Haxe

Haxe is a high-level cross-platform programming language and compiler that can produce applications and source code for many different computing platforms from one code-base. It is free and open-source software, released under an MIT License. The compiler, written in OCaml, is released under the GNU General Public License (GPL) version 2.

Haxe includes a set of features and a standard library supported across all platforms, including numeric data types, strings, arrays, maps, binary, reflective programming, maths, Hypertext Transfer Protocol (HTTP), file system and common file formats. Haxe also includes platform-specific application programming interfaces (APIs) for each compiler target. Kha, OpenFL, and Heaps.io are popular Haxe frameworks that enable creating multi-platform content from one codebase.

Haxe originated with the idea of supporting client-side and server-side programming in one language, and simplifying the communication logic between them. Code written in Haxe can be compiled into JavaScript, C++, Java, JVM, PHP, C#, Python, Lua and Node.js. Haxe can also directly compile SWF, HashLink, and NekoVM bytecode and also runs in interpreted mode.

Haxe supports externs (definition files) that can contain data type information of extant libraries to describe target-specific interaction in a type-safe manner, like C++ header files can describe the structure of existing object files. This enables to use the values defined in the files as if they were statically typed Haxe entities. Beside externs, other solutions exist to access each platform's native capabilities.

Many popular IDEs and source code editors have support available for Haxe development. No particular development environment or tool set is officially recommended by the Haxe Foundation, although VS Code, IntelliJ IDEA and HaxeDevelop have the most support for Haxe development. The core functionalities of syntax highlighting, code completion, refactoring, debugging, etc. are available to various degrees.

History
Development of Haxe began in October 2005. The first alpha version was released on November 14, 2005. Haxe 1.0 was released in April 2006, with support for Adobe Flash, JavaScript, and NekoVM programs. Support for PHP was added in 2008, and C++ was added in 2009. More platforms such as C# and Java were added with a compiler overhaul in 2012.

Haxe was developed by Nicolas Cannasse and other contributors, and was originally named haXe because it was short, simple, and "has an X inside", which the author asserts humorously is needed to make any new technology a success.

Haxe is the successor to the open-source ActionScript 2 compiler MTASC, also built by Nicolas Cannasse and is released under the GNU General Public License version 2 or later.

Compiler
The Haxe language can compile into bytecode that can be executed directly by the virtual machines it targets. It can compile to source code in C++, JavaScript, PHP, C#, Java, Python, and Lua. Haxe also has an interpreter called eval. This same interpreter is also used compile-time to run macros, which allow modification of the abstract syntax tree (AST).

This strategy of compiling to multiple source code languages is inspired by the write once, run anywhere paradigm. It also allows the programmer to choose the best platform for the job. Typical Haxe programs run identically on all platforms, but developers can specify platform-specific code and use conditional compilation to prevent it from compiling on other platforms.

The Haxe compiler is an optimizing compiler, and uses field and function inlining, tail recursion elimination, constant folding, loop unrolling and dead code elimination (DCE) to optimize the run-time performance of compiled programs. The Haxe compiler offers opt-in null-safety, it checks compile-time for nullable values.

Targets
In Haxe, supported platforms are known as "targets", which consist of the following modules:
 * The compiler-backends that are responsible for generating the respective code.
 * The run-time specific APIs that go beyond the core language support (platform-targets).

The following table documents platform and language support in Haxe. The Haxe language allows developers to gain access to many platform features, but Haxe is not a full featured engine, they might need frameworks that enable create content for certain platforms.

Advantages

 * Ability to target multiple platforms and devices using the same language
 * Ability to use strictly-typed code
 * Ability to use macros (syntax transformation) which can be done with Haxe language
 * Added language features such as extension methods and functional programming
 * The run-time performance of Haxe programs is at comparable speed to handwritten sources.

Language
Haxe is a general-purpose programming language supporting object-oriented programming, generic programming, and various functional programming constructs. Features such as iterations, exceptions, and reflective programming (code reflection) are also built-in functions of the language and libraries. Unusual among programming languages, Haxe contains a type system which is both strong and dynamic. The compiler will check types implicitly via type inference and give compile-time errors, but it also enables programs to bypass type-checking and rely on a target platform's dynamic type-handling. All of the native target APIs can be used.

Type system
Haxe has a sophisticated and flexible type system. The type kinds it offers are classes, interfaces, function-method types, anonymous types, algebraic data types (called enum in Haxe), and abstract types. Parametric polymorphism is possible with classes, algebraic types and function types, giving the language support for generic programming based on type erasure. This includes support for variance in polymorphic functions, although not in type constructors.

The type system is static unless annotations for dynamic typing are present, for use with targets that support them. Type checking follows nominal typing with the exception of anonymous types where structural typing is used instead. Finally, type inference is supported, allowing for variable declarations without type annotations.

Modules and namespaces
All Haxe code is organized in modules, which are addressed using paths. In essence, each .hx file represents a module which may contain several types. For example, to create the type  in the package my.pack as shown, the folder structure should be my\pack and the file could be A.hx in the folder pack. In other modules, other types can be imported by putting  statements below the package definition, e.g.

A module can contain multiple types, such as the following. It is possible to import one type at a time from that module, using. A type may be, in which case only its containing module can access it.

Classes
Classes (keyword ) in Haxe are similar to those in Java or TypeScript. Their fields can be either methods, variables, or properties, each static or per instance respectively. Haxe supports the accessors  and , and more advanced methods for access control that are denoted using annotations. Methods and static constant variables can be inlined using the keyword. Fields can be marked as  to declare a constant that must be initialized immediately or in the constructor and cannot be written to, in case of function   will mark as non-overridable in subclasses.

Interfaces in Haxe are very similar to those in, for example, Java.

Generics
Haxe supports generic programming. The following is an example of the identity function.

Enumerated types
Enumerated types are an important feature of the language; they can have type parameters and be recursive. They provide basic support for algebraic data types, allowing the inclusion of product types, in a fashion similar to Haskell and ML. A  expression can apply pattern matching to an enum value, allowing for elegant solutions to complex programming problems:

Examples of parametric enum types are the Haxe standard library types Option and Either:

Haxe also supports generalized algebraic data types (GADTs).

Anonymous types
Anonymous types are defined by denoting their structure explicitly, using a syntax that follows the mathematical record-based representation of a type. They can be used to implement structural typing for function arguments (see below), and can be given an alias with the keyword :

Function types
Functions are first-class values in Haxe. Their type is denoted by using arrows between argument types, and between the argument type(s) and return type, as common in many functional languages. However, unlike in prominent examples like Haskell or the ML language family, not all functions are unary functions (functions with one argument only), and in Haxe, functions can't be partially applied per default. Thus, the following type signatures have different semantics than in the aforementioned languages. The type  is a function that takes a   as arguments and returns a value of type.

Types  and   denote the same type, except that   uses labelled parameter, which is useful for completion and documentation.

Types  and   denote the same type. Both are binary functions that return a binary function of type. For  the syntax to declare a function type within a function type is used.

Abstract types
A relatively new addition to the Haxe type system is a concept termed abstract types. As used in Haxe, this refers to something different from a conventional abstract type. They are used to make conversions between types implicit, allowing reuse of existing types for specific purposes, like implementing types for units of measurement. This greatly reduces the risk of mixing up values of the same underlying type, but with different meanings (e.g., miles vs. km).

The following example assumes that the metric system is the default, while a conversion to miles is needed for legacy data. Haxe can automatically convert miles to kilometers, but not the reverse.

As the example shows, no explicit conversion is needed for the assignment "km = one100Miles;" to do the right thing.

Abstract types are entirely a compile-time feature of Haxe and do not exist at all at program runtime. As an example, both variables using abstract types above  and   will be of the type   at runtime.

Structural typing
In many functional programming languages, structural typing plays a major role. Haxe employs it in the presence of anonymous types, using the nominative typing of object-oriented programming, when only named types are involved. Anonymous types in Haxe are analogous to the implicit interfaces of the language Go as to typing. In contrast with Go interfaces, it is possible to construct a value using an anonymous type.

The Haxe compiler is divided into one frontend and multiple backends. The frontend creates an abstract syntax tree (AST) from the source code, and performs type checking, macro expansion, and optimization on the AST. The various backends translate the processed AST into source code or generate bytecode, depending on their target.

The compiler is written in OCaml. It can be run in server-mode to provide code completion for integrated development environments (IDEs) and maintain a cache, to further speed compiling.