Just is the Maybe case for a real, present value. It wraps the value so later steps can keep using the same safe container instead of handling raw null checks. section 5.3
In the Maybe pattern, Just pairs with Nothing: Just carries the value, while Nothing represents absence. A lookup can return either one, letting chained operations continue safely.
Safe lookup with Just
A student lookup can return Maybe(Student); when the record exists, mapping the address gives Just(Address(...)) instead of a null crash.
const findStudent = R.curry(function(db, ssn) {
return wrap(find(db, ssn)); #1
});
const getAddress = function(student) {
return wrap(student.fmap(R.prop('address'))); #2
}
Shows a wrapped lookup result that later composes with address extraction, making the safe flow concrete. section 5.3
