon give it some exported, upper-case methods. |
| |
| First, though, here is a factory to create a "File": |
| |
| --PROG progs/file.go /newFile/ /^}/ |
| |
| This returns a pointer to a new "File" structure with the file descriptor and name |
| filled in. This code uses Go's notion of a ''composite literal'', analogous to |
| the ones used to build maps and arrays, to construct a new heap-allocated |
| object. We could write |
| |
| n := new(File) |
| n.fd = fd |
| n.name = name |
| return n |
| |
| but for simple structures like "File" it's easier to return the address of a |
| composite literal, as is done here on line 21. |
| |
| We can use the factory to construct some familiar, exported variables of type "*File": |
| |
| --PROG progs/file.go /var/ /^.$/ |
| |
| The "newFile" function was not exported because it's internal. The proper, |
| exported factory to use is "OpenFile" (we'll explain that name in a moment): |
| |
| --PROG progs/file.go /func.OpenFile/ /^}/ |
| |
| There are a number of new things in these few lines. First, "OpenFile" returns |
| multiple values, a "File" and an error (more about errors in a moment). |
| We declare the |
| multi-value return as a parenthesized list of declarations; syntactically |
| they look just like a second parameter list. The function |
| "syscall.Open" |
| also has a multi-value return, which we can grab with the multi-variable |
| declaration on line 31; it declares "r" and "e" to hold the two values, |
| both of type "int" (although you'd have to look at the "syscall" package |
| to see that). Finally, line 35 returns two values: a pointer to the new "File" |
| and the error. If "syscall.Open" fails, the file descriptor "r" will |
| be negative and "newFile" will return "nil". |
| |
| About those errors: The "os" library includes a general notion of an error. |
| It's a good idea to use its facility in your own interfaces, as we do here, for |
| consistent error handling throughout Go code. In "Open" we use a |
| conversion to translate Unix's integer "errno" value into the integer type |
| "os.Errno", which implements "os.Error". |
| |
| Why "OpenFile" and not "Open"? To mimic Go's "os" package, which |
| our exercise is emulating. The "os" package takes the opportunity |
| to make the two commonest cases - open for read and create for |
| write - the simplest, just "Open" and "Create". "OpenFile" is the |
| general case, analogous to the Unix system call "Open". Here is |
| the implementation of our "Open" and "Create"; they're trivial |
| wrappers that eliminate common errors by capturing |
| the tricky standard arguments to open and, especially, to create a file: |
| |
| --PROG progs/file.go /^const/ /^}/ |
| |
| --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 "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.*IntArray/ /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. |