scalametaprogrammingabstract-syntax-treescala-macrosscala-quasiquotes

Using ClassSymbol in a quasiquote

I have a ClassSymbol and want to generate a zero-argument method throwing ???. Here are my attempts:

Assume that object Test is the type we have ClassSymbol of.

I.

val sym = //the ClassSymbol
val tpe = tq"$sym.type"
q"def foo(): $tpe = ???"

Result:

[error]  stable identifier required, but Test.type found.

II.

val sym = //the ClassSymbol
val tpe = tq"${sym.name}.type"
q"def foo(): $tpe = ???"

Result:

[error]  found   : c.universe.TypeName
[error]  required: c.universe.TermName
[error]         val tpe = tq"${sym.name}.type"

III.

val sym = //the ClassSymbol
val tpe = tq"${TermName(sym.name.toString)}.type"
q"def foo(): $tpe = ???"

Result:

Compiles successfully

So I ended up using the method III which looks pretty scary.

Is there a "native" way to use ClassSymbol in a quasiquote?

Solution

  • We can save your approach II

    val tpe = tq"${sym.name.toTermName}.type"

    This is similar to III but without manual handling strings.

    Also don't forget that besides quasiquotes you can always build trees with manual parsing

    val tree = tb.parse(s"def foo(): ${sym.name}.type = ???") // for macros c.parse instead of tb.parse

    Regarding the "native" way, now it's better to use a ModuleSymbol

    val sym = rm.moduleSymbol(Test.getClass)

    or

    val sym = typeOf[Test.type].termSymbol.asModule

    rather than ClassSymbol

    val sym0 = rm.classSymbol(Test.getClass)

    or

    val sym0 = typeOf[Test.type].typeSymbol.asClass

    Testing:

    val tpe = tq"$sym.type"
val tree = q"def foo(): $tpe = ???"
tb.typecheck(tree) // no exception

    I used runtime reflection but for macros it's similar.

    If you already have a ClassSymbol it can be converted to a ModuleSymbol

    val sym = sym0.companionSymbol // not sym0.companion but .companionSymbol is deprecated

    or

    val sym = sym0.asInstanceOf[scala.reflect.internal.Symbols#ClassSymbol].sourceModule.asInstanceOf[ModuleSymbol]

    or

    val sym = sym0.owner.info.decl(sym0.name.toTermName)

    Get the module symbol, given I have the module class, scala macro