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Fix #2591: Incorrect documentation on general type projections #2603

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Merged
merged 2 commits into from
May 30, 2017
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Fix #2591: Incorrect documentation on general type projections #2603

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Fix #2591: Incorrect documentation on general type projections
smarter May 30, 2017
0272c7c
Add back info on typelevel combinator calculus
smarter May 30, 2017
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14 changes: 8 additions & 6 deletions docs/docs/reference/dropped/type-projection.md
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Expand Up @@ -6,10 +6,12 @@ title: Dropped: General Type Projection
Scala so far allowed general type projection `T#A` where `T` is an arbitrary type
and `A` names a type member of `T`.

Dotty allows this only if `T` is a class type. The change was made because
unrestricted type projection is [unsound](https://github.com/lampepfl/dotty/issues/1050).
Dotty disallows this if `T` is an abstract type (class types and type aliases
are fine). This change was made because unrestricted type projection
is [unsound](https://github.com/lampepfl/dotty/issues/1050).

The restriction rule out the [type-level encoding of compbinator
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I'd leave the "restructions rule out" part in. Otherwise LGTM.

calculus](https://michid.wordpress.com/2010/01/29/scala-type-level-encoding-of-the-ski-calculus/). It also rules out the previous encodings of type
lambdas using structural types with projection as application. Type
lambdas are now [directly supported](../type-lambdas.md) in Dotty.
This restriction rules out the [type-level encoding of a combinator
calculus](https://michid.wordpress.com/2010/01/29/scala-type-level-encoding-of-the-ski-calculus/).

To rewrite code using type projections on abstract types, consider using
path-dependent types or implicit parameters.