One of the advantages interpreted languages have over compiled languages is fast prototyping: you save changes to a file and hit refresh. If we want to make any changes to our Yesod apps above, we’ll need to call runhaskell from scratch, which can be a bit tedious.

Fortunately, there’s a solution to this: yesod devel automatically rebuilds and reloads your code for you. This can be a great way to develop your Yesod projects, and when you’re ready to move to production, you still get to compile down to incredibly efficient code. The Yesod scaffolding automatically sets things up for you. This gives you the best of both worlds: rapid prototyping and fast production code.