Is it possible to declare a constrained data type using record syntax?

Question

Is it possible to declare a constrained data type using record syntax? My attempt

data ConstrainedRecord a where
  ConstrainedRecord :: Num a => { first :: a, second :: a }

causes GHC to complain "Record syntax is illegal here".

Answer 1

Yes, but in GADT syntax you always have to explicitly specify the return type of the constructor:

data ConstrainedRecord a where
  ConstrainedRecord :: Num a => { first :: a, second :: a } -> ConstrainedRecord a

(Also, this may come handy:

deriving instance (Show a) => Show (ConstrainedRecord a)

using StandaloneDeriving, since a normal deriving does not work.)