By: blahedo

blahedo — Tue, 15 Dec 2009 20:44:37 +0000

Buglet addendum: when you fixed James Iry’s buglet (with the “case e:_”) you didn’t update the surrounding text, which still refers to the “underscore” and “wildcard”.

By: Stefan Wagner

Stefan Wagner — Tue, 28 Apr 2009 19:47:38 +0000

Pardon – cut’n'paste-error, and wrong code-Tags.
public boolean checkPrime(int number) { // checks if a number between 1 and 10 is prime switch (number) { case 1: case 2: case 3: case 5: case 7: return true; default: return false; } }

By: Stefan Wagner

Stefan Wagner — Tue, 28 Apr 2009 19:45:37 +0000

To be fair, the javacode (listing 1) needn’t look too verbose:
[code]public boolean checkPrime(int number) {
// checks if a number between 1 and 10 is prime
switch (number) {
case 1: return true;
case 2: return true;
case 3: return true;
case 5: return true;
case 7: return true;

default: return false;
}
}[/code]

By: Daniel Spiewak

Daniel Spiewak — Thu, 12 Feb 2009 17:40:40 +0000

@James

Indeed, that is a bug, possibly a regression against an earlier version of Scala (I’m almost positive that I tested that snippet when I wrote it). I’m correcting the article; thanks for the catch!

By: James Iry

James Iry — Thu, 12 Feb 2009 17:36:28 +0000

Small buglet

try {
checkPrime(12)
} catch {
case e:_ => e.printStackTrace()
}

doesn’t work. It should be
try {
checkPrime(12)
} catch {
case e => e.printStackTrace()
}

By: Daniel Spiewak

Daniel Spiewak — Wed, 09 Apr 2008 16:59:15 +0000

Ah, I didn’t know that! Very useful.

By: Seth Tisue

Seth Tisue — Wed, 09 Apr 2008 16:35:15 +0000

Note that you can write:
case 1 | 2 | 3 | 5 | 7 => true
instead of having a line for each case.

By: Daniel Spiewak

Daniel Spiewak — Thu, 06 Mar 2008 23:05:59 +0000

Well, I wasn’t able to repeat your error, but I have confirmed that the following code misbehaves:

case class Color(r:Int, g:Int, b:Int)
case class Red(v:Int) extends Color(v, 0, 0)

println(Red(33)) // prints “Color(33, 0, 0)”

I’m fairly certain that this is a problem with toString() as reported in this ticket (https://lampsvn.epfl.ch/trac/scala/ticket/607). As you mentioned, getClass does in fact show that the Red class does exist, and pattern matching would be impossible without separate classes.

By: John Simmons

John Simmons — Thu, 06 Mar 2008 22:11:45 +0000

I am not sure what is going on, but it is very strange. Someone more knowledgeable than I about Scala needs to investigate it. I have tried to boil it down to a simple example. I created a script file redcolor.scala containing this code:

case class Color(r: Int, g: Int, b: Int)
case class Red(v: Int) extends Color(v, 0, 0)

val r: Color = Red(33)
println(r.getClass)

r match {
case Red(v) => println(“red ” + v)
case _ => println(“some other color”)
}

//println(r)

In this form, it prints:
class Main$Red$2
red 33

If I uncomment the final println(r), I get what looks like a compile error:

java.lang.Error: symbol variable Red$module$1 does not exist in Main.main
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:914)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.genLoadArguments(Ge
nICode.scala:1112)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:829)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.genLoadQualifier(Ge
nICode.scala:1059)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:826)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:416)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genStat(GenICode.scala:181)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$genStat$1.
apply(GenICode.scala:144)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$genStat$1.
apply(GenICode.scala:143)
at scala.List.foreach(List.scala:763)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.genStat(GenICode.sc
ala:143)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:940)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:441)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:941)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genStat(GenICode.scala:181)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$genStat$1.
apply(GenICode.scala:144)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$genStat$1.
apply(GenICode.scala:143)
at scala.List.foreach(List.scala:763)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.genStat(GenICode.sc
ala:143)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala$tools$nsc$bac
kend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:940)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:
112)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$gen$1.appl
y(GenICode.scala:70)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$gen$1.appl
y(GenICode.scala:70)
at scala.List.foreach(List.scala:763)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:
70)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:
134)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:
88)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$gen$1.appl
y(GenICode.scala:70)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun$gen$1.appl
y(GenICode.scala:70)
at scala.List.foreach(List.scala:763)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:
70)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:
80)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.gen(GenICode.scala:
66)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.apply(GenICode.scal
a:62)
at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:247)
at scala.tools.nsc.Global$GlobalPhase$$anonfun$run$1.apply(Global.scala:
233)
at scala.tools.nsc.Global$GlobalPhase$$anonfun$run$1.apply(Global.scala:
233)
at scala.Iterator$class.foreach(Iterator.scala:387)
at scala.collection.mutable.ListBuffer$$anon$1.foreach(ListBuffer.scala:
255)
at scala.tools.nsc.Global$GlobalPhase.run(Global.scala:233)
at scala.tools.nsc.backend.icode.GenICode$ICodePhase.run(GenICode.scala:
55)
at scala.tools.nsc.Global$Run.compileSources(Global.scala:545)
at scala.tools.nsc.Global$Run.compile(Global.scala:629)
at scala.tools.nsc.StandardCompileServer.session(CompileServer.scala:137
)
at scala.tools.util.SocketServer$$anonfun$run$1.apply(SocketServer.scala
:69)
at scala.tools.util.SocketServer$$anonfun$run$1.apply(SocketServer.scala
:69)
at scala.util.DynamicVariable.withValue(DynamicVariable.scala:73)
at scala.Console$.withOut(Console.scala:84)
at scala.Console$.withOut(Console.scala:102)
at scala.tools.util.SocketServer.run(SocketServer.scala:68)
at scala.tools.nsc.StandardCompileServer.main(CompileServer.scala:179)
at scala.tools.nsc.CompileServer.main(CompileServer.scala)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at scala.tools.nsc.ObjectRunner$$anonfun$run$1.apply(ObjectRunner.scala:
75)
at scala.tools.nsc.ObjectRunner$.withContextClassLoader(ObjectRunner.sca
la:49)
at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:74)
at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:161)
at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)
error: fatal error (server aborted): symbol variable Red$module$1 does not exist
in Main.main
one error found

By: Ismael Juma

Ismael Juma — Thu, 06 Mar 2008 17:29:09 +0000

John,

Are you sure no Red class is created or is it just a bug in toString? See:

https://lampsvn.epfl.ch/trac/scala/ticket/607

Ismael

Comments on: Scala for Java Refugees Part 4: Pattern Matching and Exceptions

By: blahedo

By: Stefan Wagner

By: Stefan Wagner

By: Daniel Spiewak

By: James Iry

By: Daniel Spiewak

By: Seth Tisue

By: Daniel Spiewak

By: John Simmons

By: Ismael Juma