I was just lucky enough to see Peter Hawkins present a particularly compelling synthesis language: data structure synthesis:
We consider the problem of specifying combinations of data structures with complex sharing in a manner that is both declarative and results in provably correct code. In our approach, abstract data types are specified using relational algebra and functional dependencies. We describe a language of decompositions that permit the user to specify different concrete representations for relations, and show that operations on concrete representations soundly implement their relational specification. It is easy to incorporate data representations synthesized by our compiler into existing systems, leading to code that is simpler, correct by construction, and comparable in performance to the code it replaces.
As somebody experimenting with data structure representations, bringing it down to the language level is great! It'll be appearing at PLDI next month.