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Portal de la comunidad. Libro destacado La ingeniería del conocimiento es aquella disciplina moderna que forma parte de la inteligencia artificial y cuyo fin es el diseño y desarrollo de sistemas expertos. Language Design in the Service of Software Engineering explains Go's origins and the motivation behind its design. Although Go has types and methods and allows an object-oriented style of programming, there is no type hierarchy.

There are also ways to embed types in other types to provide something analogous—but not identical—to subclassing. They are not restricted to structs classes.

The only way to have dynamically dispatched methods is through an interface. Methods on a struct or any other concrete type are always resolved statically. Object-oriented programming, at least in the best-known languages, involves too much discussion of the relationships between types, relationships that often could be derived automatically.

Rather than requiring the programmer to declare ahead of time that two types are related, in Go a type automatically satisfies any interface that specifies a subset of its methods.

Besides reducing the bookkeeping, this approach has real advantages. Types can satisfy many interfaces at once, without the complexities of traditional multiple inheritance. Interfaces can be very lightweight—an interface with one or even zero methods can express a useful concept. Interfaces can be added after the fact if a new idea comes along or for testing—without annotating the original types.

Because there are no explicit relationships between types and interfaces, there is no type hierarchy to manage or discuss. It's possible to use these ideas to construct something analogous to type-safe Unix pipes.

For instance, see how fmt. All these ideas stem from a single interface io. Writer representing a single method Write. And that's only scratching the surface. Go's interfaces have a profound influence on how programs are structured. It takes some getting used to but this implicit style of type dependency is one of the most productive things about Go.

We debated this issue but decided implementing len and friends as functions was fine in practice and didn't complicate questions about the interface in the Go type sense of basic types. Method dispatch is simplified if it doesn't need to do type matching as well. Experience with other languages told us that having a variety of methods with the same name but different signatures was occasionally useful but that it could also be confusing and fragile in practice.

Matching only by name and requiring consistency in the types was a major simplifying decision in Go's type system. Regarding operator overloading, it seems more a convenience than an absolute requirement. Again, things are simpler without it. A Go type satisfies an interface by implementing the methods of that interface, nothing more. This property allows interfaces to be defined and used without needing to modify existing code.

It enables a kind of structural typing that promotes separation of concerns and improves code re-use, and makes it easier to build on patterns that emerge as the code develops. The semantics of interfaces is one of the main reasons for Go's nimble, lightweight feel.

You can ask the compiler to check that the type T implements the interface I by attempting an assignment using the zero value for T or pointer to T , as appropriate:. If you wish the users of an interface to explicitly declare that they implement it, you can add a method with a descriptive name to the interface's method set. A type must then implement the ImplementsFooer method to be a Fooer , clearly documenting the fact and announcing it in godoc 's output.

Most code doesn't make use of such constraints, since they limit the utility of the interface idea. Sometimes, though, they're necessary to resolve ambiguities among similar interfaces. Consider this simple interface to represent an object that can compare itself with another value:. Unlike the analogous situation in some polymorphic type systems, T does not implement Equaler. The argument type of T. Equal is T , not literally the required type Equaler.

In Go, the type system does not promote the argument of Equal ; that is the programmer's responsibility, as illustrated by the type T2 , which does implement Equaler:. Even this isn't like other type systems, though, because in Go any type that satisfies Equaler could be passed as the argument to T2. Equal , and at run time we must check that the argument is of type T2. Some languages arrange to make that guarantee at compile time. In Go, T3 does not satisfy Opener , although it might in another language.

While it is true that Go's type system does less for the programmer in such cases, the lack of subtyping makes the rules about interface satisfaction very easy to state: Go's rule is also easy to implement efficiently. We feel these benefits offset the lack of automatic type promotion. Should Go one day adopt some form of polymorphic typing, we expect there would be a way to express the idea of these examples and also have them be statically checked.

It is disallowed by the language specification because the two types do not have the same representation in memory. It is necessary to copy the elements individually to the destination slice. In Go, types are closely tied to methods, in that every named type has a possibly empty method set.

The general rule is that you can change the name of the type being converted and thus possibly change its method set but you can't change the name and method set of elements of a composite type. Go requires you to be explicit about type conversions. Under the covers, interfaces are implemented as two elements, a type T and a value V. V is a concrete value such as an int , struct or pointer, never an interface itself, and has type T.

The value V is also known as the interface's dynamic value, since a given interface variable might hold different values V and corresponding types T during the execution of the program. Such an interface value will therefore be non- nil even when the pointer value V inside is nil.

This situation can be confusing, and arises when a nil value is stored inside an interface value such as an error return:. This means that if the caller compares the returned error to nil , it will always look as if there was an error even if nothing bad happened. To return a proper nil error to the caller, the function must return an explicit nil:. As an example, os. Similar situations to those described here can arise whenever interfaces are used. Just keep in mind that if any concrete value has been stored in the interface, the interface will not be nil.

For more information, see The Laws of Reflection. Variant types, also known as algebraic types, provide a way to specify that a value might take one of a set of other types, but only those types.

A common example in systems programming would specify that an error is, say, a network error, a security error or an application error and allow the caller to discriminate the source of the problem by examining the type of the error. Another example is a syntax tree in which each node can be a different type: We considered adding variant types to Go, but after discussion decided to leave them out because they overlap in confusing ways with interfaces.

What would happen if the elements of a variant type were themselves interfaces? Also, some of what variant types address is already covered by the language. The error example is easy to express using an interface value to hold the error and a type switch to discriminate cases.

The syntax tree example is also doable, although not as elegantly. In Go method types must match exactly, so Value does not implement Copyable. Go separates the notion of what a type does—its methods—from the type's implementation. If two methods return different types, they are not doing the same thing.

Programmers who want covariant result types are often trying to express a type hierarchy through interfaces. In Go it's more natural to have a clean separation between interface and implementation. The convenience of automatic conversion between numeric types in C is outweighed by the confusion it causes. When is an expression unsigned? How big is the value? Is the result portable, independent of the machine on which it executes?

For reasons of portability, we decided to make things clear and straightforward at the cost of some explicit conversions in the code. The definition of constants in Go—arbitrary precision values free of signedness and size annotations—ameliorates matters considerably, though.

A related detail is that, unlike in C, int and int64 are distinct types even if int is a bit type. The int type is generic; if you care about how many bits an integer holds, Go encourages you to be explicit. Although Go is strict about conversion between variables of different numeric types, constants in the language are much more flexible.

Literal constants such as 23 , 3. Pi occupy a sort of ideal number space, with arbitrary precision and no overflow or underflow. For instance, the value of math. Pi is specified to 63 places in the source code, and constant expressions involving the value keep precision beyond what a float64 could hold.

Only when the constant or constant expression is assigned to a variable—a memory location in the program—does it become a "computer" number with the usual floating-point properties and precision. Also, because they are just numbers, not typed values, constants in Go can be used more freely than variables, thereby softening some of the awkwardness around the strict conversion rules.

One can write expressions such as. A blog post titled Constants explores this topic in more detail. The same reason strings are: We believe that Go's implementation of maps is strong enough that it will serve for the vast majority of uses. If a specific application can benefit from a custom implementation, it's possible to write one but it will not be as convenient syntactically; this seems a reasonable tradeoff.

Map lookup requires an equality operator, which slices do not implement. They don't implement equality because equality is not well defined on such types; there are multiple considerations involving shallow vs. We may revisit this issue—and implementing equality for slices will not invalidate any existing programs—but without a clear idea of what equality of slices should mean, it was simpler to leave it out for now. In Go 1, unlike prior releases, equality is defined for structs and arrays, so such types can be used as map keys.

Slices still do not have a definition of equality, though. There's a lot of history on that topic. Early on, maps and channels were syntactically pointers and it was impossible to declare or use a non-pointer instance. Also, we struggled with how arrays should work. Eventually we decided that the strict separation of pointers and values made the language harder to use.

Changing these types to act as references to the associated, shared data structures resolved these issues. This change added some regrettable complexity to the language but had a large effect on usability: Go became a more productive, comfortable language when it was introduced.

There is a program, godoc , written in Go, that extracts package documentation from the source code and serves it as a web page with links to declarations, files, and so on.

An instance is running at golang. In fact, godoc implements the full site at golang. A godoc instance may be configured to provide rich, interactive static analyses of symbols in the programs it displays; details are listed here.

For access to documentation from the command line, the go tool has a doc subcommand that provides a textual interface to the same information. Go has established conventions to guide decisions around naming, layout, and file organization. The document Effective Go contains some advice on these topics. More directly, the program gofmt is a pretty-printer whose purpose is to enforce layout rules; it replaces the usual compendium of do's and don'ts that allows interpretation. All the Go code in the repository, and the vast majority in the open source world, has been run through gofmt.

The document titled Go Code Review Comments is a collection of very short essays about details of Go idiom that are often missed by programmers. It is a handy reference for people doing code reviews for Go projects. The library sources are in the src directory of the repository. If you want to make a significant change, please discuss on the mailing list before embarking. See the document Contributing to the Go project for more information about how to proceed. For GitHub accounts, the password can be a personal access token.

Since the inception of the project, Go has had no explicit concept of package versions, but that is changing. Versioning is a source of significant complexity, especially in large code bases, and it has taken some time to develop an approach that works well at scale in a large enough variety of situations to be appropriate to supply to all Go users.

For more information, see the Go 1. Regardless of the actual package management technology, "go get" and the larger Go toolchain does provide isolation of packages with different import paths.

This observation leads to some advice for package authors and package users. Packages intended for public use should try to maintain backwards compatibility as they evolve. The Go 1 compatibility guidelines are a good reference here: If different functionality is required, add a new name instead of changing an old one. If a complete break is required, create a new package with a new import path.

If you're using an externally supplied package and worry that it might change in unexpected ways, but are not yet using Go modules, the simplest solution is to copy it to your local repository. This is the approach Google takes internally and is supported by the go command through a technique called "vendoring".

This involves storing a copy of the dependency under a new import path that identifies it as a local copy. See the design document for details. As in all languages in the C family, everything in Go is passed by value. That is, a function always gets a copy of the thing being passed, as if there were an assignment statement assigning the value to the parameter. For instance, passing an int value to a function makes a copy of the int , and passing a pointer value makes a copy of the pointer, but not the data it points to.

See a later section for a discussion of how this affects method receivers. Map and slice values behave like pointers: Copying a map or slice value doesn't copy the data it points to.

Copying an interface value makes a copy of the thing stored in the interface value. If the interface value holds a struct, copying the interface value makes a copy of the struct. If the interface value holds a pointer, copying the interface value makes a copy of the pointer, but again not the data it points to. Note that this discussion is about the semantics of the operations. Actual implementations may apply optimizations to avoid copying as long as the optimizations do not change the semantics.

Pointers to interface values arise only in rare, tricky situations involving disguising an interface value's type for delayed evaluation.

It is a common mistake to pass a pointer to an interface value to a function expecting an interface. The compiler will complain about this error but the situation can still be confusing, because sometimes a pointer is necessary to satisfy an interface.

The insight is that although a pointer to a concrete type can satisfy an interface, with one exception a pointer to an interface can never satisfy an interface. The printing function fmt. Fprintf takes as its first argument a value that satisfies io. Writer —something that implements the canonical Write method. Thus we can write. Even so, it's almost certainly a mistake if the value is a pointer to an interface; the result can be confusing.

For programmers unaccustomed to pointers, the distinction between these two examples can be confusing, but the situation is actually very simple. When defining a method on a type, the receiver s in the above examples behaves exactly as if it were an argument to the method.

Whether to define the receiver as a value or as a pointer is the same question, then, as whether a function argument should be a value or a pointer.

There are several considerations. First, and most important, does the method need to modify the receiver? If it does, the receiver must be a pointer. Slices and maps act as references, so their story is a little more subtle, but for instance to change the length of a slice in a method the receiver must still be a pointer.

In the examples above, if pointerMethod modifies the fields of s , the caller will see those changes, but valueMethod is called with a copy of the caller's argument that's the definition of passing a value , so changes it makes will be invisible to the caller. By the way, in Java method receivers are always pointers, although their pointer nature is somewhat disguised and there is a proposal to add value receivers to the language.

It is the value receivers in Go that are unusual. Second is the consideration of efficiency. If the receiver is large, a big struct for instance, it will be much cheaper to use a pointer receiver.

If some of the methods of the type must have pointer receivers, the rest should too, so the method set is consistent regardless of how the type is used.

See the section on method sets for details. For types such as basic types, slices, and small structs , a value receiver is very cheap so unless the semantics of the method requires a pointer, a value receiver is efficient and clear. See the relevant section of Effective Go for more details. The sizes of int and uint are implementation-specific but the same as each other on a given platform.

For portability, code that relies on a particular size of value should use an explicitly sized type, like int On bit machines the compilers use bit integers by default, while on bit machines integers have 64 bits. Historically, this was not always true. On the other hand, floating-point scalars and complex types are always sized there are no float or complex basic types , because programmers should be aware of precision when using floating-point numbers.

The default type used for an untyped floating-point constant is float For a float32 variable initialized by an untyped constant, the variable type must be specified explicitly in the variable declaration:.

Alternatively, the constant must be given a type with a conversion as in foo: From a correctness standpoint, you don't need to know. Each variable in Go exists as long as there are references to it. The storage location chosen by the implementation is irrelevant to the semantics of the language.

The storage location does have an effect on writing efficient programs. When possible, the Go compilers will allocate variables that are local to a function in that function's stack frame. However, if the compiler cannot prove that the variable is not referenced after the function returns, then the compiler must allocate the variable on the garbage-collected heap to avoid dangling pointer errors.

Also, if a local variable is very large, it might make more sense to store it on the heap rather than the stack. In the current compilers, if a variable has its address taken, that variable is a candidate for allocation on the heap.

However, a basic escape analysis recognizes some cases when such variables will not live past the return from the function and can reside on the stack. The Go memory allocator reserves a large region of virtual memory as an arena for allocations.

This virtual memory is local to the specific Go process; the reservation does not deprive other processes of memory. A description of the atomicity of operations in Go can be found in the Go Memory Model document. These packages are good for simple tasks such as incrementing reference counts or guaranteeing small-scale mutual exclusion.

For higher-level operations, such as coordination among concurrent servers, higher-level techniques can lead to nicer programs, and Go supports this approach through its goroutines and channels. For instance, you can structure your program so that only one goroutine at a time is ever responsible for a particular piece of data.

That approach is summarized by the original Go proverb ,. See the Share Memory By Communicating code walk and its associated article for a detailed discussion of this concept. Whether a program runs faster with more CPUs depends on the problem it is solving. The Go language provides concurrency primitives, such as goroutines and channels, but concurrency only enables parallelism when the underlying problem is intrinsically parallel.

Problems that are intrinsically sequential cannot be sped up by adding more CPUs, while those that can be broken into pieces that can execute in parallel can be sped up, sometimes dramatically. Sometimes adding more CPUs can slow a program down. In practical terms, programs that spend more time synchronizing or communicating than doing useful computation may experience performance degradation when using multiple OS threads.

This is because passing data between threads involves switching contexts, which has significant cost, and that cost can increase with more CPUs. For instance, the prime sieve example from the Go specification has no significant parallelism although it launches many goroutines; increasing the number of threads CPUs is more likely to slow it down than to speed it up. For more detail on this topic see the talk entitled Concurrency is not Parallelism.

How can I control the number of CPUs? Programs with the potential for parallel execution should therefore achieve it by default on a multiple-CPU machine. To change the number of parallel CPUs to use, set the environment variable or use the similarly-named function of the runtime package to configure the run-time support to utilize a different number of threads.

Setting it to 1 eliminates the possibility of true parallelism, forcing independent goroutines to take turns executing. Go's goroutine scheduler is not as good as it needs to be, although it has improved over time.

In the future, it may better optimize its use of OS threads. Goroutines do not have names; they are just anonymous workers. They expose no unique identifier, name, or data structure to the programmer. Some people are surprised by this, expecting the go statement to return some item that can be used to access and control the goroutine later.

The fundamental reason goroutines are anonymous is so that the full Go language is available when programming concurrent code. By contrast, the usage patterns that develop when threads and goroutines are named can restrict what a library using them can do. Here is an illustration of the difficulties. Once one names a goroutine and constructs a model around it, it becomes special, and one is tempted to associate all computation with that goroutine, ignoring the possibility of using multiple, possibly shared goroutines for the processing.

Moreover, experience with libraries such as those for graphics systems that require all processing to occur on the "main thread" has shown how awkward and limiting the approach can be when deployed in a concurrent language. The very existence of a special thread or goroutine forces the programmer to distort the program to avoid crashes and other problems caused by inadvertently operating on the wrong thread. For those cases where a particular goroutine is truly special, the language provides features such as channels that can be used in flexible ways to interact with it.

Doing so would allow a method to modify the contents of the value inside the interface, which is not permitted by the language specification. Even in cases where the compiler could take the address of a value to pass to the method, if the method modifies the value the changes will be lost in the caller. As an example, if the Write method of bytes. Buffer used a value receiver rather than a pointer, this code:. This is almost never the desired behavior.

One might mistakenly expect to see a, b, c as the output. What you'll probably see instead is c, c, c. This is because each iteration of the loop uses the same instance of the variable v , so each closure shares that single variable.

When the closure runs, it prints the value of v at the time fmt. Println is executed, but v may have been modified since the goroutine was launched. To help detect this and other problems before they happen, run go vet. To bind the current value of v to each closure as it is launched, one must modify the inner loop to create a new variable each iteration.

One way is to pass the variable as an argument to the closure:. In this example, the value of v is passed as an argument to the anonymous function. That value is then accessible inside the function as the variable u. Even easier is just to create a new variable, using a declaration style that may seem odd but works fine in Go:.

This behavior of the language, not defining a new variable for each iteration, may have been a mistake in retrospect. It may be addressed in a later version but, for compatibility, cannot change in Go version 1. There is no ternary testing operation in Go.

You may use the following to achieve the same result:. The if-else form, although longer, is unquestionably clearer. A language needs only one conditional control flow construct. Put all the source files for the package in a directory by themselves.

Source files can refer to items from different files at will; there is no need for forward declarations or a header file. Other than being split into multiple files, the package will compile and test just like a single-file package.

Inside that file, import "testing" and write functions of the form. Run go test in that directory. That script finds the Test functions, builds a test binary, and runs it. See the How to Write Go Code document, the testing package and the go test subcommand for more details.

Go's standard testing package makes it easy to write unit tests, but it lacks features provided in other language's testing frameworks such as assertion functions. An earlier section of this document explained why Go doesn't have assertions, and the same arguments apply to the use of assert in tests.

Proper error handling means letting other tests run after one has failed, so that the person debugging the failure gets a complete picture of what is wrong. It is more useful for a test to report that isPrime gives the wrong answer for 2, 3, 5, and 7 or for 2, 4, 8, and 16 than to report that isPrime gives the wrong answer for 2 and therefore no more tests were run.

The programmer who triggers the test failure may not be familiar with the code that fails. Time invested writing a good error message now pays off later when the test breaks. A related point is that testing frameworks tend to develop into mini-languages of their own, with conditionals and controls and printing mechanisms, but Go already has all those capabilities; why recreate them?

We'd rather write tests in Go; it's one fewer language to learn and the approach keeps the tests straightforward and easy to understand. If the amount of extra code required to write good errors seems repetitive and overwhelming, the test might work better if table-driven, iterating over a list of inputs and outputs defined in a data structure Go has excellent support for data structure literals.

The work to write a good test and good error messages will then be amortized over many test cases. The standard Go library is full of illustrative examples, such as in the formatting tests for the fmt package. There is no clear criterion that defines what is included because for a long time, this was the only Go library.

There are criteria that define what gets added today, however.

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This makes it much easier to build tools such as debuggers, dependency analyzers, automated documentation extractors, IDE plug-ins, and so on.

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