Aladdin - Scala Bugtracking
[#1297] project: compiler priority: medium category: bug
submitter assigned to status date submitted
Adriaan Martin moved to trac 2007-08-28 09:07:23.0
subject undoTo gets head of empty list
code 
adriaan-moors-computer:~ adriaan$ ledit ~/src/scala/trunk/build/quick/bin/scala
Welcome to Scala version 2.6.0.12630.20070822-153214.
Type in expressions to have them evaluated.
Type :help for more information.

scala> def foo[A, B, C](l: List[A], f: A => B=>B, g: B=>B=>C): List[C] = l map (g compose f)
what happened 
Exception in thread "main" java.util.NoSuchElementException: head of empty list
        at scala.Nil$.head(List.scala:1193)
        at scala.Nil$.head(List.scala:1190)
        at scala.tools.nsc.symtab.Types$class.undoTo(Types.scala:2726)
        at scala.tools.nsc.symtab.Types$class.isSubType(Types.scala:2888)
        at scala.tools.nsc.symtab.SymbolTable.isSubType(SymbolTable.scala:11)
        at scala.tools.nsc.symtab.Types$Type.$less$colon$less(Types.scala:469)
        at scala.tools.nsc.symtab.Types$class.isSubArgs$0(Types.scala:2950)
        at scala.tools.nsc.symtab.Types$class.isSubArgs$0(Types.scala:2951)
        at scala.tools.nsc.symtab.Types$class.isSubType0(Types.scala:2955)
        at scala.tools.nsc.symtab.Types$class.isSubType(Types.scala:2886)
        at scala.tools.nsc.symtab.SymbolTable.isSubType(SymbolTable.scala:11)
        at scala.tools.nsc.symtab.Types$Type.$less$colon$less(Types.scala:469)
        at scala.tools.nsc.typechecker.Infer$Inferencer.isCompatible(Infer.scala:360)
        at scala.tools.nsc.typechecker.Infer$Inferencer.isWeaklyCompatible(Infer.scala:364)
        at scala.tools.nsc.typechecker.Infer$Inferencer.protoTypeArgs(Infer.scala:437)
        at scala.tools.nsc.typechecker.Typers$Typer.doTypedApply(Typers.scala:1601)
        at scala.tools.nsc.typechecker.Typers$Typer.typedApply$0(Typers.scala:2276)
        at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2836)
        at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
        at scala.tools.nsc.typechecker.Typers$Typer.typedArg(Typers.scala:1497)
        at scala.tools.nsc.typechecker.Typers$Typer.typedArgToPoly$0(Typers.scala:1606)
        at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$69.apply(Typers.scala:1615)
        at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$69.apply(Typers.scala:1615)
        at scala.List$.map2(List.scala:277)
        at scala.tools.nsc.typechecker.Typers$Typer.doTypedApply(Typers.scala:1615)
        at scala.tools.nsc.typechecker.Typers$Typer.typedApply$0(Typers.scala:2276)
        at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2836)
        at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
        at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2982)
        at scala.tools.nsc.typechecker.Typers$Typer.transformedOrTyped(Typers.scala:3031)
        at scala.tools.nsc.typechecker.Typers$Typer.typedDefDef(Typers.scala:1241)
        at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2679)
        at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
        at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2971)
        at scala.tools.nsc.typechecker.Typers$Typer.typedStat$0(Typers.scala:1463)
        at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$57.apply(Typers.scala:1493)
        at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$57.apply(Typers.scala:1493)
        at scala.List$.loop$0(List.scala:244)
        at scala.List$.mapConserve(List.scala:261)
what expected no exception (if I uncomment my change to undoTo, the crash disappears, but it does not type check) Burak pointed out why it doesn't type check.. this works fine (i.e. no type error and no crash): 
def foo[A, B, C](l: List[A], f: A => B=>B, g: (B=>B)=>C): List[C] = l map (g compose f)
[back to overview]
Changes of this bug report
Adriaan  edited on  2007-08-28 10:28:06.0
improved "what expected" (the bug still stands though, IMO)
Gilles  edited on  2007-08-29 09:53:10.0