[swift-evolution] Generic Alaises

[Ross O'Brien]

It's not the same topic. Let's take an example: suppose we have a data
structure for a graph of nodes and edges, where the nodes and edges are
indexed and both have values. So we have a Graph<NodeIndex : Hashable,
EdgeIndex : Hashable, NodeValue, EdgeValue>.
Now suppose we want a shortest path from one node to another, and we have a
data structure to represent that. Now we have a Path<NodeIndex : Hashable,
EdgeIndex : Hashable, NodeValue, EdgeValue>. They both have the same
'generic signature'. If you're navigating a Graph<String, Int, City,
Motorway>, you're going to want a Path<String, Int, City, Motorway> as
output.

Right now you might write that as:
func shortestPath<NodeIndex, EdgeIndex, NodeValue,
EdgeValue>(graph:Graph<NodeIndex, EdgeIndex, NodeValue, EdgeValue>,
startNode: NodeIndex, endNode: NodeIndex> -> Path<NodeIndex, EdgeIndex,
NodeValue, EdgeValue>

It might save a fair amount of typing if we had a generic equivalent to
both typealias and associatedtype.

associatedgenerics GraphElements = <NodeIndex : Hashable, EdgeIndex :
Hashable, NodeValue, EdgeValue>
func shortestPath<GraphElements>(graph: Graph<GraphElements>,
startNode<GraphElements.NodeIndex>, endNode<GraphElements.NodeIndex>) ->
Path<GraphElements>

genericalias NavigationGraphElements = GraphElements<String, Int, City,
Motorway>

typealias NavigationGraph = Graph<NavigationGraphElements>
// navigationGraph.shortestPath() now returns a
Path<NavigationGraphElements>
// this last part is closest to the proposal under review.


On Wed, Apr 6, 2016 at 9:05 PM, Milos Rankovic via swift-evolution <
swift-evolution at swift.org> wrote:

> Chris Lattner has a proposal under review
> <https://github.com/apple/swift-evolution/blob/master/proposals/0048-generic-typealias.md> on
> this topic.
>
> milos
>
> On 6 Apr 2016, at 20:41, James Campbell via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> This was inspired from the topic about moving where clauses out of
> parameter lists.
>
> Certain generics get very long winded, I was wondering if we could create
> some sort of alias for generics.
>
> func anyCommonElements <T: SequenceType, U: SequenceType where
> T.Generator.Element: Equatable, T.Generator.Element == U.Generator.Element>
> (lhs: T, _ rhs: U) -> Bool
>
> could be shared across functions like so:
>
> genericalias SequencesWithSameElements<T, U> =  <T: SequenceType, U:
> SequenceType where T.Generator.Element: Equatable, T.Generator.Element ==
> U.Generator.Element>
>
> func anyCommonElements <SequencesWithSameElements> (lhs: T, _ rhs: U) ->
> Bool
> func == <SequencesWithSameElements> (lhs: T, _ rhs: U) -> Bool
>
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>
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>
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