Add Export as a Dual of Import #6169

odersky · 2019-03-26T10:36:20Z

This is a pre-SIP proposal and its implementation to introduce export clauses to the language.

Motivation and details are in the export.md file of this PR.

I co-developed proposal and implementation because the type checking details are a bit tricky, so it paid to have a concrete algorithm to work on that can be tested.

This pre-SIP addresses what must be the longest standing proposal to improve the Scala language. The suggestion to have something like this was first brought up by Jamie Webb in 2005.

Cf also the pre-SIP by David Barry: https://github.com/japgolly/SIP/blob/master/SIP-xx-export/SIP.md

julienrf · 2019-03-26T10:52:24Z

This is interesting. Can we use it to define aliases within the same class?

trait IterableOps[+A, +CC[_], +C] {

  def concat[B >: A](other: IterableOnce[B]): CC[B]

  export this.{concat => ++}

}

Note that a difference with the examples you mentioned is that class members can be refined, in which case the alias should be refined accordingly:

trait X[+A] extends IterableOps[A, Seq, List[A]] {

  def concat[B >: A](other: IterableOnce[B]): List[B] = ... // note that return type is `List[B]` instead of the inherited `Seq[B]`

}

odersky · 2019-03-26T11:18:07Z

Note that a difference with the examples you mentioned is that class members can be refined, in which case the alias should be refined accordingly

No, export is not made for that. I think what you are looking for in this use case is @alpha (#5975).

sjrd · 2019-03-26T14:16:59Z

Should we have a Contributors thread to discuss the proposal, independently of its implementation?

In the meantime:

Type members are aliased by type definitions, and term members are aliased by method definitions. Export aliases copy the type and value parameters of the members they refer to.

So, exports basically copy members. IIUC that means we cannot use export to re-export towers of implicits with decreasing priorities in a hierarchy of classes (the XYZLowPriorityImplicits pattern), as the exports will all end up in the same level of priority, even when the imported members had different levels of priority. This means that it is impossible to use export to replace the use of package object with extends that use the LowPriorityImplicits pattern. Now, that pattern is the only strong reason that we could not already replace all package objects by top-level definitions (albeit with more boilerplate than with export). So IIUC export does not actually solve the loss of API design expressivity incurred by dropping package object ... extends.

odersky · 2019-03-26T14:50:38Z

@srjd Yes, I was going to open a thread on contributors.

Regarding implicits, we fortunately will have better ways now to prioritze than by location of original definition.: #6071 (comment).

OlivierBlanvillain · 2019-04-02T10:01:03Z

docs/docs/reference/other-new-features/export.md

+It is a compile-time error if a simple or renaming selector does not identify any eligible
+members.
+
+Type members are aliased by type definitions, and term members are aliased by method definitions. Export aliases copy the type and value parameters of the members they refer to.


What about annotations, access modifiers, and the inline modifier?

None of them are copied (I believe that's implied by the text).

OlivierBlanvillain · 2019-04-02T11:35:11Z

docs/docs/reference/other-new-features/export.md

+ 1. Elaborate any annotations of the class.
+ 2. Elaborate the parameters of the class.
+ 3. Elaborate the self type of the class, if one is given.
+ 4. Enter all definitions of the class as class members, with types to be completed


From this it's not very clear if definitions in classes can use exported aliases...

They can, but paths to exported aliases cannot refer to other exported aliases.

OlivierBlanvillain · 2019-04-02T11:47:41Z

compiler/src/dotty/tools/dotc/typer/Namer.scala

+                val maybeStable = if (mbr.symbol.isStableMember) StableRealizable else EmptyFlags
+                ctx.newSymbol(
+                  cls, alias,
+                  Method | Final | maybeStable | mbr.symbol.flags & ImplicitOrImplied,


Synthetic?

No, I don't think we want to make them Synthetic .

OlivierBlanvillain · 2019-04-02T11:51:42Z

compiler/src/dotty/tools/dotc/typer/Namer.scala

+          val sym = mbr.symbol
+          if (sym.is(ImplicitOrImplied) != exp.impliedOnly) s"is ${if (exp.impliedOnly) "not " else ""}implied"
+          else if (!sym.isAccessibleFrom(path.tpe)) "is not accessible"
+          else if (sym.isConstructor || sym.is(ModuleClass) || sym.is(Bridge)) "_"


Missing error message?

No, it's intended to be skipped later. I'll change it to something less obscure.

OlivierBlanvillain · 2019-04-02T12:23:42Z

tests/run/exports.scala

+  Copier.cfg
+  Copier.config
+  Copier.config2
+}


It would be nice to also have an example with inline since it was mentioned in the meeting.

inline is not implemented in this PR. Not clear we want it.

soronpo · 2019-04-02T13:39:25Z

Proposal for error test cases:

class Foo {
  lazy foo : Foo = new Foo
  export foo._
}

class Foo {
  lazy bar: Bar = new Bar
  export bar._
}
class Bar {
  lazy foo: Foo = new Foo
  export foo._
}

So far, this gave a "member already exists" error. Also, fix info of type export aliases.

- Clearer code for skipped whynoMatchStr - Surivive missing ExportForwarders attachments - Add tests with self-referential exports

odersky force-pushed the add-export-2 branch 2 times, most recently from e404bd9 to f36c5d9 Compare March 28, 2019 15:00

liufengyun requested a review from OlivierBlanvillain April 1, 2019 14:36

liufengyun assigned OlivierBlanvillain Apr 1, 2019

OlivierBlanvillain approved these changes Apr 2, 2019

View reviewed changes

OlivierBlanvillain assigned odersky and unassigned OlivierBlanvillain Apr 2, 2019

odersky added 14 commits April 3, 2019 11:39

Export: syntax and parsing

3f73fad

Export: Type checking

426bbf7

Test

aabf4a2

Bug fixes

9eb006a

Allow exports as toplevel statements

aeff199

Make export aliases of stable values stable

a40ad34

Docs and tests

b325f93

Doc tweaks

4bb7a6b

Rename classes to avoid 'compiled twice' errors

37aa7c0

Better error messages when no export aliases exist

713468b

Fix typo

570ed9f

Allow implementing deferred members by export aliases

87eff21

So far, this gave a "member already exists" error. Also, fix info of type export aliases.

Address review comments

3022cf6

- Clearer code for skipped whynoMatchStr - Surivive missing ExportForwarders attachments - Add tests with self-referential exports

Update checkfile

1731ba7

odersky force-pushed the add-export-2 branch from 801e3ea to 1731ba7 Compare April 3, 2019 09:44

odersky merged commit 5b0a7ce into scala:master Apr 4, 2019

allanrenucci deleted the add-export-2 branch April 4, 2019 17:01

biboudis added this to the 0.14 Tech Preview milestone Apr 5, 2019

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Add Export as a Dual of Import #6169

Add Export as a Dual of Import #6169

odersky commented Mar 26, 2019 •

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julienrf commented Mar 26, 2019 •

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odersky commented Mar 26, 2019 •

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sjrd commented Mar 26, 2019

odersky commented Mar 26, 2019

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Add Export as a Dual of Import #6169

Add Export as a Dual of Import #6169

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odersky commented Mar 26, 2019 • edited Loading

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soronpo commented Apr 2, 2019 • edited Loading

odersky commented Mar 26, 2019 •

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julienrf commented Mar 26, 2019 •

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odersky commented Mar 26, 2019 •

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soronpo commented Apr 2, 2019 •

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