Support implicitNotFound on parameters #6340

lrytz · 2018-02-19T22:55:39Z

This allows a more specific message for individual implicit parameters
that are not found.

This allows improving error message, for example in overloading
resolution:

scala> trait Iterable[A] { def map[B](f: A => B): Iterable[B] }
scala> trait SortedSet[A] { def map[B: Ordering](f: A => B): SortedSet[B] }

scala> def t(s: SortedSet[Int]) = s.map(x => x) // OK
t: (s: SortedSet[Int])SortedSet[Int]

scala> def t[T](s: SortedSet[T]) = s.map(x => x) // Confusing error
                                        ^
       error: No implicit Ordering defined for T.

better:

scala> trait SortedSet[A] { def map[B](f: A => B)(implicit @annotation.implicitNotFound("No implicit Ordering[${B}] found to build a SortedSet[${B}]. You can build an unordered Set[${B}] by calling `unsorted` before `map`.") ord: Ordering[B]): SortedSet[B] }
defined trait SortedSet

scala> def t[T](s: SortedSet[T]) = s.map(x => x)
                                        ^
       error: No implicit Ordering[T] found to build a SortedSet[T]. You can build an unordered Set[T] by calling `unsorted` before `map`.

lrytz · 2018-02-19T22:56:05Z

See also scala/collection-strawman#432

soronpo · 2018-02-28T13:23:24Z

What happens in the following case?

@implicitNotFound("BarError")
trait Bar
@implicitNotFound("FooError")
trait Foo
object Foo {
  implicit def ev(implicit @implicitNotFound("BarParamError") bar : Bar) : Foo = ???
}

implicitly[Foo]

lrytz · 2018-02-28T20:54:47Z

It tells you

scala> implicitly[Foo]
                 ^
       error: FooError

With -Xlog-implicits

scala> implicitly[Foo]
                 ^
       Foo.ev is not a valid implicit value for Foo because:
       hasMatchingSymbol reported error: BarParamError
                 ^
       error: FooError

So it's picked up here as well.

szeiger · 2018-04-23T19:00:03Z

test/files/neg/t2462b.check

 trait Meh2[-From, +To]
      ^
+t2462b.scala:13: warning: Invalid implicitNotFound message for value theC:
+The type parameter Uuh referenced in the message of the @implicitNotFound annotation is not defined by value theC.


I find this message confusing. The type parameter is supposed to be defined by m, not by theC.

szeiger · 2018-04-23T19:02:18Z

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

-    private lazy val typeParamNames: List[String] = sym.typeParams.map(_.decodedName)
-    private def typeArgsAtSym(paramTp: Type) = paramTp.baseType(sym).typeArgs
+    private lazy val typeParamNames: List[String] = {
+      val s = if (sym.isParameter) sym.enclMethod else sym


Can (and should) we get the full scope here instead of only the method's type parameters? This doesn't work:

scala> trait I[X, Y] defined trait I scala> trait T[A] { def f[B](implicit @scala.annotation.implicitNotFound("Missing I[${A}, ${B}]") i: I[A, B]): Unit = () } ^ warning: Invalid implicitNotFound message for value i: The type parameter A referenced in the message of the @implicitNotFound annotation is not defined by value i. defined trait T

This allows a more specific message for individual implicit parameters that are not found. This allows improving error message, for example in overloading resolution: ``` scala> trait Iterable[A] { def map[B](f: A => B): Iterable[B] } scala> trait SortedSet[A] { def map[B: Ordering](f: A => B): SortedSet[B] } scala> def t(s: SortedSet[Int]) = s.map(x => x) // OK t: (s: SortedSet[Int])SortedSet[Int] scala> def t[T](s: SortedSet[T]) = s.map(x => x) // Confusing error ^ error: No implicit Ordering defined for T. ``` better: ``` scala> trait SortedSet[A] { def map[B](f: A => B)(implicit @annotation.implicitNotFound("No implicit Ordering[${B}] found to build a SortedSet[${B}]. You can build an unordered Set[${B}] by calling `unsorted` before `map`.") ord: Ordering[B]): SortedSet[B] } defined trait SortedSet scala> def t[T](s: SortedSet[T]) = s.map(x => x) ^ error: No implicit Ordering[T] found to build a SortedSet[T]. You can build an unordered Set[T] by calling `unsorted` before `map`. ```

lrytz · 2018-06-15T08:38:06Z

Updated. When putting an @implicitNotFound on a parameter, the message can now refer to any type paramters in scope. (hmm, that makes me think if it should support type members too.. that might be hard to implement).

lrytz · 2018-06-15T08:39:41Z

test/files/neg/t2462c.check

+t2462c.scala:36: error: I see no C[Foo[Int]]
+  h[Foo[Int]]
+   ^
+t2462c.scala:40: error: String List[?] List[C[_]] Int Option[Long] -- .


The List[?] is not great here. The formatParameterMessage method could be improved for the case of type constructors. cc @adriaanm.

consider me sniped :-) I pushed a commit that adds some ad-hoc pretty printing for these. Feel free to g push -f HEAD^

lrytz · 2018-06-15T08:41:48Z

Maybe we can say this is good enough to merge? It's only about error messages. We can always accept contributions that improve it :) I'd like to not spend much more time on this.

soronpo · 2018-06-15T09:18:01Z

What will happen when an implicit parameter is missing from an implicit conversion? :

@implicitNotFound("BarError")
trait Bar
@implicitNotFound("FooError")
trait Foo
object Foo {
  implicit def fromBar(bar : Bar)(implicit @implicitNotFound("FooParamError") foo : Foo) : Foo = foo
}

val foo : Foo = new Bar{}

Currently (and sadly) scalac does not even report FooError. Will it now report FooParamError ?

adriaanm · 2018-06-15T09:29:24Z

What will happen when an implicit parameter is missing from an implicit conversion?

Same as before. No (user-defined) error. Having some form of interactive implicit search debugger would be great, but for error messages I don't think we should try to guess secondary issues.

soronpo · 2018-06-15T09:46:26Z

Same as before. No (user-defined) error. Having some form of interactive implicit search debugger would be great, but for error messages I don't think we should try to guess secondary issues.

In many cases I don't think it's guessing and I would have loved a custom error instead of just a type mismatch. More specifically, in case where an implicit conversion only fails due to a missing implicit parameter and there is only one possible implicit conversion definition in scope.

adriaanm · 2018-06-15T09:25:58Z

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

+      val paramSyms = paramNames.map(lookupTypeParam).filterNot(_ == NoSymbol)
+      val paramTypeRefs = paramSyms.map(sym => typeRef(NoPrefix, sym, sym.typeParams.map(_ => WildcardType)))
+      val prefix = fun match {
+        case TypeApply(Select(qual, _), _) => qual.tpe


could collapse these two cases (and also deal with repeated Apply) using: case treeInfo.Applied(Select(qual, _), _, _) => qual.tpe

adriaanm · 2018-06-15T10:01:39Z

test/files/neg/t2462c.check

+t2462c.scala:36: error: I see no C[Foo[Int]]
+  h[Foo[Int]]
+   ^
+t2462c.scala:40: error: String List[?] List[C[_]] Int Option[Long] -- .


consider me sniped :-) I pushed a commit that adds some ad-hoc pretty printing for these. Feel free to g push -f HEAD^

adriaanm · 2018-06-15T10:02:28Z

We could discuss that in a ticket and a different PR, but it's definitely out of scope for this one.

adriaanm · 2018-06-15T10:03:30Z

@lrytz, others: the first commit LGTM, but I'll let you decide on my follow-up :-)

soronpo · 2018-06-15T10:54:53Z

We could discuss that in a ticket and a different PR, but it's definitely out of scope for this one.

@adriaanm I created a separate ticket for that
scala/bug#10946

With some help I can create a PR, if you'd consider accepting it.

lrytz · 2018-06-15T12:19:33Z

test/files/neg/t2462c.check

  h[Foo[Int]]
   ^
-t2462c.scala:40: error: String List[?] List[C[_]] Int Option[Long] -- .
+t2462c.scala:40: error: String List [?T0, ZZ] -> List[C[_]] Int Option[Long] -- .


People will complain why this syntax doesn't work in their code 😏

adriaanm · 2018-06-15T12:23:11Z

Neither do wildcards :-p

…

On Fri, Jun 15, 2018 at 14:19 Lukas Rytz ***@***.***> wrote: ***@***.**** commented on this pull request. ------------------------------ In test/files/neg/t2462c.check <#6340 (comment)>: > @@ -10,7 +10,7 @@ t2462c.scala:33: error: No C of Foo[Int] t2462c.scala:36: error: I see no C[Foo[Int]] h[Foo[Int]] ^ -t2462c.scala:40: error: String List[?] List[C[_]] Int Option[Long] -- . +t2462c.scala:40: error: String List [?T0, ZZ] -> List[C[_]] Int Option[Long] -- . People will complain why this syntax doesn't work in their code 😏 — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#6340 (review)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAFjy1Wkw8M7zeEoEIYyMedNqqaLfdtmks5t86ZagaJpZM4SLMvC> .

adriaanm · 2018-06-15T12:55:53Z

With some help I can create a PR, if you'd consider accepting it.

let's first discuss on the ticket -- I'm not against on principle or anything, but it's probably worth talking through before embarking on the implementation

scala-jenkins added this to the 2.13.0-M4 milestone Feb 19, 2018

lrytz requested a review from szeiger February 20, 2018 15:27

szeiger reviewed Apr 23, 2018

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szeiger modified the milestones: 2.13.0-M4, 2.13.0-M5 May 2, 2018

lrytz force-pushed the implicitNotFoundParam branch from 77c974a to 3d2b107 Compare June 15, 2018 08:34

lrytz commented Jun 15, 2018

View reviewed changes

implicitNotFound: prettier printing for type funs

e907e31

adriaanm force-pushed the implicitNotFoundParam branch from 8badb82 to e907e31 Compare June 15, 2018 10:00

adriaanm reviewed Jun 15, 2018

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adriaanm approved these changes Jun 15, 2018

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lrytz commented Jun 15, 2018

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lrytz merged commit 76c4ef4 into scala:2.13.x Jun 15, 2018

SethTisue added the release-notes worth highlighting in next release notes label Jun 15, 2018

smarter mentioned this pull request Aug 22, 2018

Support implicitNotFound on parameters, like Scala 2.13.0-M5 scala/scala3#4986

Closed

ktoso mentioned this pull request Aug 23, 2018

Once 2.13 lands use implicitNotFound for parameters akka/akka-core#25506

Open

Support implicitNotFound on parameters #6340

Support implicitNotFound on parameters #6340

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