Mike Slinn Mike Slinn - 8 months ago 87
Scala Question

Scala ADT Type Mismatch

I'm puzzled by the compile error for the line that defines

endState
. Here is the code:

import java.util.UUID

object Evaluation {
def default: Evaluation[Unit, Unit] = new Evaluation[Unit, Unit](identity)
}

case class Evaluation[From, +To](evaluate: (From) => To)

object FSMLike {
val uuidEmpty: UUID = new UUID(0L, 0L)

val endState: Evaluation[Unit, FSMLike[Nothing]] = new Evaluation(() => FSMEmpty)

lazy val stop: FSMEntry[Unit, Unit, Nothing] = FSMEntry(uuidEmpty, Evaluation.default, endState)

def apply[From1, From2, To](
action: Evaluation[From1, Unit],
nextState: Evaluation[From2, FSMLike[To]]
): (UUID, FSMLike[To]) = {
val uuid = UUID.randomUUID
uuid -> FSMEntry(uuid, action, nextState)
}
}

sealed trait FSMLike[+A]

case object FSMEmpty extends FSMLike[Nothing]

case class FSMEntry[From1, From2, +To](
id: UUID,
action: Evaluation[From1, Unit],
nextState: Evaluation[From2, FSMLike[To]]
) extends FSMLike[To] {
def transition(arg1: From1, arg2: From2): FSMLike[To] = {
action.evaluate(arg1)
nextState.evaluate(arg2)
}

override def toString: String = s"^$id^"
}


Here is the error:

Error:(14, 72) type mismatch;
found : () => FSMEmpty.type (with underlying type () => FSMEmpty.type)
required: Unit => FSMLike[Nothing]
val endState: Evaluation[Unit, FSMLike[Nothing]] = new Evaluation(() => FSMEmpty)

Answer Source

You are trying to pass () => FSMEmpty, a function with no arguments, where a function with one argument of type Unit is expected. Sure, when you use () as an expression, it's the only value of type Unit, but the left-hand side of => isn't an expression.

You should write _ => FSMEmpty instead. { case () => FSMEmpty } would work as well, I think, but there isn't much point to using it.

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