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Concur UI Lib is a brand new client side Web UI framework that explores an entirely new paradigm. It does not follow FRP (think Reflex or Reactive Banana), or Elm architecture, but aims to combine the best parts of both. This repo contains the Concur implementation for Purescript, using the React backend.


Work in progress tutorials are published in the Concur Documentation site

API documentation is published on Pursuit.


Purescript-Concur is reasonably light. The entire uncompressed JS bundle, including react and all libraries, for the entire example application in this repo clocks in at 180KB. You can build this bundle yourself with the command npm run prod (currently broken due to the move to spago).

This leads to pretty fast initial load times. Running the Chrome audit on produces -

Ports to other languages

Concur's model translates well to other platforms.

  1. Concur for Haskell - The original version of Concur written in Haskell.
  2. Concur for Javascript - An official but experimental port to Javascript.
  3. Concur for Python - An unofficial and experimental port to Python. Uses ImgUI for graphics. Created and Maintained by potocpav.


You can quickly get a production setup going (using Spago and Parcel) by cloning the Purescript Concur Starter.

Else, if you use Spago -

spago install purescript-concur-react

Or if you use Bower -

bower install purescript-concur-react

Building examples from source

git clone
cd purescript-concur-react
npm install
# Build library sources
npm run build
# Build examples
npm run examples
# Start a local server
npm run start
# Check examples
open localhost:1234 in the browser

External React Components

It's easy to add external React components to Concur. Usually all you would require to wrap an external component is to import it as a ReactClass, and then wrapping it with one of the el functions.

For example, let's say you want to wrap the Button component provided by the material-ui library.

Step 1: First write an FFI module that exposes the ReactClass component -

// Button.js
exports.classButton =  require('@material-ui/core/Button').default

And import it into your purescript program

-- Button.purs
foreign import classButton :: forall a. ReactClass a

If you are using the Purescript React MUI bindings, then you can simply import the class component from the library without defining the FFI module -

import MaterialUI.Button (classButton)

Step 2: Then wrap up the imported ReactClass into a widget to make it usable within Concur -

import Concur.React.DOM (El, el')
import React (unsafeCreateElement)
import React.DOM.Props (unsafeFromPropsArray)

button :: El
button = el' (unsafeCreateElement classButton <<< unsafeFromPropsArray)

Step 3: Now you can use button normally within Concur. For example -

import Concur.React.DOM as D
import Concur.React.Props as P

helloButton = button [P.onClick] [D.text "Hello World!"]

Note that you can mix in the default widgets and props with the MUI ones.


Demo and Source for composing all the examples in one page.

Individual example sources -

  1. Hello World! Shows simple effectful widgets with state using StateT. Source.
  2. A simple counter widget without using StateT. Source.
  3. Focus counter demonstrates a stateful widget, with multiple event handlers, and no action types needed! Source.
  4. Virtual Keyboard An onscreen virtual keyboard. Demonstrates FFI as well as handling document level events inside nested widgets. Source.
  5. A login widget. Source.
  6. Concur has Signals! Sample counting widget implemented with Signals! Source.
  7. A Full-featured TodoMVC implementation with LocalStorage Persistence built with Signals. Source.
  8. A Fully editable tree in ~30 lines of code (with Signals). Source.
  9. A Postfix calculator. Source.
  10. Using AJAX and handling JSON responses. Source.
  11. A small widget to Visualise CSS color codes. Source.
  12. Asynchronous timers which can be cancelled. Source.
  13. A Routed widget which demonstrates routing. Source.
  14. The Elm Architecture example demonstrates how Concur subsumes "The Elm Architecture". Source.
  15. Performance test - A huge list of 50 thousand parallel buttons. This has two variants, fast (uses slightly lower level interface) and slow (idiomatic concur code). Source.
  16. Tail Recursion demo - Since Concur is purely functional in nature, its primary mode of iteration is via recursion. Purescript in general is NOT stack stafe with tail recursion; It uses tricks like tailRec and tailRecM. However, Concur performs trampolining to make monadic recursion completely stack safe. This example demonstrates that by making a huge number of tail recursive calls in a short span of time. Source.