Nested (non-capturing) type aliases #405
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lukellmann
reviewed
Dec 21, 2024
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| ```kotlin | ||
| class Dijkstra { | ||
| typelias VisitedNodes = Set<Node> |
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Suggested change
| typelias VisitedNodes = Set<Node> | |
| typealias VisitedNodes = Set<Node> |
typo
| // = capture(Inner<Int>, capture(Example<S>, { T })) | ||
| // = capture(Inner<Int>, { S }) | ||
| // = capture(Int) + { S } = { S } ⊆ { S } => OK | ||
| typelias Boo<S> = Example<S>.Inner |
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| typelias Boo<S> = Example<S>.Inner | |
| typealias Boo<S> = Example<S>.Inner |
same here
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| // capture(Inner<Int>, { T }) | ||
| // = capture(Int) + { T } | ||
| // = { T } ⊈ { T } => not allowed | ||
| typealias Moo = Inner<Int> |
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| // capture(Inner<Int>, { T }) | |
| // = capture(Int) + { T } | |
| // = { T } ⊈ { T } => not allowed | |
| typealias Moo = Inner<Int> | |
| // capture(Inner<Int>, { T }) | |
| // = capture(Int, { T }) + { T } | |
| // = { T } ⊈ { } => not allowed | |
| typealias Moo = Inner<Int> |
capture has two "parameters", Moo has no type parameters
| > [!TIP] | ||
| > As a rule of thumb, a nested type alias is correct if it could be used as the supertype or a parameter type within a nested class living within the same classifier. | ||
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| We formally define the set of captured type parameters of a type `T` with enclosing parameters `P`, `captured(T, P)`, as follows. |
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| We formally define the set of captured type parameters of a type `T` with enclosing parameters `P`, `captured(T, P)`, as follows. | |
| We formally define the set of captured type parameters of a type `T` with enclosing parameters `P`, `capture(T, P)`, as follows. |
typo
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| // capture(Example<S>.Inner<Int>, { T }) | ||
| // = capture(Inner<Int>, capture(Example<S>, { T })) | ||
| // = capture(Inner<Int>, { S }) | ||
| // = capture(Int) + { S } = { S } ⊆ { S } => OK | ||
| typelias Boo<S> = Example<S>.Inner |
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Suggested change
| // capture(Example<S>.Inner<Int>, { T }) | |
| // = capture(Inner<Int>, capture(Example<S>, { T })) | |
| // = capture(Inner<Int>, { S }) | |
| // = capture(Int) + { S } = { S } ⊆ { S } => OK | |
| typelias Boo<S> = Example<S>.Inner | |
| // capture(Example<S>.Inner<Int>, { T }) | |
| // = capture(Inner<Int>, capture(Example<S>, { T })) | |
| // = capture(Inner<Int>, { S }) | |
| // = capture(Int, { S }) + { S } = { S } ⊆ { S } => OK | |
| typelias Boo<S> = Example<S>.Inner<Int> |
capture has two "parameters", Inner has a type parameter
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| // correctUsage.kt | ||
| import A.* // imports `I` | ||
| import C.* // imports `D` | ||
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| val d = A().D() |
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it seems like this is missing the usage of I:
val i = A().I()
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Only comment here about the text itself. The discussion about the feature is done in #406.
Right now type aliases can only be used at the top level. The goal of this document is to propose a design to allow them within other classifiers, in case they do not capture any type parameters of the enclosing declaration.