vinyl records that are specialized to take a set of key-value pairs (i.e. (Symbol,*)), which means that each field has a name and a type.

Rec (f :: Symbol -> Type -> Type) (kvs :: [(Symbol,Type)])

the motivation was writing a FromJSON instance for records. but I got too many kind errors, whether I represented the "record" (i.e. JavaScript object or Python dict), as Rec (f :. ElField) ['("a",String), '("b",Bool)] or, like most other vinyl libraries, as Rec f [ElField '("a",String), ElField '("b",Bool)].

while the representation should be possible, it's still not convenient. I've been exclusively using vinyl with Symbol, so this specialization should only help, both with writing record functions and with the error messages. also, once GHC gets actual records (through some particular language extension, a type system plug-in, dependent types, or whatever), i'll want to keep the style of a type level set (i.e. MAP Symbol *) anyways.