Reflex exercises: Events

Posted on September 21, 2017

If you haven’t done it already, I’d work through the first exercise in order to get everything set up and working.

The problem

We’re going to start to build a HTML vending machine.

It’s a very capitalist goal, so we’ll start with a type alias to focus ourselves:

-- src/Ex01/Common.hs
type Money = Int

We’re going to need to track what items we have for sale and what they cost:

-- src/Ex01/Common.hs
data Product =
  Product {
    pName :: Text
  , pCost :: Money

and we’ll set up some products based on the fact that we’re aiming our vending machine at the healthy eaters out there:

-- src/Ex01/Common.hs
carrot :: Product
carrot = Product "Carrot" 1

celery :: Product
celery = Product "Celery" 2

cucumber :: Product
cucumber = Product "Cucumber" 3

We have inputs for the buttons on the machine corresponding to our products, and for the “Refund” button:

-- src/Ex01/Common.hs
data Inputs t =
  Inputs {
    ieCarrot   :: Event t ()
  , ieCelery   :: Event t ()
  , ieCucumber :: Event t ()
  , ieRefund   :: Event t ()

and we have outputs for the product to put in the tray, the amount of money that purchase consumes, the change to issue, and whether someone tried to purchase something they couldn’t afford:

-- src/Ex01/Common.hs
data Outputs t =
  Outputs {
    oeVend           :: Event t Text
  , oeSpend          :: Event t Money
  , oeChange         :: Event t Money
  , oeNotEnoughMoney :: Event t ()

You are given a function to fill in that takes the amount of money in the system and a value of type Inputs, and produces a value of type Outputs:

-- src/Ex01/Exercise.hs
ex01 :: Reflex t 
     => Money 
     -> Inputs t 
     -> Outputs t
ex01 money (Inputs eCarrot eCelery eCucumber eRefund) =
    eVend =
    eSpend =
    eChange =
    eNotEnoughMoney =
    Outputs eVend eSpend eChange eNotEnoughMoney

To see what this looks like in the context of our HTML vending machine, run:

> nix-shell
nix-shell> ./

from the exercises directory and then visit http://localhost:9090 in your browser (although there is currently a bug in jsaddle-warp effecting Firefox).

It should update the browser every time that you save your code while it is in a compilable state.

The goal

The specification

Here are the things we are after.

When the Event is triggered for a product:

  • if there is enough money in the system, eVend should fire with the name of the product and eSpend should fire with the cost of the product.
  • if there is not enough money in the system, eNotEnoughMoney should fire

When eRefund is triggered:

  • eChange should fire with the amount of money that is in the system

Everything else is taken care of for you.

Run the script:

> nix-shell
nix-shell> ./

open src/Ex01/Exercise.hs in an editor and give it a try.

What it should look like

If you achieve those things, you should get something that behaves like this:


A single Event t Product would be much easier to deal with than the trio of eCarrot , eCelery and eCucumber.

There is a solution in src/Ex01/Solution.hs, but it’s worth spending a little while on this if you get stuck before you have a look.

We’re preparing educational materials about the reflex library, and using it to see what exciting things we can do with FRP.

> Dave Laing

Dave is a programmer working at the Queensland Functional Programming Lab.