Thursday, January 19, 2012

Parboiled, Case Classes and Headaches

I had the lovely task today of figuring out how to not use case classes in Scala for my AST.  My AST demanded as structure that resulted in conflicts of overriding constructor parameters when case classes were used. This meant I needed to build the class myself, and it took all day to get it right. My first attempt was the following as I found a blog post stating that Parboiled only depends on the apply method:

object ImportViral {
  def apply( packagespec:PS ):ImportViral =
    new ImportViral( packagespec )
}
class ImportViral( packagespec:PS ) extends ImportStatement( packagespec ) {}


And my respective rule in the parser:

def ZImportViral = rule { "importviral" ~ WhiteSpace ~ ZPS ~~> ast.ImportViral }

But this resulted in a rather ugly error:

error: overloaded method value ~~> with alternatives:

[X, Y, Z, R](f: (X, Y, Z, ast.PS) => R)org.parboiled.scala.rules.ReductionRule3[X,Y,Z,R] and

[Y, Z, R](f: (Y, Z, ast.PS) => R)org.parboiled.scala.rules.ReductionRule2[Y,Z,R] and

[Z, R](f: (Z, ast.PS) => R)org.parboiled.scala.rules.ReductionRule1[Z,R] and

[R](f: ast.PS => R)org.parboiled.scala.rules.Rule1[R]

cannot be applied to (net.codingwell.weave.languages.silk.ast.ImportViral.type)

I figured out pretty quick that the issue was f: ast.PS => R meaning it wanted some sort of conversion from my PackageSpecification to "R" (ImportViral). So obviously I did something wrong. I fiddled for half the day before trying hardcoding the rule type.

def ZImportViral:Rule1[ast.ImportViral]

This gave me a new only minorly more helpful error.

found   : net.codingwell.weave.languages.silk.ast.ImportViral.type (with underlying type object net.codingwell.weave.languages.silk.ast.ImportViral)

required: net.codingwell.weave.languages.silk.ast.PackageSpecification => net.codingwell.weave.languages.silk.ast.ImportViral

So again, my object isn't a conversion. After another several hour sprint trying to figure out what a case class is equivalent to in code. I found a decompiler http://java.decompiler.free.fr/?q=preview
Plugged in a case class from my AST and looked for differences. Took me a little bit to notice but plain as day, the object was inheriting from Function1. The magical glue to make a function object in scala. Now I am no Scala expert so this also taught me that objects can inherit from different things than their companion class. Who Knew?

Final working code:

object ImportViral extends Function1[PS,ImportViral] {
   def apply( packagespec:PS ):ImportViral =
     new ImportViral( packagespec )
}
class ImportViral( packagespec:PS ) extends ImportStatement( packagespec ) {}


This code can be found on github: Github - Deathbobomega/Weave - ast.scala

1 comment:

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