Beautiful language and beautiful library for web programming. It imports all the practical stuff from heavy theory and mixes it in a coherent environement.<p>I've seen at least : monads for effects, functionnal reactive programming for reactive interfaces depending on signals (generating javascript behind the scene) and type level functions and records for powerful metaprogaming / generic programming in the haskell sense (independant of the types but dependant of their structure).<p>The last point is a very powerful one, making Ur's type system more expressive than Haskell's (but I saw recently that Haskell will also introduce kinds).<p>I'm not knowledgable enough, so I would be interested in an explanation of what the advantage of those compared to SYB-like generics (I would say no runtime type info attached and so more static checking ?). I think generics are not used enough and maybe Ur has a good point here : making easier and safe all the boilerplate of manipulating data representations, which is especially important in a world like web programming.<p>And I would also like to know what's with in the XML and SQL DSL's : is it a combinator library with syntactic sugar (I guess Scala does this) or is there more type hackery in it also using type transformations ?