The eve package

[ Tags: bsd3, framework, library ] [ Propose Tags ]

An extensible event-driven application framework in haskell for building embarassingly modular software.

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Versions 0.1.0, 0.1.1, 0.1.2, 0.1.6, 0.1.7, 0.1.8
Dependencies base (>=4.9 && <5), containers, data-default, free, lens, mtl [details]
License BSD3
Copyright 2017 Chris Penner
Author Chris Penner
Category Framework
Home page
Source repository head: git clone
Uploaded Sat Apr 15 23:44:49 UTC 2017 by ChrisPenner
Distributions NixOS:0.1.8
Downloads 274 total (18 in the last 30 days)
Rating 0.0 (0 ratings) [clear rating]
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Status Docs available [build log]
Last success reported on 2017-04-15 [all 1 reports]
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Readme for eve-0.1.8

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An extensible event-driven application framework in haskell for building embarassingly modular software.


You can find hackage documentation for eve HERE

Getting started

Building A Game in Eve

Here's a guide which walks you through building your first application in Eve from start to finish, it's quite thorough and it's a great place to start!

If you have any issues (and I'm sure there'll be a few; it's a new project!) please report them here.

Core Principles

Eve's core principle is making it easy to build programs in a modular way. There are two key concepts in Eve which you should be aware of:

  • Events
  • State


Eve provides many useful combinators for dispatching events and adding listeners to events, events are a broad concept in Eve and can be triggered by user-interaction, file-changes, even network sockets! Anything you can think of really! Each time an event is fired, your app 'reacts' by running any associated listeners on the given event.

The functions you need to know are (with simplified types, see the real type in the hackage docs):

  • dispatchEvent :: forall eventType result m. (Monad m, Monoid result) => eventType -> m result
  • addListener :: forall eventType result m. (Monad m, Monoid result) (eventType -> m result) -> m ListenerId

As I mention above, these types are simplified a bit (and yet they still look complicated!). Actually, the types look so complex so that they're simpler to use! The forall makes it so that you can call dispatchEvent with ANY Typeable type and it will run the proper event listeners which were registered by addListener; those listeners can alter app state, or even dispatch more events! If the listeners return some (monoidal) value then the results from all listeners are combined with mappend and are returned. That's pretty much it!

Here's a quick example for those who need to see some code:

import Eve
import Data.Monoid

-- Define an event to listen for, in this case we don't even need any data alongside it.
data ComputeScore = ComputeScore

-- Define some computations which calculate some aspect of score.
-- We accept an argument of 'ComputeScore' to define what this is a listener for
scoreContributor1, scoreContributor2 :: ComputeScore -> App (Sum Int)
scoreContributor1 _ = do
  ... -- do some calculation over app state to determine one aspect of score
  return (Sum score)

scoreContributor2 _ = do
  ... -- Calculate some other aspect of the score
  return (Sum score)

-- In eve's initialization block we register the listeners, we could add these listeners anywhere
main :: IO ()
main = eve_ $ do
  ... -- other initialization (e.g. key listeners, etc.)
  addListener_ scoreContributor1
  addListener_ scoreContributor2

  -- This dispatches the triggering event and monoidally sums all the individual score components!
computeTotalScore :: App (Sum Int)
computeTotalScore = do
  Sum score <- dispatchEvent ComputeScore
  return score


Next we see how Eve handles state. Eve seeks to be as extensible as possible so it makes very few assumptions about the type of state that you (or your extensions) plan to store. You can define a type of state yourself using data and then provide actions which alter that state using a MonadState instance (from mtl). Don't worry if you don't know what that means, here's a real quick example which uses the combinators from the lens library to make a few simple state changes.

import Eve
import Control.Lens
data MyState = MyState
  { _myInt :: Int
  , _myString :: String
makeLenses ''MyState

-- This alters some state and returns the old string for some reason.
doSomething :: Action MyState String
doSomething = do
  oldString <- use myString
  myString .= "Hi!"
  myInt += 1
  return oldString

So what does this gain us? Well now if we have a MyState somewhere in our app we can run that Action on it! We can also register that Action as a listener for some event!

Now for the interesting part; handling state for extensions. This is usually a bit tricky since the types that an extension might use aren't known by you (the app author). Eve takes care of this by providing an interface for extensions to store and keep track of arbitrary types, while still allowing other extensions to run actions that it exports. This is where the HasStates typeclass comes in; here's the honest to goodness implementation:

class HasStates s  where
  states :: Lens' s States

If your state implements that typeclass, then extensions can store their own states inside it! It's pretty easy to implement too, let's add it to our MyState.

import Eve
import Control.Lens
data MyState = MyState
  { _myInt :: Int
  , _myString :: String
  , _myStates :: States
makeLenses ''MyState

instance HasStates MyState where
    states = myStates

Done! We added a new field which has the type States which is exported by Eve. Then we just took the lens created by makeLenses and used it in our instance. That's it! Now extensions can store their own state inside Action MyState by using the stateLens; check out the hackage docs on that for more info on how to do it!

Those are the basics, but you can do much more than that if you like! Eve also lets you add listeners and dispatch events on an Object specific basis! If you have a copy of some state (let's say a single instance of an Enemy in a game) you can dispatch events over that enemy individually and any registered (Action Enemy) callbacks will be run without affecting any other enemies! Check out HasEvents to see how that works.

One last cool feature is that event listeners can return information! If your event listener results in a return value that's a Monoid (like a list, or string for example) you can collect the responses of all the listeners when you call dispatchEvent. This is a great way for your application to 'ask' extensions about their state.

When designing applications in Eve; it's crucial to think about how the state of you application will be stored, and how different components interact. Eve works best when components are separated and communicate with each-other through events. This is because it allows those who will eventually write extensions to your application to 'hook' into those events to add functionality.

There are some definite Pros and Cons to Eve's approach:


  • Implementing most core functionality using the event system your app remains extensible.
  • Flexibility & Adaptability; applications can be written in such a way that users can replace entire components with alternate versions.


  • Module cross-dependencies makes the community infrastructure more fragile,
  • This architecture takes some getting used-to.



Eve uses Stack for reproducible builds.

  1. Install stack
  2. Clone this repo and cd into the directory
  3. Run stack build

Running Tests

  • stack test


Chatting about features is a key part of Eve's development; come join us in the Chat Room to discuss features or improvements!