String.toInt, .toDouble should allow a default parameter

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Aladdin: [http://scala-webapps.epfl.ch/bugtracking/bugs/displayItem.do?id=1299 bug 1299], [http://scala-webapps.epfl.ch/bugtracking/contribs/display.do?id=773 contrib 773]

== Code ==

object Foo {
   // this is overly verbose
   def getInt(s : String) = try {
     s.toInt
   } catch {
     case e : NumberFormatException => 0
   }

   // I'd much rather write this
   def getInt2(s : String) = s.toInt(0)
}

== What happened ==

n/a

== What expected ==

To be able to optionally provide a default value for string-to-primitive conversion methods, much like Option.get.

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Imported From: https://issues.scala-lang.org/browse/SI-16?orig=1
Reporter: @michelou

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Aaron Harnly (aaronharnly) said:
Or perhaps more straightforwardly, returning an Option[Int].

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@odersky said:
I am against the proliferation of default parameters or option results in the library.
If Java decided against this, I do not see why Scala should reach a different decision. Nothing prevents anyone from defining a method

numSafe[A](foo: => A): Option[A] =
try { Some(foo) } catch { ex: NumberFormatException => None }

to deal with these situations once and for all in a systematic manner.

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@paulp said:
Replying to [comment:6 odersky]:

numSafe[A](foo: => A): Option[A] =
try { Some(foo) } catch { ex: NumberFormatException => None }

to deal with these situations once and for all in a systematic manner.

I agree, it's that wrapper (or something like it, and things similar to it) which I do think belong in the standard library somewhere. The mere fact that the exact same wrapping is needed in multiple places in scalac itself is a pretty reasonable argument for it, not to mention the likely requirements of the remainder of the world.

"Once and for all" + "systematic" == "let's do this once for everyone..."

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@odersky said:
Yes, your idea of making a wrapper sounds like the right thing to do. How about `optional'? It probably could do into Predef. Or do you have a better idea?

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@paulp said:
Replying to [comment:8 odersky]:

Yes, your idea of making a wrapper sounds like the right thing to do. How about `optional'? It probably could do into Predef. Or do you have a better idea?

I'm torn, as I think there's already too much in Predef, which I would rather see broken down. This issue is well intertwined with (at least) my hopes for a) a much easier import/export mechanism b) a generalized exception handling mechanism. So it's hard to say whether I have a better idea, it depends a bit on the appropriate level of ambition.

I would definitely prefer to call it "opt" though, since making this lightweight is key.

Just thinking out loud now, because what better place than closed trac ticket #16 could there be for me to flesh out my ideas, but:

def opt[T](f: => T)(implicit pf: PartialFunction[Either[Throwable,T]] => Boolean): Option[T] = ...

The gist being that somehow a function must be supplied which partitions both return values and exceptions into Some, None, or neither (that last being exceptions which you did not opt to catch, which will simply fly right on through.)

Now very nice wrappers can be written. This approach also generalizes the oft-rewritten

def onull[T](x: T): Option[T] = if (x == null) None else Some(x)

...but now that could be composed with other criteria or exceptions.

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aunndroid said:
The issue is raised to avoid the verbose exception handling. How about provide a way to detect the exception beforehand ? Like RichChar's isDigit. In this case, isNumber.

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@soc said:
I don't know if there is any interest left, but at least the original proposal has the problem that the style suggested is normally used for specifying radixes, not default values.

[SethTisue]

there is an active discussion on this at scala/collection-strawman#431

[SethTisue]

has PR now, reviewers wanted: scala/scala#6538

[SethTisue]

for fixing the last open two-digit Scala issue, @martijnhoekstra officially gets to wear a big, shiny golden crown whenever he wants.

https://github.com/scala/bug/issues?q=is%3Aopen+is%3Aissue+sort%3Acreated-asc shows 16 remaining open three-digit issues, for anyone interested in earning the right to wear an only slightly smaller and slightly less jewel-encrusted crown