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A scripting language for D. Inspired heavily by JavaScript.

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# DMildew

Mildew is a scripting language for the D programming language inspired by Lua and JavaScript. While there are other scripting languages for D such as Lua, one cannot easily use D delegates as C function pointers in those languages. Other languages require modification of the D class in order to bind. With Mildew, bindings for any public method or property can be written without touching the original D class module. The downside is that there is no way to trivialize this binding process with metaprogramming yet. The prototype inheritance system of Mildew allows scripts to extend D classes in powerful ways.

The ideal use case for this software is for embedding in D applications with an event loop that require scriptable components. It is not intended to replace Node.js. Usage of this library on embedded devices may be limited due to file size.

This software is licensed under the GNU General Public License version 3.0 so that it may be used in free and open software. For a commercial software usage license, please contact the author. In the future, the license will be changed to LGPL3 when the software is more stable and ready for production use.

One can try DMildew from their browser [here](http://pillager86.xyz). Note that variables cannot be saved between commands in the online REPL. In the future this will change when the REPL server is implemented with vibe.d

## Usage

The examples/ folder contains example scripts. It should look familiar to anyone who knows JavaScript. However, Mildew is not a full feature ES6 JavaScript implementation.

This project is in its early stages so one should probably use the ~main version to get the latest bug fixes. The release tags are only so that it is usable in dub.

The REPL sub-project dmildew:run shows how to instantiate an Interpreter instance and evaluate lines of Mildew code. Documentation for the D library API can be found [here](https://dmildew.dpldocs.info/mildew.html). The main interface for the API is `mildew.interpreter.Interpreter`. The asynchronous callback API is still a work in progress and will require the host application to have some sort of event loop that calls the appropriate Interpreter method at the end each cycle, or an Interpreter method that runs all pending Fibers to completion before exiting.

## Mildew Standard Library Documentation

The documentation for the standard library usable by scripts, which is only loaded if the host application chooses to do so, can be found [here](https://pillager86.github.io/dmildew/).

## Building

Building the library is as simple as writing `dub build` in a terminal in the main project directory. To build the REPL and script runner one can write `dub build dmildew:run` in the same directory as the main project. Add `-b release` to the build and run commands to generate an optimized binary that performs slightly better than the default debugging build.

To build the fs library for reading and writing files, `cd` to the fs directory and run `dub build`. It will automatically place the shared library in the directory above it. The fs library is not available on Windows.

## Compiling and Running Bytecode Files

A script can be compiled with `dub run dmildew:bccompiler -- <name of script file.mds> -o <name of binary.mdc>` and the resulting binary bytecode file can be run directly with the REPL as if it were a normal text file of source code. The API is not stable yet so bytecode programs may need to be recompiled each pre-release.

## Running the Examples

In a terminal in the main project directory run `dub run dmildew:run -- examples/<nameofexample>.mds`. To try the interactive shell simply type `dub run dmilew:run`. In the interactive shell it is only possible to continue a command on a new line by writing a single backslash at the end of a line. Functions, variables, and classes declared in one REPL command will not be accessible in the next unless stored in a var. To store a class such as `class Foo {}` one must write `var Foo = Foo;` immediately after. One can also store anonymous class expressions in a global variable such as `var Foo = class {};`.

The option `-d` prints bytecode disassembly before running each chunk of code. The option `-v` prints highly verbose step by step execution of bytecode in the virtual machine. For example, to print detailed information while running the REPL, the dub command would be `dub run dmildew:run -- -v -d`.

If the fs library was built, the option `--lib=fs` can be added to load the fs library and use functions in the fs namespace. This feature is not available on Windows.

## Binding

See source/mildew/stdlib/global.d for how to bind free functions and source/mildew/stdlib/console.d for how to store free functions in a "namespace." Classes are bound by assigning the native D object to the thisObj's nativeObject field after casting the thisObj to ScriptObject. The constructor ScriptFunction should have a "prototype" field (accessed with ["prototype"] and not to be confused with the .prototype D property) containing functions to serve as the bound D class methods. The "prototype" field itself should have a field called "constructor" set to the constructor so that the `instanceof` operator will work. The Date and RegExp libraries are a good example of how class binding works.

Binding structs can only be done by wrapping the struct inside a class and storing the class object in a ScriptObject. The RegExp library is an example of this, as it wraps the D std.regex.Regex!char struct.

The function or delegate signature that can be wrapped inside a ScriptAny (and thus ScriptFunction) is `ScriptAny nameOfBinding(Environment, ScriptAny* thisObj, ScriptAny[] args, ref NativeFunctionError);` And such a function is wrapped by `ScriptAny someFunc = new ScriptFunction("name of function", &nameOfBinding);`. This is analogous to how Lua bindings work.

`bindingexample2.zip` in the examples folder contains a simple program that binds a class and its public methods and properties. D classes that are bound can be extended by the script as long as the native function constructor checks that the `thisObj` parameter is an object and assigns the native object to its `nativeObject` field. `bindingexample3.zip` shows a more advanced example of binding D classes that have an inheritance hierarchy. The power of Mildew is that methods written for the base class will automatically work on the bound subclasses.

## Caveats

This language is stricter than JavaScript. Global variables cannot be redeclared unless they are undefined by setting them to `undefined`. Local variables cannot be redeclared in the same scope likewise. However, it is possible to check if a variable is defined using the `isdefined` function of the standard library, which takes a string value equal to the variable name. Semicolons are required in a manner similar to C# or Java.

Since all programs are run in a new scope, the `var` keyword declares variables that are stored in the global scope, while `let` and `const` work the same as in ES6. This is more similar to Lua.

For-in loops over Strings iterate for each unicode code point, rather than each 8-bit character, although Strings are internally stored as UTF-8. Array indexing of Strings can access individual 8-bit characters as long as they form a complete code point.

To declare a function to be stored in an object, one must write `objectName.fieldName = function(...)...` because `function objectName.fieldName(...)...` declarations do not work.

Binding classes by extending ScriptObject will not work and is not supported. Script classes that extend native D classes must call `super` in a constructor for it to work even if there are no parameters.

Closure functions that refer to variables in an outer scope beyond the immediate function declaration scope can have variables shadowed by declaring them in the same scope. See examples/thistest.mds for the issue.

The "super" keyword cannot be used to access static base class methods.

When using the destructuring variable declarations, whichever variable name is associated with the spread operator is placed as the last variable name in the list during compilation. The spread variable in an object destructuring receives the entire object. Destructuring in other contexts besides variable declaration is not implemented.

Arguments with default values must be the last arguments in an argument list.

If any complex data type is used as a key to a Map, modifying the key causes undefined behavior.

Mildew is not optimized for computationally heavy tasks. The design of the language focuses on interoperability with D native functions and CPU intensive operations should be moved to native implementations and called from the scripting language.

Mildew has only been tested on Windows and Linux x86_64 operating systems and built with DMD v2.094.2. Please test on other operating systems and compilers to report problems. Note that compiled bytecode is platform dependent (endianness matters) and bytecode scripts must be compiled for each type of CPU, similar to Lua.

## Help

There is now a ##dmildew channel on the Freenode IRC network. If no one is there, leave a question or comment on the github project page.

## Current Goals

* Possibly support importing other scripts from a script. However, most host applications would probably prefer to do this with XML/JSON tables of contents and their own solution. The `runFile` stdlib function exists but is not intended for production use and will be replaced or removed.
* Implement certain future script libraries as shared libraries that can be dynamically loaded at runtime. The only libraries that would qualify are classes that are not used by the core runtime. For example, Generator and RegExp are part of the core runtime and cannot be separated into dynamic libraries.
* Optional possibly asynchronous file I/O library not loaded by the default library loading function due to security. This would be enabled in the REPL only by a specific option and possibly implemented as a shared library module.
* The Promise class for wrapping asynchronous APIs. (In progress.)
* Bind native classes and functions with one line of code with mixins and template metaprogramming. Or write software that will analyze D source files and generate bindings.
* Write a more complete and robust standard library for the scripting language. (In progress.)
* Allow all alphanumeric unicode characters as components of identifier and label tokens. Currently identifiers are limited to ASCII characters.
* Implement asynchronous functions and function calls. (In progress.)
* Destructuring of function parameters and the ability to parse `{}` expressions as the left hand side of a binary expression.

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