I reckon this is based on Daan Leijen's awesome (and very readable) paper "Extensible records with scoped labels" [1]. It's also quite simple to implement, I've done it here [2] (in OCaml).

[1] http://research.microsoft.com/apps/pubs/default.aspx?id=6540... [2] https://github.com/tomprimozic/type-systems/tree/master/exte...