Handling and throwing Exceptions in Scala - scala

Question

I have the following implementation:
val dateFormats = Seq("dd/MM/yyyy", "dd.MM.yyyy")
implicit def dateTimeCSVConverter: CsvFieldReader[DateTime] = (s: String) => Try {
val elem = dateFormats.map {
format =>
try {
Some(DateTimeFormat.forPattern(format).parseDateTime(s))
} catch {
case _: IllegalArgumentException =>
None
}
}.collectFirst {
case e if e.isDefined => e.get
}
if (elem.isDefined)
elem.get
else
throw new IllegalArgumentException(s"Unable to parse DateTime $s")
}
So basically what I'm doing is that, I'm running over my Seq and trying to parse the DateTime with different formats. I then collect the first one that succeeds and if not I throw the Exception back.
I'm not completely satisfied with the code. Is there a better way to make it simpler? I need the exception message passed on to the caller.

Answer 1

The one problem with your code is it tries all patterns no matter if date was already parsed. You could use lazy collection, like Stream to solve this problem:
def dateTimeCSVConverter(s: String) = Stream("dd/MM/yyyy", "dd.MM.yyyy")
.map(f => Try(DateTimeFormat.forPattern(format).parseDateTime(s))
.dropWhile(_.isFailure)
.headOption
Even better is the solution proposed by jwvh with find (you don't have to call headOption):
def dateTimeCSVConverter(s: String) = Stream("dd/MM/yyyy", "dd.MM.yyyy")
.map(f => Try(DateTimeFormat.forPattern(format).parseDateTime(s))
.find(_.isSuccess)
It returns None if none of patterns matched. If you want to throw exception on that case, you can uwrap option with getOrElse:
...
.dropWhile(_.isFailure)
.headOption
.getOrElse(throw new IllegalArgumentException(s"Unable to parse DateTime $s"))
The important thing is, that when any validation succeedes, it won't go further but will return parsed date right away.

Answer 2

This is a possible solution that iterates through all the options
val dateFormats = Seq("dd/MM/yyyy", "dd.MM.yyyy")
val dates = Vector("01/01/2019", "01.01.2019", "01-01-2019")
dates.foreach(s => {
val d: Option[Try[DateTime]] = dateFormats
.map(format => Try(DateTimeFormat.forPattern(format).parseDateTime(s)))
.filter(_.isSuccess)
.headOption
d match {
case Some(d) => println(d.toString)
case _ => throw new IllegalArgumentException("foo")
}
})
This is an alternative solution that returns the first successful conversion, if any
val dateFormats = Seq("dd/MM/yyyy", "dd.MM.yyyy")
val dates = Vector("01/01/2019", "01.01.2019", "01-01-2019")
dates.foreach(s => {
dateFormats.find(format => Try(DateTimeFormat.forPattern(format).parseDateTime(s)).isSuccess) match {
case Some(format) => println(DateTimeFormat.forPattern(format).parseDateTime(s))
case _ => throw new IllegalArgumentException("foo")
}
})

Answer 3

I made it sweet like this now! I like this a lot better! Use this if you want to collect all the successes and all the failures. Note that, this might be a bit in-efficient when you need to break out of the loop as soon as you find one success!
implicit def dateTimeCSVConverter: CsvFieldReader[DateTime] = (s: String) => Try {
val (successes, failures) = dateFormats.map {
case format => Try(DateTimeFormat.forPattern(format).parseDateTime(s))
}.partition(_.isSuccess)
if (successes.nonEmpty)
successes.head.get
else
failures.head.get
}