> On Apr 14, 2016, at 4:58 PM, Jens Alfke via swift-users
> <swift-users@swift.org> wrote:
>
> It appears that you can’t extend a generic class/struct with a requirement
> that a type parameter inherit from a non-protocol:
> extension Dictionary where Key : String { … }
> The above produces the error “Type ‘Key’ constrained to non-protocol type
> ‘String’”.
>
> So is there a way I can add a method to a class, where the method depends on
> one of the type parameters being a specific type? For example, let’s say I
> want a “concatenatedKeys” method that returns all the Dictionary’s keys
> concatenated into a string with commas between them.
>
> (The actual reason I’m doing this is more complicated, but it has to do with
> JSON encoding — JSON dictionary keys have to be strings, so I want to be
> type-safe and make my encoding methods available only for Dictionary<String,
> ___>.)
>
> —Jens
I thought I replied last week but I don’t see it.
It looks like you can make it work using StringLiteralConvertible. Based on
this:
http://stackoverflow.com/questions/32610150/extension-of-dictionary-where-string-anyobject
This works:
extension Dictionary where Key: StringLiteralConvertible, Value: Any {
func testMethod() -> String {
return "test"
}
}
var d: [String: Int] = [:]
d["abc"] = 1
print(d.testMethod())
HTH,
Dave Reed
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