There is undefined
from the Undefined Package - but for just setting up something like this will work too
undefined :: forall a. a
undefined = go unit
where go unit = go unit
And yes it seems you can do
tt :: forall r. Record ( label :: String | Persistable Int )
but not add | r
Once you import type (+)
and change your Persistable
a bit it works:
import Type.Row (type (+))
type Persistable a r = (persist :: a -> Unit | r)
tt :: forall r. Record ( label :: String | Persistable Int + r)
tt = undefined
this type-checks for me.
Of course this is a bit silly as + r
is a bit like x + 0
in common arithmetic - so you could do
type Persistable a r = (persist :: a -> Unit | r)
tt :: forall r. Record ( label :: String | Persistable Int r )
tt = undefined
here too - but once you have more parts I’d go with +
PS: Don’t forget the label names 
PPS: I guess you could generalize my last example there to Record ( moreLabals | ExtensionOne (ExtensionTwo (... ExtensionN r)...)) )
as was hinted in Jordan’s Reference.