I am fairly new to Scala and am having a build error which states Public method must have explicit type. Just wanted to know if anyone has come across this kind of error and if so how to fix it?
def pk = primaryKey("passive_prompts_primary_key", (uuid, promptType, trigger))
Is where the error is forming
My best guess is to add the return type in but I am not sure how to do that
You should specify the return type because the method pk is public (instead of ???)
def pk: ??? = primaryKey("passive_prompts_primary_key", (uuid, promptType, trigger))
It's hard to say what exactly to write here instead of ??? because only you can know what type primaryKey(...) returns.
Normally Public method must have explicit type should be a warning, not error. Probably you have switched on in build.sbt something like -Xfatal-warnings (Scala 2.12) or -Werror (Scala 2.13). One of options is to switch it off but probably this is not recommended because it's a useful option.
Now about how you can find yourself the type of primaryKey(...) (in order to know what to write instead of ???).
You can look at the definition of def primaryKey....
Alternatively you can start the REPL with sbt console and write the right hand side of pk, something like
scala> import SomeObject._ // where primaryKey, uuid, promptType, trigger are defined
scala> primaryKey("passive_prompts_primary_key", (uuid, promptType, trigger))
The REPL then write the value and type of pk.
Or you can use functionality of IDE. I'm pressing Alt+Enter in ItelliJ Idea and choose "Add type annotation to definition".
But Idea can sometimes guess types incorrectly.
Or you can use self-defined getType in the project
// add to build.sbt: libraryDependencies += scalaOrganization.value % "scala-reflect" % scalaVersion.value
import scala.reflect.runtime.universe.{TypeTag, Type, typeOf}
def getType[T: TypeTag](t: T): Type = typeOf[T]
println(getType(
primaryKey("passive_prompts_primary_key", (uuid, promptType, trigger))
))
Also there is Scalafix rule that adds explicit return types
https://scalacenter.github.io/scalafix/docs/rules/ExplicitResultTypes.html
Related
Edit: I updated the question to be more descriptive.
Note: I use the Scala 2.11 compiler, because that is the compiler version used by the LMS tutorial project.
I am porting a DSL written in Haskell to Scala. The DSL is an imperative language, so I used monads with do-notation, namely WriterT [Stmt] (State Label) a. I had trouble porting this to Scala, but worked around it by using the ReaderWriterState monad and just using Unit for the Reader component. I then went about searching for an alternative for the do-notation found in Haskell. For-comprehensions are supposed to be this alternative in Scala, but they are tailored towards sequences, I e.g. could not pattern match a tuple, it would insert a call to filter. So I started looking for alternatives and found multiple libraries: effectful, monadless, and each. I tried effectful first which does exactly what I wanted to achieve, I even prefer it over Haskell's do-notation, and it worked well with the ReaderWriterState monad from ScalaZ I had been using. In my DSL I have actions like Drop() (a case class) that I want to be able to use directly as a statement. I hoped to use implicits for this, but because the ! method of effectful (or monadless equivalent for that matter) is too general, I cannot get Scala to automatically convert my Action case classes to something of type Stmt (the ReaderWriterState that returns Unit).
So if not implicits, would there be a different way to achieve it?
As shown in Main.passes2, I did figure out a workaround that I would not mind using, but I am curious whether I stumbled upon is some limitation of the language, or is simply my lack of experience with Scala.
With Main.fails1 I will get the following error message: Implicit not found: scalaz.Unapply[scalaz.Monad, question.Label]. Unable to unapply type question.Label into atype constructor of kind M[_] that is classified by the type class scalaz.Monad. Check that the type class is defined by compiling implicitly[scalaz.Monad[type constructor]] and review the implicits in object Unapply, which only cover common type 'shapes.'
Which comes from ScalaZ its Unapply: https://github.com/scalaz/scalaz/blob/d2aba553e444f951adc847582226d617abae24da/core/src/main/scala/scalaz/Unapply.scala#L50
And with Main.fails2 I will just get: value ! is not a member of question.Label
I assume it is just a matter of writing the missing implicit definition, but I am not quite sure which one Scala wants me to write.
The most important bits of my build.sbt are the version:
scalaVersion := "2.11.2",
And the dependencies:
libraryDependencies += "org.scala-lang" % "scala-compiler" % "2.11.2",
libraryDependencies += "org.scala-lang" % "scala-library" % "2.11.2",
libraryDependencies += "org.scala-lang" % "scala-reflect" % "2.11.2",
libraryDependencies += "org.pelotom" %% "effectful" % "1.0.1",
Here is the relevant code, containing the things I tried and the code necessary to run those things:
package question
import scalaz._
import Scalaz._
import effectful._
object DSL {
type GotoLabel = Int
type Expr[A] = ReaderWriterState[Unit, List[Action], GotoLabel, A]
type Stmt = Expr[Unit]
def runStmt(stmt: Stmt, startLabel: GotoLabel): (Action, GotoLabel) = {
val (actions, _, updatedLabel) = stmt.run(Unit, startLabel)
val action = actions match {
case List(action) => action
case _ => Seq(actions)
}
(action, updatedLabel)
}
def runStmt(stmt: Stmt): Action = runStmt(stmt, 0)._1
def getLabel(): Expr[GotoLabel] =
ReaderWriterState((_, label) => (Nil, label, label))
def setLabel(label: GotoLabel): Stmt =
ReaderWriterState((_, _) => (Nil, Unit, label))
implicit def actionStmt(action: Action): Stmt =
ReaderWriterState((_, label) => (List(action), Unit, label))
}
import DSL._
final case class Label(label: String) extends Action
final case class Goto(label: String) extends Action
final case class Seq(seq: List[Action]) extends Action
sealed trait Action {
def stmt(): Stmt = this
}
object Main {
def freshLabel(): Expr[String] = effectfully {
val label = getLabel.! + 1
setLabel(label).!
s"ants-$label"
}
def passes0() =
freshLabel()
.flatMap(before => Label(before))
.flatMap(_ => freshLabel())
.flatMap(after => Label(after));
def passes1() = effectfully {
unwrap(actionStmt(Label(unwrap(freshLabel()))))
unwrap(actionStmt(Label(unwrap(freshLabel()))))
}
def fails1() = effectfully {
unwrap(Label(unwrap(freshLabel())))
unwrap(Label(unwrap(freshLabel())))
}
def pasess2() = effectfully {
Label(freshLabel.!).stmt.!
Label(freshLabel.!).stmt.!
}
def fails2() = effectfully {
Label(freshLabel.!).!
Label(freshLabel.!).!
}
def main(args: Array[String]): Unit = {
runStmt(passes0())
}
}
Question quality
I want to start with complaining about the question quality. You provide almost no textual description of what you are trying to achieve and then just show us a wall of code but don't explicitly reference your dependencies. This is far from what could count as a Minimal, Complete, and Verifiable example. And typically you get much more chances to get some answers if you provide a clear problem that is easy to understand and reproduce.
Back to the business
When you write something like
unwrap(Label(unwrap(freshLabel())))
you ask too much from the Scala compiler. Particularly unwrap can unwrap only something that is wrapped into some Monad, but Label is not a monad. It is not just the fact that there is no Monad[Label] instance, it is the fact that it structurally doesn't fit that kills you. Simply speaking an instance of ScalaZ Unapply is an object that allows you to split an applied generic type MonadType[SpecificType] (or other FunctorType[SpecificType]) into an "unapplied"/"partially-applied" MonadType[_] and SpecificType even when (as in your case) MonadType[_] is actually something complicated like ReaderWriterState[Unit, List[Action], GotoLabel, _]. And so the error says that there is no known way to split Label into some MonadType[_] and SpecifictType. You probably expected that your implicit actionStmt would do the trick to auto-convert Label into a Statement but this step is too much for the Scala compiler because for it to work, it also implies splitting the composite type. Note that this conversion is far from obvious for the compiler as the unwrap is itself a generic method that can handle any Monad. Actually in some sense ScalaZ needs Unapply exactly because the compiler can't do such things automatically. Still if you help the compiler just a little bit by specifying generic type, it can do the rest of the work:
def fails1() = effectfully {
// fails
// unwrap(Label(unwrap(freshLabel())))
// unwrap(Label(unwrap(freshLabel())))
// works
unwrap[Stmt](Label(unwrap(freshLabel())))
unwrap[Stmt](Label(unwrap(freshLabel())))
}
There is also another possible solution but it is a quite dirty hack: you can roll out your custom Unapply to persuade the compiler that Label is actually the same as Expr[Unit] which can be split as Expr[_] and Unit:
implicit def unapplyAction[AC <: Action](implicit TC0: Monad[Expr]): Unapply[Monad, AC] {
type M[X] = Expr[X]
type A = Unit
} = new Unapply[Monad, AC] {
override type M[X] = Expr[X]
override type A = Unit
override def TC: Monad[Expr] = TC0
// This can't be implemented because Leibniz really witness only exactly the same types rather than some kind of isomorphism
// Luckily effectful doesn't use leibniz implementation
override def leibniz: AC === Expr[Unit] = ???
}
The obvious reason why this is a dirty hack is that Label is actually not the same as Expr[Unit] and you can see it by the fact that you can't implement leibniz in your Unapply. Anyway, if you import unapplyAction, even your original fails1 will compile and work because effectful doesn't use leibniz inside.
As for your fails2, I don't think you can make it work in any simple way. Probably the only way that you may try is to create another macro that would convert in-place call to ! on your action (or its implicit wrapper) into the effectful's ! on the action.stmt. This might work but I didn't try it.
I am using json4s library in my scala program.
my build.sbt looks like
libraryDependencies ++= Seq(
"org.json4s" % "json4s-native_2.11" % "3.3.0"
)
in my code, i have a function
import org.json4s._
import org.json4s.native.JsonMethods._
import org.json4s.JValue
class Foo {
def parse(content: String) = {
val json = parse(content)
}
}
but the IDE complains "Recursive method parse needs result type"
The scala compiler usually infers the return type of methods based on their implementations, but it has trouble inferring the type of recursive methods.
The message recursive method parse needs result type is due to this shortcoming. Your def parse(content: String) recurses by calling parse(content). This makes the method recursive (infinitely so, but I'm assuming you were planning on changing it later). In order for it to compile, you'll need to explicitly state the return type, e.g. def parse(content: String): Unit.
I'm going to take a further guess and say that there is a parse method being imported from either json4s or JsonMethods. This is being shadowed by your own parse method due to it having the same method signature. If you actually want to call JsonMethods.parse, then you'll need to actually say JsonMethods.parse to clarify the ambiguity.
I wonder if it is possible to do something like the following:
import scala.reflect.runtime.universe._
class Bar[T]
def foo[T]()(implicit ctag: reflect.ClassTag[T]) {
val clazz = classOf[Bar[ctag.runtimeClass.asInstanceOf[Class[T]]]]
}
Here the Scala compiler complains:
error: stable identifier required, but ctag.runtimeClass found.
Is there a way to get the class type with type parameters inserted from the runtime type information available in the function?
Is there a way to get the class type with type parameters inserted from the runtime type information available in the function?
classOf[Bar[T]] works for a very simple reason: it doesn't insert any runtime information! classOf[Bar[T]], classOf[Bar[String]], classOf[Bar[Int]], classOf[Bar[_]] are all the same; that's what type erasure means in JVM context (to avoid misleading, I prefer always using classOf[Bar[_]] where possible). Note that there is actually a single exception: if Bar is Array, because classOf[Array[Int]] and classOf[Array[Object]] (e.g.) are different!
classOf[T] obviously would need runtime information and so it doesn't work.
Thanks to the comment by #Mr. V I realized that its actually easier than I initially thought:
import scala.reflect.runtime.universe._
class Bar[T]
def foo[T]() {
val clazz = classOf[Bar[T]]
}
Suppose I have:
class X
{
val listPrimitive: List[Int] = null
val listX: List[X] = null
}
and I print out the return types of each method in Scala as follows:
classOf[ComplexType].getMethods().foreach { m => println(s"${m.getName}: ${m.getGenericReturnType()}") }
listPrimitive: scala.collection.immutable.List<Object>
listX: scala.collection.immutable.List<X>
So... I can determine that the listX's element type is X, but is there any way to determine via reflection that listPrimitive's element type is actually java.lang.Integer? ...
val list:List[Int] = List[Int](123);
val listErased:List[_] = list;
println(s"${listErased(0).getClass()}") // java.lang.Integer
NB. This seems not to be an issue due to JVM type erasure since I can find the types parameter of List. It looks like the scala compiler throws away this type information IFF the parameter type is java.lang.[numbers] .
UPDATE:
I suspect this type information is available, due to the following experiment. Suppose I define:
class TestX{
def f(x:X):Unit = {
val floats:List[Float] = x.listPrimitive() // type mismatch error
}
}
and X.class is imported via a jar. The full type information must be available in X.class in order that this case correctly fails to compile.
UPDATE2:
Imagine you're writing a scala extension to a Java serialization library. You need to implement a:
def getSerializer(clz:Class[_]):Serializer
function that needs to do different things depending on whether:
clz==List[Int] (or equivalently: List[java.lang.Integer])
clz==List[Float] (or equivalently: List[java.lang.Float])
clz==List[MyClass]
My problem is that I will only ever see:
clz==List[Object]
clz==List[Object]
clz==List[MyClass]
because clz is provided to this function as clz.getMethods()(i).getGenericReturnType().
Starting with clz:Class[_] how can I recover the element type information that was lost?
Its not clear to me that TypeToken will help me because its usages:
typeTag[T]
requires that I provide T (ie. at compile time).
So, one path to a solution... Given some clz:Class[_], can I determine the TypeTokens of its method's return types? Clearly this is possible as this information must be contained (somewhere) in a .class file for a scala compiler to correctly generate type mismatch errors (see above).
At the java bytecode level Ints have to be represented as something else (apparently Object) because a List can only contain objects, not primitives. So that's what java-level reflection can tell you. But the scala type information is, as you infer, present (at the bytecode level it's in an annotation, IIRC), so you should be able to inspect it with scala reflection:
import scala.reflect.runtime.universe._
val list:List[Int] = List[Int](123)
def printTypeOf[A: TypeTag](a: A) = println(typeOf[A])
printTypeOf(list)
Response to update2: you should use scala reflection to obtain a mirror, not the Class[_] object. You can go via the class name if need be:
import scala.reflect.runtime.universe._
val rm = runtimeMirror(getClass.getClassLoader)
val someClass: Class[_] = ...
val scalaMirrorOfClass = rm.staticClass(someClass.getName)
// or possibly rm.reflectClass(someClass) ?
val someObject: Any = ...
val scalaMirrorOfObject = rm.reflectClass(someObject)
I guess if you really only have the class, you could create a classloader that only loads that class? I can't imagine a use case where you wouldn't have the class, or even a value, though.
I'd like to experiment with the use of a dynamic data model with a reflective library that uses typeOf[].
I've defined a class at runtime with a Scala reflection ToolBox in 2.11:
import scala.tools.reflect.ToolBox
import scala.reflect.runtime.universe._
import scala.reflect.runtime.{ currentMirror => cm }
def cdef() = q"case class C(v: String)"
val tb = cm.mkToolBox()
val csym = tb.define(cdef())
def newc(csym: Symbol) = q"""new ${csym}("hi")"""
val obj = tb.eval(newc(csym))
I'm able to circumvent the typeOf[] call by entering Scala reflection via the ClassSymbol instead, but that requires modifying a library over which I have no immediate control.
Is there any way that I can use it as a type parameter in a library whose entry point is typeOf[]?
I've tried:
The only way I found to go from a value to something that I could use in the type position was use Java reflection to invoke the companion class' apply method and call .type on the result:
val method_apply = obj.getClass.getMethod("apply", "".getClass)
val typeTemplate = method_apply.invoke(obj, "hello")
type MyType = typeTemplate.type
(Keeping with the naming scheme of #xeno_by and #travisbrown 's menagerie of odd types, I might call this "Frankenstein's Type", because it is made from parts, given life at the wrong time, not quite a substitute for the original, and given that this is all happening at runtime, should probably be burned with fire.)
This type alias works as a type parameter is some cases. But in the case of typeOf[MyType], the the compiler makes a TypeTag before the runtime type is defined, so typeOf[MyType] returns a type member that doesn't correspond to the runtime type/class (e.g. TypeTag[package.Example.MyType] instead of TypeTag[package.C])
Should I expect the ToolBox to have generated a TypeTag, and if so, how do I use it?
If I have to make a TypeTag at runtime, this question shows me how, but then how do I attach it to whatever I use as a type parameter?
Thanks for any ideas,
-Julian