If I have a record type:
type Rec_ab = { a :: String, b :: Int }
Can I get from it type which is effectfully extended with {c :: Boolean} : { a :: String, b :: Int, c :: Boolean }?
If I had a row, I could do it:
Row_ab = ( a :: String, b :: Int )
Rec_abc = { c :: Boolean | Row_ab }
but if I have a record Rec_ab how this should be solved? I'm not aware of the way of going from Record -> Row.
In general, you can't.
If you need that kind of extensibility, your approach with Row_ab is good enough, but another possible solution is to parametrize the record and then use the parameter to extend it:
type Rec_ab r = { a :: String, b :: Int | r }
type Rec_abc = Rec_ab ( c :: Boolean )
This also allows you to write functions that would work with both Rec_ab and Rec_abc:
add42 :: forall r. Rec_ab r -> Rec_ab r
add42 r = r { b = r.b + 42 }