Gren Playground

Create pictures, animations, and games with Gren!

This is a port of elm-playground.

Start by putting shapes on screen and work up to making games. I hope this package will be fun for a broad range of ages and backgrounds!


A picture is a list of shapes. For example, this picture combines a brown rectangle and a green circle to make a tree:

import Playground exposing (..)

main =
    [ rectangle brown 40 200
    , circle green 100
        |> moveUp 100

Play around to get familiar with all the different shapes and transformations in the library.


An animation is a list of shapes that changes over time. For example, here is a spinning triangle:

import Playground exposing (..)

main =
  animation view

view time =
  [ triangle orange 50
      |> rotate (spin 8 time)

It will do a full spin every 8 seconds.

Maybe try making a car with spinning octogons as wheels? Try using wave to move things back-and-forth? Try using zigzag to fade things in-and-out?


A game lets you use input from the mouse and keyboard to change your picture. For example, here is a square that moves around based on the arrow keys:

import Playground exposing (..)

main =
  game view update { x = 0, y = 0 }

view computer { x, y } =
  [ square blue 40
      |> move x y

update computer { x, y } =
  ( x + toX computer.keyboard
  , y + toY computer.keyboard

Every game has three important parts:

  1. memory - Store information. Our examples stores { x, y } coordinates.
  2. update - Update the memory based on mouse movements, key presses, etc. Our example moves the (x,y) coordinate around based on the arrow keys.
  3. view - Turn the memory into a picture. Our example just shows one blue square at the (x,y) coordinate we have been tracking in memory.

When you start making fancier games, you will store fancier things in memory. There is a lot of room to develop your programming skills here: Making lists, using records, creating custom types, etc.

I started off trying to make Pong, then worked on games like Breakout and Space Invaders as I learned more and more. It was really fun, and I hope it will be for you as well!


See the examples/ directory for some examples.

You can run them like this:

npm install
cd examples
npx gren make Mario.gren
open index.html