The and method of Check will create a new Check that calls apply on both
instances. However, we soon hit the problem of what to do if they both return a
Left:
We need a way to combine values of type E, which hints towards the need for a
Semigroup instance for E. We’re assuming that we don’t want to short-circuit
but rather accumulate all errors.
For the and implementation, we follow the algebraic data type style that is
recommended by the book:
Validated is a more appropriate data type to accumulate errors than Either.
We can also rely on the Applicative instance for Validated to avoid the
pattern match:
The or combinator should return a Valid if the left hand side is Valid or
if the left hand side is Invalid but the right hand side is Valid. If both
are Invalid, it should return an Invalid combining both errors. Due to the
latter, we can’t rely on orElse but rather have a slightly more complicated
implementation:
Exercise 10.4.2: Checks
With our previous Check renamed to Predicate, we can implement the new
Check with the proposed interface as follows, using an algebraic data type
approach as before:
flatMap is a bit weird to implement because we don’t have a flatMap for
Validated. Fortunately, we have flatMap in Either and a withEither
method in Validated that allows us to apply a function over an Either that
gets converted back to a Validated.
andThen gets implemented very similarly to flatMap, except that we don’t use
the output of the first Check to decide which other Check to use. The next
Check is already statically provided to us:
Exercise 10.4.3: Recap
The helper predicates that are introduced in this exercise make use of a lift
method on Predicate that we haven’t implemented yet. Its implementation can be
something like the following:
A Check for username can be implemented as follows, making use of the
longerThan and alphanumeric predicates.
A Check for the email address can be implemented as follows. We first check
that the string contains at least one @, then split the string, check each of
the sides and combine them back at the end:
Exercise 10.5: Kleislis
The run method on Predicate must return a A => Either[E, A]. We must rely
on the existing apply method so we also need a Semigroup instance for E:
Our checks don’t change much. We have decided to implement the email address
check slightly differently here, applying the checks directly in the split step: