swift - Access Computed Property of a Protocol in a Protocol Extension -

assume have protocol like

protocol typedstring : customstringconvertible, equatable, hashable { }  func == <s : typedstring, t : typedstring> (lhs : t, rhs : s) -> bool {     return lhs.description == rhs.description } 

i want able provide default implementation of hashable:

extension typedstring {     var hashvalue = { return self.description.hashvalue } } 

but problem error use of unresolved identifier self.

how create default implementation of hashable using string representation given description property defined in customstringconvertible protocol?

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the motivation want create shallow wrappers around strings, can semantically type-check code. example instead of writing func login (u : string, p : string) -> bool instead write func login (u : username, p : password) -> bool (failable) initialisers username , password validation. example might write

struct username : typedstring {     let description : string      init?(username : string) {         if username.isempty { return nil; }          self.description = username     } } 

what want totally possible, error message you're receiving isn't making clear problem is: syntax computed property incorrect. computed properties need explicitly typed , don't use equals sign:

extension typedstring {     var hashvalue: int { return self.description.hashvalue } } 

this works fine!