[swift-evolution] Generic Alaises

[Milos Rankovic]

Hi Ross,

That was a hell of an example! However, even with the types as they are now, the code needn’t look so dreadful. With following protocols in place:

protocol Indexed {
	associatedtype Index : Hashable
	var index: Index { get }
}

protocol NodeType : Indexed { }
protocol EdgeType: Indexed { }

protocol GraphType {
	associatedtype Node : NodeType
	associatedtype Edge : EdgeType
	
	var nodes: [Node] { get }
	var edges: [Edge] { get }
}

We can define `Graph` and `Path` as:

struct Graph<Node:NodeType, Edge:EdgeType> : GraphType {
	let nodes: [Node] = [] // todo
	let edges: [Edge] = [] // todo
}

struct Path<Node:NodeType, Edge:EdgeType> {}

So the function with the endless signature you’ve got would become:

extension Graph {
	func shortestPath(from from: Node, to: Node) -> Path<Node, Edge> {
		fatalError("todo")
	}
}

As for my link to Chris’s proposal, all I meant is to point out that there is a proposal under review with an identical name as this thread. I certainly did not mean to discourage discussion James started!

milos

> On 6 Apr 2016, at 21:45, Ross O'Brien <narrativium+swift at gmail.com> wrote:
> 
> 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 <mailto: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 <mailto: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
>> ___________________________________
>> 
>> James⎥
>> 
>> james at supmenow.com <mailto:james at supmenow.com>⎥supmenow.com <http://supmenow.com/>
>> Sup
>> 
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>> 
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