App 2: Handling View (Scala 3 Video)
Next up, the “View” use case is a little more complicated than the first two. Basically this code needs to attempt to read the database and then take one of two possible actions from there:
- In the success case, it prints the list of to-do items
- In the failure case, it prints an error message about the database being inaccessible
This is straightforward to do because we already wrote the selectAll database method previously, and it returns a Try value, so we just need to handle its Success and Failure cases in a match expression.
Here’s the code that does what I just described, with a few comments to make those steps clear:
def handleView(): Try[Unit] = Try {
// attempt to read all the tasks from the database.
// this function returns a list of strings, wrapped inside
// a Try:
val res: Try[Seq[String]] = db.selectAll()
// handle the Success and Failure cases:
res match
case Success(tasks) =>
// in the Success case we get a list of tasks (strings),
// so print those out in a nice format:
printTasks(tasks)
case Failure(t) =>
// in the Failure case, where we can’t access the database,
// print the exception (a Throwable) we get from selectAll:
System.err.println(t)
}
private def printTasks(tasks: Seq[String]): Unit =
for
(task, count) <- tasks.zip(Stream from 1)
do
println(s"${count}. $task")
Here’s a shorter version of handleView, without the comments:
def handleView(): Try[Unit] = Try {
val res: Try[Seq[String]] = db.selectAll()
res match
case Success(tasks) =>
for
(task, count) <- tasks.zip(Stream from 1)
do
println(s"${count}. $task")
case Failure(t) =>
System.err.println(t)
}
Note that the cases of the match expression print to STDOUT and STDERR, so they both have the return type Unit. And because the entire function body is wrapped in a Try constructor call:
def handleView(): Try[Unit] = Try { // body here ... }
------------------------
this means that both cases of this function correctly result in returning the Try[Unit] type.
Again, if any of this seems like overkill, remember to imagine that we’re accessing a remote database or REST service, and in the real world, those things may be down.
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