Hack is a programming language for the HipHop Virtual Machine (HHVM), created by Facebook as a dialect of PHP.The language implementation is open-source, licensed under the MIT License.
|Designed by||Julien Verlaguet, Alok Menghrajani, Drew Paroski, and others|
|Typing discipline||Static, dynamic, weak, gradual|
|PHP, OCaml, Java, C#, Scala, Haskell|
Hack is a programming language for the HipHop Virtual Machine (HHVM), created by Facebook as a dialect of PHP. The language implementation is open-source, licensed under the MIT License.
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- Programming productivity without breaking things.
- 'I don't always write PHP, but when I do, I use Hack.'
- Hack 在你编写代码的时候会检查你的代码文件并提供即时类型检查，并且它通常在 200 毫秒内即可完成，不会有很明显的延迟，因此你可以很轻松地集成到你的开发流程中去.
Hack allows programmers to use both dynamic typing and static typing. This kind of a type system is called gradual typing, which is also implemented in other programming languages such as ActionScript. Hack's type system allows types to be specified for functionarguments, function return values, and class properties; however, types of local variables are always inferred and cannot be specified.
Hack was introduced on March 20, 2014. Before the announcement of the new programming language, Facebook had already implemented the code and 'battle tested' it on a large portion of its web site.
Hack is designed to interoperate seamlessly with PHP, which is a widely used open-source scripting language that has a focus on web development and can be embedded into HTML. A majority of valid PHP scripts are also valid in Hack; however, numerous less frequently used PHP features and language constructs are not supported in Hack.
Hack extends the type hinting available in PHP 5 through the introduction of static typing, by adding new type hints (for example, for scalar types such as integer or string), as well as by extending the use of type hints (for example, for class properties or function return values). However, types of local variables cannot be specified. Since Hack uses a gradual typing system, in the default mode, type annotations are not mandatory even in places they cannot be inferred; the type system will assume the author is correct and admit the code. However, a 'strict' mode is available which requires such annotations, and thus enforces fully sound code.
Syntax and semantics
The basic file structure of a Hack script is similar to a PHP script with a few changes. A Hack file does not include the
<?php opening markup tag and forbids using top-level declarations. Code must be placed in an entrypoint function. These are automatically executed if they are in the top-level file, but not if the file is included via
require, or the autoloader. Like other functions in Hack, the function names must be unique within a project – i.e. projects with multiple entrypoints can not call both
The above script, similar to PHP, will be executed and the following output is sent to the browser:
Hackland Aloe Farm
Unlike PHP, Hack and HTML code do not mix; either XHP or another template engine needs to be used.
Like PHP 7, Hack allows types to be specified for function arguments and function return values. Functions in Hack are thus annotated with types like the following:
- ^Bryan O'Sullivan (2014-03-28). 'Where Credit Belongs for Hack'. Retrieved 2019-02-06.
- ^ ab'facebook/hhvm: hhvm / hphp / hack / LICENSE'. github.com. Facebook. 2018-04-11. Retrieved 2019-02-06.
- ^ abJosh Lockhart (2014-04-03). 'Facebook's Hack, HHVM, and the future of PHP'. O'Reilly Media. Retrieved 2019-02-06.
- ^Cade Metz (2014-03-20). 'Facebook Introduces 'Hack,' the Programming Language of the Future'. Wired. Retrieved 2019-02-06.
- ^Aseem Rastogi; Avik Chaudhuri; Basil Hosmer (January 2012). 'The Ins and Outs of Gradual Type Inference'(PDF). Association for Computing Machinery (ACM). Retrieved 2019-02-06.
- ^ ab'Hack Manual: Hack and HHVM – Type Annotations'. docs.hhvm.com. Retrieved 2019-02-06.
- ^Verlaguet, Julien; Menghrajani, Alok (2014-03-20). 'Hack: a new programming language for HHVM'. Facebook. Retrieved 2019-02-06.
- ^ ab'Inconsistencies: Introduction'. docs.hhvm.com. Retrieved 2019-04-04.
- ^'Hack Manual: Partial Mode'. docs.hhvm.com. Retrieved 2019-02-06.
- ^'Hack Manual: Strict Mode'. docs.hhvm.com. Retrieved 2019-02-06.
- ^Emmott, Fred (2019-02-11). 'HHVM 4.0.0'. hhvm.com. Retrieved 2019-05-02.
- Official website and language reference
Sealed classes and interfaces represent restricted class hierarchies that provide more control over inheritance. All subclasses of a sealed class are known at compile time. No other subclasses may appear after a module with the sealed class is compiled. For example, third-party clients can't extend your sealed class in their code. Thus, each instance of a sealed class has a type from a limited set that is known when this class is compiled.
The same works for sealed interfaces and their implementations: once a module with a sealed interface is compiled, no new implementations can appear.
In some sense, sealed classes are similar to
enum classes: the set of values for an enum type is also restricted, but each enum constant exists only as a single instance, whereas a subclass of a sealed class can have multiple instances, each with its own state.
To declare a sealed class or interface, put the
sealed modifier before its name:
A sealed class is abstract by itself, it cannot be instantiated directly and can have
Constructors of sealed classes can have one of two visibilities:
protected (by default) or
Location of direct subclasses
Direct subclasses of sealed classes and interfaces must be declared in the same package. They may be top-level or nested inside any number of other named classes, named interfaces, or named objects. Subclasses can have any visibility as long as they are compatible with normal inheritance rules in Kotlin.
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Hacklang Vec Map
Subclasses of sealed classes must have a proper qualified name. They can't be local nor anonymous objects.
enum classes can't extend a sealed class (as well as any other class), but they can implement sealed interfaces.
Inheritance in multiplatform projects
There is one more inheritance restriction in multiplatform projects: direct subclasses of sealed classes must reside in the same source set. It applies to sealed classes without the
If a sealed class is declared as
expect in a common source set and have
actual implementations in platform source sets, both
actual versions can have subclasses in their source sets. Moreover, if you use a hierarchical structure, you can create subclasses in any source set between the
Learn more about the hierarchical structure of multiplatform projects.
Sealed classes and when expression
The key benefit of using sealed classes comes into play when you use them in a
when expression. If it's possible to verify that the statement covers all cases, you don't need to add an
else clause to the statement. However, this works only if you use
when as an expression (using the result) and not as a statement:
when expressions on
expect sealed classes in the common code of multiplatform projects still require an
else branch. This happens because subclasses of
actual platform implementations aren't known in the common code.