Darn it, apparently I'm not "getting" the more complex cases of for
comprehensions at all. Returning a Box[Tag] from that function
affected several other functions which expected Tag or null. So rather
than pulling the value out of the box, I decided to modify the
functions, borrowing heavily from the rewrite provided by David.
Here's a fragment of a function which is supposed to get me a sequence
of Master objects. Note that getMastersWithTags returns a (possibly
empty) List[Master]:
def masters = for {
sy <- selectedYear
sm <- selectedMonth
m <- Master.getMastersWithTags(sy, sm)
} yield m
Intuitively, this should produce a (possibly empty) sequence of object
of type Master, right? First, selectedYear would be unboxed, then (if
there's a year) the selected month, and then, if both values are
available, the for loop should in theory iterate over the list
produced by Master.getMastersWithTags(sy, sm) and produce a sequence
of objects.
NetBeans says:
type mismatch:
found: List[Master]
required: net.liftweb.common.Box[?]
and I'm not sure what exactly this means. Could someone clarify?
On Nov 16, 10:51 pm, DMB <[email protected]> wrote:
> Nice! And you're right. I'm trying to program in Scala as if it were
> C# with a weird implementation of LINQ. And C# it is not.
>
> But the thread was not really about this. The thread was about a bug
> in the compiler, which still stands. And you can pry my return
> statements out of my cold, dead hands. :-)
>
> On Nov 16, 9:42 pm, David Pollak <[email protected]>
> wrote:
>
> > If you see the "return" statement in your program, you are likely doing
> > something wrong. It means that it's time to refactor your code so there's a
> > single return point from your method. Using null is a red flag (use Option
> > or Box) and using vars (rather than vals) is a yellow flag.
>
> > So, let's go through your code:
>
> > def findTagWithName(tags: List[Tag], name: String) : Tag = {
> > val t = tags.find(x => x.name == name)
> > return t.getOrElse(null)
> > }
>
> > becomes:
>
> > def findTagWithName(tags: List[Tag], name: String): Box[Tag] =
> > tags.find(x => x.name == name)
>
> > def getCookieValue(name: String, default: String) : String = {
> > for(cookie <- S.findCookie(name); v <- cookie.value) {
> > return v
> > }
> > return default
> > }
>
> > becomes:
>
> > def getCookieValue(name: String, default: String): String =
> > (for(cookie <- S.findCookie(name); v <- cookie.value)
> > yield v) openOr default
>
> > def getParamValue(name: String, default: String) : String = {
> > for(v <- S.param(name)) {
> > return v
> > }
> > return default
> > }
>
> > becomes:
>
> > def getParamValue(name: String, default: String): String =
> > S.param(name) openOr default
> > - Show quoted text -
>
> > def getSelectedTag(tags: List[Tag], cookieName: String, urlParam: String)
> > : Tag = {
> > var t : Tag = null
> > try {
> > if(tags.length == 0) {
> > return null
> > }
>
> > // See if tag name is present in the URL
> > val tagName = getParamValue(urlParam, "")
> > // If yes, check if a tag with this name is present in tags
> > t = findTagWithName(tags, tagName)
> > // If it is, write out the right cookie and return
> > if(t != null) {
> > return t
> > }
>
> > // if no, check the corresponding cookie
> > val cv = getCookieValue(cookieName, "")
> > if(cv != null) {
> > // and check if the tag with a name stored in cookie is
> > present in tags
> > t = findTagWithName(tags, cv)
> > if(t != null) {
> > return t
> > }
> > }
>
> > // otherwise just select the last tag from the list and write out
> > the cookie
> > t = tags.last
> > return t
> > } finally {
> > if(t != null) {
> > var c = HTTPCookie(cookieName, t.name).setMaxAge(365 * 24 *
> > 60 * 60)
> > S.addCookie(c)
> > }
> > }
> > }
>
> > becomes:
>
> > def getSelectedTag(tags: List[Tag], cookieName: String, urlParam:
> > String): Box[Tag] = {
> > // See if tag name is present in the URL
> > def tagForParam =
> > for {
> > param <- S.param(urlParam)
> > tag <- tags.find(_.name == param)
> > } yield tag
>
> > // check the corresponding cookie
> > def tagForCookie =
> > for {
> > cookie <- S.findCookie(cookieName)
> > value <- cookie.value
> > tag <- tags.find(_.name == value)
> > } yield tag
>
> > val tag = tagForParam or tagForCookie or tags.lastOption
>
> > tag.foreach( t =>
> > S.addCookie(HTTPCookie(cookieName, t.name).setMaxAge(365 * 24 * 60 *
> > 60)))
>
> > tag
> > }
>
> > Note how the revised code is cleaner and easier to read. The logic is
> > grouped together and the expression of what tag to select is very clear.
>
> > I hope this helps.
>
> > Thanks,
>
> > David
>
> > On Mon, Nov 16, 2009 at 8:45 PM, DMB <[email protected]> wrote:
> > > I wonder if anyone else finds this stack trace familiar. As often is
> > > the case with non-mainstream languages, Google turns up nothing of
> > > value. The code fragment goes below. If I comment out the code inside
> > > the finally statement, everything compiles fine. If I keep it, I get
> > > the stack trace below. I have tried to create a minimal repro, but did
> > > not have any success. I understand this is not a Lift issue, but this
> > > group has a sizable chunk of Scala enthusiasts, so hopefully someone
> > > has encountered something like this
>
> > > block #9 :
> > > CONSTANT (Constant(null))
> > > LOAD_LOCAL variable t
> > > DUP
> > > STORE_LOCAL variable eqEqTemp$2
> > > CZJUMP (REFERENCE(java.lang.Object))EQ ? 13 : 15
> > > Successors: 3 15 13 3
> > > trying to emit: DROP REFERENCE(scala.Null)
> > > java.lang.reflect.InvocationTargetException
> > > at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
> > > at sun.reflect.NativeMethodAccessorImpl.invoke
> > > (NativeMethodAccessorImpl.java:39)
> > > at sun.reflect.DelegatingMethodAccessorImpl.invoke
> > > (DelegatingMethodAccessorImpl.java:25)
> > > at java.lang.reflect.Method.invoke(Method.java:597)
> > > at org.scala_tools.maven.executions.MainHelper.runMain
> > > (MainHelper.java:151)
> > > at org.scala_tools.maven.executions.MainWithArgsInFile.main
> > > (MainWithArgsInFile.java:26)
> > > Caused by: java.lang.AssertionError: assertion failed: BasicBlock
> > > closed
> > > at scala.Predef$.assert(Predef.scala:92)
> > > at scala.tools.nsc.backend.icode.BasicBlocks$BasicBlock.emit
> > > (BasicBlocks.scala:327)
> > > at scala.tools.nsc.backend.icode.BasicBlocks$BasicBlock.emit
> > > (BasicBlocks.scala:317)
> > > at scala.tools.nsc.backend.icode.GenICode$ICodePhase.adapt
> > > (GenICode.scala:1006)
> > > at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala
> > > $tools$nsc$backend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:
> > > 496)
> > > at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala
> > > $tools$nsc$backend$icode$GenICode$ICodePhase$$genLoad(GenICode.scala:
> > > 447)
> > > at scala.tools.nsc.backend.icode.GenICode$ICodePhase.scala
> > > $tools$nsc$backend$icode$GenICode$ICodePhase$$genStat(GenICode.scala:
> > > 183)
> > > at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun
> > > $genStat$1.apply(GenICode.scala:146)
> > > at scala.tools.nsc.backend.icode.GenICode$ICodePhase$$anonfun
> > > $genStat$1.apply(GenICode.scala:145)
> > > etc, etc...
>
> > > The code fragment:
>
> > > def findTagWithName(tags: List[Tag], name: String) : Tag = {
> > > val t = tags.find(x => x.name == name)
> > > return t.getOrElse(null)
> > > }
>
> > > def getCookieValue(name: String, default: String) : String = {
> > > for(cookie <- S.findCookie(name); v <- cookie.value) {
> > > return v
> > > }
> > > return default
> > > }
>
> > > def getParamValue(name: String, default: String) : String = {
> > > for(v <- S.param(name)) {
> > > return v
> > > }
> > > return default
> > > }
>
> > > def getSelectedTag(tags: List[Tag], cookieName: String, urlParam:
> > > String) : Tag = {
> > > var t : Tag = null
> > > try {
> > > if(tags.length == 0) {
> > > return null
> > > }
>
> > > // See if tag name is present in the URL
> > > val tagName = getParamValue(urlParam, "")
> > > // If yes, check if a tag with this name is present in
> > > tags
> > > t = findTagWithName(tags, tagName)
> > > // If it is, write out the right cookie and return
> > > if(t != null) {
> > > return t
> > > }
>
> > > // if no, check the corresponding cookie
> > > val cv = getCookieValue(cookieName, "")
> > > if(cv != null) {
> > > // and check if the tag with a name stored in cookie
> > > is present in tags
> > > t = findTagWithName(tags, cv)
> > > if(t != null) {
> > > return t
> > > }
> > > }
>
> > > // otherwise just select the last tag from the list and
> > > write out the cookie
> > > t = tags.last
> > > return t
> > > } finally {
> > > if(t != null) {
> > > var c = HTTPCookie(cookieName, t.name).setMaxAge(365 *
> > > 24 * 60 * 60)
> > > S.addCookie(c)
> > > }
> > > }
> > > }
>
> > > --
>
> > > You received this message because you are subscribed to the Google Groups
> > > "Lift" group.
> > > To post to this group, send email to [email protected].
> > > To unsubscribe from this group, send email to
> > > [email protected]<liftweb%[email protected]>
> > > .
> > > For more options, visit this group at
> > >http://groups.google.com/group/liftweb?hl=.
>
> > --
> > Lift, the simply functional web frameworkhttp://liftweb.net
> > Beginning Scalahttp://www.apress.com/book/view/1430219890
> > Follow me:http://twitter.com/dpp
> > Surf the harmonics
>
> ...
>
> read more »
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