helm-0.3.0: README.md

# Helm

## Introduction

Helm is a functionally reactive game engine written in Haskell and built around
the [Elerea FRP framework](https://github.com/cobbpg/elerea). Helm is
heavily inspired by the [Elm programming language](http://elm-lang.org) (especially the API).
All rendering is done through a vector-graphics based API. At the core, Helm is
built on SDL and the Cairo vector graphics library. The plan is to change to a more
robust setup in the future, such as a lightweight homebrewed renderer built on OpenGL.
But for now, Cairo performs pretty well.

In Helm, every piece of input that can be gathered from a user (or the operating system)
is hidden behind a signal. For those unfamiliar with FRP, signals are essentially
a value that changes over time. This sort of architecture used for a game allows for pretty
simplistic (and in my opinion, artistic) code.

## Features

* Allows you to express game logic dependent on input in a straightforward manner,
  treating events as first class values (the essence of FRP).

* Vector graphics based rendering, allow you to either write art
  designed for any resolution or still load generic images and render
  those as you would with any pixel-based direct blitting game engine.

* Straightforward API heavily inspired by the Elm programming language. The API
  is broken up into the following areas:

  * `FRP.Helm` contains the main code for interfacing with the game engine but
    also includes some utility functions and the modules `FRP.Helm.Color` and `FRP.Helm.Graphics`
    in the style of a sort of prelude library, allowing it to be included and readily
    make the most basic of games.

  * `FRP.Helm.Automaton` contains the `Automaton` data structure and functions
    for composing, creating and calculating them. Automatons are a useful
    abstraction of a dynamic process that is fed input from a signal
    and feeds output through a signal. This is really useful for things
    like animation systems, accumulating network packets and other
    stateful but input dependent things.

  * `FRP.Helm.Color` contains the `Color` data structure, functions for composing
    colors and a few pre-defined colors that are usually used in games.

  * `FRP.Helm.Graphics` contains all the graphics data structures, functions
    for composing these structures and other general graphical utilities.

  * `FRP.Helm.Joystick` contains signals for working with joystick state.

  * `FRP.Helm.Keyboard` contains signals for working with keyboard state.

  * `FRP.Helm.Mouse` contains signals for working with mouse state.

  * `FRP.Helm.Text` contains functions for composing text, formatting it
    and then turning it into an element.

  * `FRP.Helm.Window` contains signals for working with the game window state.

## Example

The simplest example of a Helm game that doesn't require any input from the user is the following:

```haskell
import FRP.Helm
import qualified FRP.Helm.Window as Window

render :: (Int, Int) -> Element
render (w, h) = collage w h [move (100, 100) $ filled red $ square 64]

main :: IO ()
main = run $ do
  dims <- Window.dimensions

  return $ fmap render dims
```

It renders a red square at the position `(100, 100)` with a side length of 64px.  
  
The next example is the barebones of a game that depends on input. It shows how to create
an accumulated state that depends on the values sampled from signals (e.g. mouse input).
You should see a white square on the screen and pressing the arrow keys allows you to move it.

```haskell
{-# LANGUAGE RecordWildCards #-}

import Control.Applicative
import FRP.Elerea.Simple
import FRP.Helm
import qualified FRP.Helm.Keyboard as Keyboard
import qualified FRP.Helm.Window as Window

data State = State { mx :: Double, my :: Double }

step :: (Int, Int) -> State -> State
step (dx, dy) state = state { mx = (realToFrac dx) + mx state,
                              my = (realToFrac dy) + my state }

render :: (Int, Int) -> State -> Element
render (w, h) (State { .. }) = collage w h [move (mx, my) $ filled white $ square 100]

main :: IO ()
main = run $ do
  dims <- Window.dimensions
  arrows <- Keyboard.arrows
  stepper <- transfer (State { mx = 0, my = 100 }) step arrows

  return $ render <$> dims <*> stepper
```

## Installing and Building

Helm requires GHC 7.6 (Elerea doesn't work with older versions due to a compiler bug).
To install the latest (stable) version from the Hackage repository, use:

```
cabal install helm
```

Alternatively to get the latest development version, you can clone this repository and then run:

```
cabal install
```

You may need to jump a few hoops to install the Cairo bindings (which are a dependency),
which unfortunately is out of my hands.

## License

Helm is licensed under the MIT license. See the `LICENSE` file for more details.

## Contributing

Helm would benefit from either of the following contributions:

1. Try out the engine, reporting any issues or suggestions you have.

2. Look through the source, get a feel for the code and then
   contribute some features or fixes. If you plan on contributing
   code please submit a pull request and follow the formatting
   styles set out in the current code: 2 space indents, documentation
   on every top-level function, favouring monad operators over
   do blocks, etc.

The following is a list of areas I want to tackle in the future, 
and possible targets that others could try for:

* Improve the API. There's a few API calls from Elm that would work
  just as nicely in Helm. These are marked inside TODOs in the code.
  There also other important things that it's missing,
  such as audio, joysticks and loading a larger range of
  image formats.

* Backend wise, it would be nice to use OpenGL instead of Cairo.
  Cairo isn't particuarly that well performing for graphic intensive games,
  although work is done being towards to fix that. However, using
  OpenGL would make the engine more lightweight, easier to port
  and be incredibly easier to accelerate. This means I have
  to write the full vector graphics stack myself, but the worse part
  will probably just be line styles, the rest should be moderately easy.
  This will also allow loading of multiple image formats, as the current
  reason for not using SDL_image is that it's annoying as fuck
  to integrate with Cairo. Helm also currently uses the Cairo toy text
  API for rendering, which isn't suppose to be used in production. If switched
  to OpenGL, SDL_ttf would be a better fit.

* Optimizations and testing. This is a early release of the engine so
  obviously little testing or optimizations have been done.
  It's a little hard to set up a test framework for a game engine,
  but I have a few ideas, such as writing a dummy version of the backend
  that simply renders to a PNG file that is fed fake (but predictable) input,
  which is then compared to a static PNG file to see if the final expected
  rendering outcome was achieved.

* Port and support multiple platforms. I've only been testing it on
  Linux, but there's really no reason that it wouldn't work out of the box
  on Windows or OSX after setting up the dependencies. But I'd definitely
  also like to investigate Android and iOS.