Haxe

Unified language

The most unique aspect of the Haxe architecture was the decision to support Flash, JavaScript and server-side scripting with one unified language. In typical web development projects, developers must use many different languages to build an integrated web application:

Haxe originated with the idea of supporting all such aspects in one language, and simplifying the communication logic between them. Thus, the application logic need not deal with the implementation of the communication, which usually involves working with XML files.

The original goals of Haxe were:

Compiler

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.

Targets

In Haxe, supported platforms are known as "targets", which are Haxe modules that provide access to core-APIs (language and bytecode targets), for the compiler-backends that are responsible for generating the respective code, and for runtimes with specific APIs that go beyond the core language support (platform-targets).

Performance

The Haxe compiler is an optimizing compiler, and uses function inlining, constant folding, and dead code elimination (DCE) to optimize the run-time performance of compiled programs.

The run-time performance of Haxe programs varies depending on the target platform:

Development

Development of Haxe began in October 2005. The first beta version was released in February 2006. Haxe 1.0 was released in April 2006, with support for Adobe Flash, JavaScript, and Neko programs.

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.

Haxe has much in common with ActionScript 3. The Haxe compiler is developed in the OCaml language. No knowledge of OCaml is needed to develop applications using Haxe.

Advantages to using Haxe include:

Source code

The recommended IDE for Haxe development is FlashDevelop, which supports ActionScript 2, 3 and Haxe as first-class languages with syntax highlighting, code completion, and other features. Code folding, code refactoring and interactive debugging is also supported within the IDE.

To help leverage extant code, the open-source community has created source code converters for:

To help build applications in various platforms, or convert Haxe code into other languages, the Haxe compiler can compile Haxe to: ActionScript 3, C++, C#, Java, PHP, Python.

Platform support

The Haxe language can compile into bytecode for different virtual machines such as the Adobe Flash Player and Neko, and can generate source code in ActionScript 3, JavaScript, Lua, and includes experimental support for C++ and C#.

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.

The following table documents platform and language support in Haxe. Most Haxe programs will function without changes in any given language, although Haxe programs that use a platform-specific API will function only on the given platform.

Language

Haxe is a general-purpose language supporting object-oriented programming, generic programming, and various functional programming constructs. Features such as iterations, exceptions, and 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 and give compile-time errors, but it also enables the programmer to bypass type-checking and rely on the target platform's dynamic type-handling.

Haxe is similar to ECMAScript, although almost no ECMAScript code will run on Haxe without modifications. Unlike ECMAScript, Haxe is a compiled language. Haxe is inspired by ActionScript, Java, and OCaml.

Since Haxe had its origins in ActionScript 3, all of the extant Flash API can be used, although Haxe requires better-formed code and programming standards than Adobe compilers (for example, regarding scoping and data typing).

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 (ADTs, called enum in Haxe), and abstract types. Parametric polymorphism is possible with classes, ADTs 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.

Classes

Classes (keyword class) in Haxe are similar to those in Java or ActionScript 3. Their fields can be either methods, variables, or properties, each static or per instance respectively. Haxe supports the accessors public and private, and more advanced methods for access control (ACL, link) that are denoted using annotations. Methods and static constant variables can be inlined using the keyword inline.

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

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 switch 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 typedef:

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 F is a function that takes an Int and a String as arguments, and returns a value of type Float.

The same notation in a language with unary functions only, would refer to a function that takes an Int as argument, and returns a function of type String->Float.

Types F2 and F3 denote the same type. Both are binary functions that return a binary function of type F. For F3 the syntax to declare a function type within a function type is used.

Abstract types

The latest 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 extant 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.

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.