Given a field with key K with value LV in L and value RV in R, this produces a FieldType[K, (LV, RV)]
Given a field with key K with value LV in L and no value in R, this produces a FieldType[K, LV]
Given a field with key K with no value in L and value RV in R, this produces a FieldType[K, RV]
Typeclass for merging two records
Given a field with key K with value LV in L and value RV in R, this produces a FieldType[K, (LV, RV)] Given a field with key K with value LV in L and no value in R, this produces a FieldType[K, LV] Given a field with key K with no value in L and value RV in R, this produces a FieldType[K, RV]