helm: A functionally reactive game engine.

Introduction

Helm is a functionally reactive game engine written in Haskell and built around the Elerea FRP framework. Helm is heavily inspired by the Elm programming language (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.

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.

Documentation of the Helm API is available on Hackage. There is currently a heavily work-in-progress guide on Helm's website, which is a resource aiming to give thorough explanations of the way Helm and its API work through examples. You can ask on the mailing list if you're having any trouble with using the engine for games or working on the engine itself, or if you just want to chit-chat about Helm.

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-blitting 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, FRP.Helm.Utilities 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.Animation contains a simple implementation of animations. Each animation is made up of a list of frames which render a form at a specific time.
  • 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.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.Utilities contains a few useful functions, such as lifting/folding signal generators containing signals.
  • FRP.Helm.Time contains functions for composing units of time and signals that sample from the game clock.
  • FRP.Helm.Transition contains functions for composing transitions allowing you to animate between interpolable types, e.g. colors.
  • 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:

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 = do
  engine <- startup defaultConfig

  run engine $ render <~ Window.dimensions engine

It renders a red square at the position (100, 100) with a side length of 64.

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.

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 { mx = mx, my = my }) =
  centeredCollage w h [move (mx, my) $ filled white $ square 100]

main :: IO ()
main = do
    engine <- startup defaultConfig

    run engine $ render <~ Window.dimensions engine ~~ stepper

  where
    state = State { mx = 0, my = 0 }
    stepper = foldp step state Keyboard.arrows

Checkout the demos folder for more examples.

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. Read the installing guide on the website for a few platform-specific instructions.

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 when there is a logical flow of data, spaces between operators and after commas, etc. Please also confirm that the code passes under HLint.

There are a number of issues tagged with the bounty tag, meaning they have associated bounties on Bountysource.