I want to balance some factors here. As my initial goal is rapid prototyping and experimentation, I'd like to use a language I already know well, in other words Haskell. 

But of course even with a familiar language, I'm going into a quite unfamiliar situation (web programming) and there is a learning curve with that. 

It may be that a language other than Haskell, i.e. PureScript, although requiring a learning curve, would be more suited to my app's needs and thus save me grief.  

I don't know.

Dennis