Yes, this entails moving more logic into compilers/interpreters. For example, Dhall is actually designed this way: not only is it a command line interpreter but it's also a Haskell library that you can use to interpret Dhall expressions and marshal them into Haskell programs. So the Haskell language combined with the Dhall library becomes an "interpreter builder": a way to crank out a large number of specialized applications that are customized via an effect-free programmable configuration language

For the specific example of a server, there would be two layers:

* in Haskell you would implement a server configured by a Dhall expression (i.e. a hybrid server/interpreter)

* end users program in Dhall, not Haskell

* end users provide a record of pure functions (one for each API endpoint) that translate user input to output

However, I think that's still just a superficial answer to the question. The next level is to ask: why do we even need a server? A server is just a way to distribute values, but Dhall already has a way to distribute values (and code): we just import them by reference. So why not just use that directly instead of standing up a server to reimplement what the Dhall interpreter already does internally

You can encode markup in a purely functional languages. That's actually the easy part since you can treat it as an ordinary data structure. Actually, the harder part is coming up with a user-friendly way to transform a stream of events into a stream of outputs within a purely functional language. In my opinion, there is no clear consensus on the "right" way to do this, but the field of functional reactive programming is based on searching for clean solutions to this problem

However, again, you need to step back and ask: what did we need this GUI for? In some cases, the GUI is the requirement (like in a game) so there's not much you can do to simplify things. However, in other cases -the GUI is just a poor man's interface to composing values and code, in which case we might just be able to replace it with importing URLs and paths using Dhall's built-in mechanisms (and perhaps replace all these bespoke GUIs with a general purpose GUI for manipulating Dhall expressions)