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spago install game


To test one of the examples, run npm run example-<Name>, replacing <Name> with the name of the example or test you want to run. This will create a server that auto-rebuilds every time you make a change to its source, and open a browser window. The examples are located in /test/Test/<Name>.

Open the REPL using npm run repl (npx spago -x dev.dhall repl).



Module reference is published on Pursuit.

The Game module


Reducer is for describing a reduction of a Run effect row. Its first argument extra contains the effects that will be removed, and its second argument req contains the effects that must be present in the row to perform the reduction. So, having a Reducer extra req means that you can interpret all the effects in extra either purely or in terms of any of the effects in req. mkReducer is a function you can use to construct a reducer, where you provide a function Run (Anything + extra_req) ~> Run (Anything + req) where extra_req is the union of extra and req. See the top-level docs in Run.Unsafe for a more in-depth explanation of Anything.

There is provided an identityReducer, which is a reducer that doesn't reduce any effects, useful for when your extra is an empty row. There is also composeReducer (infix version (>->)), which composes two reducers left to right.


GameUpdate is a type constructor that takes four arguments: extra ∷ # Type, req ∷ # Type, execOut ∷ # Type and a ∷ Type. It's a newtype around Reducer extra req → Run execOut a. In essence, it's a Run action that can run effects from extra, provided all effects in req are present.

If you want to make a template function to construct a GameUpdate, you can structure it like this:

  • Have update be the union of execIn and extra, and have the caller supply a Run update a that you convert to a Run execIn a using the reducer.
  • Turn Run execIn a into Run execOut a, reducing the specific effects for your update template into the more general effects supported globally in the Game.
  • execIn will in most cases be a superset of execOut

Note that it is usually recommended to make your own row type like this:

type MyUpdateExecIn r =
  ( {- your execIn effects go in here, looking like `name ∷ EFFECT_NAME` -}
  | r)`

Where you omit the Union constraint and the update type variable, and use MyUpdateExecIn extra instead of update. You can find an explanation to why this may be needed here.


A Game is simply an Array of GameUpdates.

The function mkRunGame lets you create a function that will run the game. It takes two functions as input:

  • interpret ∷ Run execOut a → Run interpreted b
    This function decides how all the updates will get interpreted individually.
  • parallelize ∷ Array (Run interpreted b) → Run interpreted b
    This function decides how all the updates will be combined together. It is called parallelize internally because it will usually run all the updates in parallel. This is what mkAffGame does.

After calling mkRunGame with these two functions, you will have a Reducer extra req → Game extra req execOut a → Run interpreted b. You can then pass in a Reducer and a Game, and it will be run in Run with the interpreted row of effects.

Using AffGame


TemplateAffGame is how AffGame relates to Game. This is how it's defined:

type ExecOut e s a =
  ( stateRef ∷ READER (Ref s)
  , env      ∷ READER e
  , end      ∷ EXCEPT a
  , effect   ∷ EFFECT
  , aff      ∷ AFF

type Req = (effect ∷ EFFECT, aff ∷ AFF)

type AffGameUpdate extra e s a =
  GameUpdate extra Req (ExecOut e s a) Unit

type TemplateAffGame extra e s a =
  { init    ∷ Run (effect ∷ EFFECT, aff ∷ AFF) { env ∷ e, initState ∷ s }
  , updates ∷ Array (AffGameUpdate extra e s a)

So TemplateAffGame can be seen as an extension of Game, where req is EFFECT and AFF, and execOut is those two effects in addition to these:

  • env ∷ READER e, which can read a custom environment that is set in a TemplateAffGame's initialization.
  • end ∷ EXCEPT a, which when used will terminate the TemplateAffGame, resolving with the provided value
  • stateRef ∷ READER (Ref s), which can read a Ref containing the state of the TemplateAffGame.

In addition to specifying req and execOut, TemplateAffGame also has an initalization that runs before the updates, and all updates must return Unit.

When using AffGame, you'll typically be structuring things in a TemplateAffGame, using mkAffGame to make an AffGame from it, and then running that using either launchGame_ or runGameAff.


An AffGame extra a is simply Reducer extra Req → Aff a. This way, it's easy to run a TemplateAffGame in it, while also making it easy to provide lots of useful primitives and building blocks. It has all the instances you'd expect and probably some more on top of that.

For examples on how to use AffGame, see the tests, or the cookbook example