SIP Meeting Minutes - 24th September 2018



The following agenda was distributed to attendees:

Topic Reviewers Accepted/Rejected
Summary of the Contributors thread “Proposal to remove XML literals from the language” Sébastien Doeraene Pending
Summary of the Contributors thread “Proposal to remove the procedure Syntax” Josh Suereth Pending
Proposal to add Intersection Types to the Language Martin Odersky Discussion opened until the 25th October 2018, comments welcomed here
Proposal to Add Union Types to the Language Martin Odersky Discussion opened until the 25th October 2018, comments welcomed here
Proposal to add Implicit Function Types to the Language Martin Odersky Discussion opened until the 25th October 2018, comments welcomed here
Proposal to add Dependent Function Types to the Language Martin Odersky Discussion opened until the 25th October 2018, comments welcomed here
Proposal to add Trait Parameters to the Language Martin Odersky Discussion opened until the 25th October 2018, comments welcomed here

Jorge Vicente Cantero was the Process Lead and Darja Jovanovic was the secretary.

Date and Location

The meeting took place on the 24th September 2018 at 5 PM CEST via Google Hangouts at EPFL in Lausanne, Switzerland as well as other locations.

Watch on Scala Center YouTube channel

Minutes were taken by Darja Jovanovic and Jorge Vicente Cantero.


Not present


Opening Remarks

Jorge opens the meeting, explains SIP dynamics: Finalising discussion about the 1st batch, action points Introducing batch two Decision / voting / postponing the discussion

Discussion of the first Scala 3 batch

“Proposal to remove XML literals from the language”

(YouTube time 3’ - 16’50’’)

Sébastien suggests to postpone the removal of XML and Procedure syntax, because when the removal takes place it will be a code breaking change, not a binary or tasty one. Adds it would be better to focus on changes that have an impact on tasty and binary format and deal with these later.

Seth (YouTube time:4’54) suggests to have a warning notes that it will eventually be removed.

Eugene asks what would be the positive effect of that change? And that we need to vote on it first.

Iulian asks what are the promises with regards to binary-compatible and source-compatible releases in Scala 3. To him it looks weird that we could break the source between 3.0 and 3.1.

Josh notes that by enforcing binary compatibility across Scala 2 and Scala 3 we are sacrificing source compatibility. He asks for this decision to be more thought over as it is a big decision with lots of impact for Scala tooling. He agrees we can make source-breaking releases nicer to use in source-based build tools like Pants or Bazel, but trading off binary compatibility by source compatibility is not a decision to take lightly more.

Martin thinks that no matter what trade-offs we do with regards to compatibility, he’d like to be able to remove XML literals in the first release of Scala 3.0 because having the XML spec inside the Scala spec gives a bad impression of complexity of the language, where he believes Scala is instead a more lightweight language than its competitors. There is no way he can make this argument if the XML spec continues to be in the relatively simple Scala language specification more.

(The discussion with regards to binary compatibility and source compatibility trade-offs is postponed.)

“Proposal to remove the procedure Syntax”

(YouTube time: 16’50’’ - 19.28’’)

Jorge reminds that in the last meeting we agreed that before moving forward with the change we needed:

  1. A better motivation
  2. A good explanation of why this change promotes the use of types (making it safer)
  3. A removal of the examples that were misleading
  4. A link to a Scalafix rewrite that could make a migration.

Jorge then points out that the changes need to be done in order to move forward, but is asking a Committee to voice their opinion about removing this feature in Scala 3. Josh underlines that there were 2 parts in the debate 1) Are procedures different than a method, do we want them visually distinctive? 2) Other issues listed by Jorge above. In particular, the fact that we want people to explicitly annotate the unit in their methods because it makes code more readable.

A decision will be taken into the future when all those items are acted on.

Discussion of the second Scala 3 batch

An overview of the second batch can be found in this Scala Contributors thread. The batches under discussion are:


Feedback on these proposals is open until the 25th October 2018, as describe in the linked Scala Contributors thread.

Proposal to add Intersection Types and Union Types to the language

(YouTube time: 20’49’’ - 24’01)

Martin presents the intersection types as per doc. He does a basic description of the feature and points out that intersection types are the duals of union types. He points out that union types have helped replace most of the lubbing mechanism and early precocious lubbing that happened in Scala 2 (which happens in less degree in the current implementation of Scala 3 but could be improved in future releases). Martin also thinks that union types are useful for null safety, where any type coming from Java could be annotated as T | Null. He then goes on describing further implementation details and trade-offs that Scala 3 does in this space. Adriaan asks what are the trade-offs between the encoding of union types that we have in Scala and the one they use in other languages like Typescript. Martin points out that performance-wise Scala union types would be more performant because T | Null wouldn’t box if T is a primitive type.

Sebastien (YouTube time: 30’03’’) gives his input based on the fact that Scala.js already has Union Types in Scala 2. He states that they are very limited; they were introduced for modeling because some libraries “desperately needed” them but turned out they were overused for no apparent reason. He advises to document the Union Types proper usage well and not get discouraged by the possible “overusage”.

There is some back-and-forth between Sebastien and Josh with regards to performance of union types and their boxing (especially in the presence of specialization). More

Proposal to add Implicit Function Types to the Language


(YouTube time: 39’01’’ - 43’11’’)

Martin explains what implicit function types are about and points out it’s a pretty “hot” feature that was published in POPL 2018. He underlines the advantages of implicit function types (like further abstraction of code that depends on a notion/representation of a context, like Scala 3’s compiler) and points out that implicit function types can replace the reader monad, even though it’s about 10x faster than the reader monad is. A comprehensive explanation of what implicit function types can do can be found in Olivier’s Blainvillain talk at ScalaDays, Berlin 2018. Martin thinks that implicit function types should be seen as the canonical way of doing scope injection, which gives you a lot of expressivity, to which Sebastien adds that what Martin means by canonical scope injection doesn’t necessarily correspond with the way people do normal scope injection, because in normal scope injection you can’t refer to identifier or parent scopes. Martin clarifies that for him comonadic abstraction are the classical way of scope injection in which we inject things into an environment, hence the use of canonical scope injection when referring to implicit function types which allow you to do the same. His definition of scope injection comes more from a typing rules perspective rather than the lexical point of view. Martin agrees that if there is a name clash with implicit function types there is a problem indeed.

Proposal to add Dependent Function Types to the Language

(YouTube time: 43’11’’ - 44’40’’)

Martin mentions that dependent function types is the last big addition to Scala’s type checker. The reason why they are added is because Scala has dependent methods and there is a need for dependent functions (the same rationale has been doing with regards to implicit methods and implicit function types). It’s an obvious win because dependent function types allow us to abstract over the idea of implicit methods in functions, so the more we can do the better. Initially he was afraid of the feature because he thought it violated this Scala principle that in the end anything is an instance of a class in some way and it turned out that a new encoding of dependent function types made this initial argument moot. Dependent function types are now encoded as implicit function types with type refinements, so this way it doesn’t violate that principle. Adriaan mentions that the last missing bit is polymorphic function types and Martin agrees and says that they are looking into that, but maybe not for Scala 3.0 (Guillaume Martres is pushing for polymorphic function types).

Proposal to add Trait Parameters to the Language

(YouTube time: 44’42’’ - end )

Martin describes trait parameters and says that they subsume a large number of use cases of early initializers. They were not added to Scala from the start because of uncertainty in the way they would work with regards to linearization and initialization of parameters. The way they solved this problem is by enforcing the rule that only the class extending a trait with parameters can pass the parameters. The motivation to add trait parameters is to regularize the language and get rid of early initializers which are an ad-hoc feature and are much harder to understand how to use correctly. Afterward, the Committee discusses some of the limitations of trait parameters. Josh (YouTube time: 48’40’’ suggests he will find one of his libraries that uses a lot of early initializers and see if trait parameters allow him to replace them. He’s curious about how clean would the code look after the change.

Jorge then wraps up the meeting, points out how feedback on these proposals would work (check the following link and finalizes the discussion.

Conclusion Next meeting will be dedicated to the Second Batch disscusion.