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Better compile errors for pattern guards in for-comprehension #3530

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Merged
merged 1 commit into from
Jun 4, 2020
Merged

Better compile errors for pattern guards in for-comprehension #3530

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4 changes: 3 additions & 1 deletion core-tests/shared/src/test/scala/zio/ZIOSpec.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -3244,7 +3244,9 @@ object ZIOSpec extends ZIOBaseSpec {
} yield n
"""
}
val expected = "Cannot prove that NoSuchElementException <:< String."

val expected =
"Pattern guards are only supported when the error type is a supertype of NoSuchElementException. However, your effect has String for the error type."
if (TestVersion.isScala2) assertM(result)(isLeft(equalTo(expected)))
else assertM(result)(isLeft(anything))
}
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19 changes: 17 additions & 2 deletions core/shared/src/main/scala/zio/ZIO.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@

package zio

import scala.annotation.implicitNotFound
import scala.concurrent.ExecutionContext
import scala.reflect.ClassTag
import scala.util.{ Failure, Success }
Expand Down Expand Up @@ -3693,6 +3694,20 @@ object ZIO extends ZIOCompanionPlatformSpecific {
self.provideLayer[E1, R0, R0 with R1](ZLayer.identity[R0] ++ layer)
}

@implicitNotFound(
"Pattern guards are only supported when the error type is a supertype of NoSuchElementException. However, your effect has ${E} for the error type."
)
sealed trait CanFilter[+E] {
def apply(t: NoSuchElementException): E
}

object CanFilter {
implicit def canFilter[E >: NoSuchElementException]: CanFilter[E] =
new CanFilter[E] {
def apply(t: NoSuchElementException): E = t
}
}

implicit final class ZIOWithFilterOps[R, E, A](private val self: ZIO[R, E, A]) extends AnyVal {

/**
Expand All @@ -3704,10 +3719,10 @@ object ZIO extends ZIOCompanionPlatformSpecific {
* positive <- io2 if positive > 0
* } yield ()
*/
def withFilter(predicate: A => Boolean)(implicit ev: NoSuchElementException <:< E): ZIO[R, E, A] =
def withFilter(predicate: A => Boolean)(implicit ev: CanFilter[E]): ZIO[R, E, A] =
self.flatMap { a =>
if (predicate(a)) ZIO.succeedNow(a)
else ZIO.fail(new NoSuchElementException("The value doesn't satisfy the predicate"))
else ZIO.fail(ev(new NoSuchElementException("The value doesn't satisfy the predicate")))
}
}

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