Almost a year has passed since my previous answer and things have changed in the world of functional reactive programming (FRP).
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In particular, I have created reactive-banana, a small library for FRP in Haskell. While still missing some features, it’s already a great way to implement GUIs in a purely functional style, as demonstrated by a collection of examples. Here a screenshot:
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As before, there is also Flapjax, a JavaScript library for FRP in the web browser.