Created
November 18, 2015 19:26
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An example of where I get confused by the interplay between associated types, typealiases and protocol extensions.
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| // I'm using the concept of bits just to make the code a bit more concrete. | |
| // The actual use case has nothing to do with implementing type level bits ... | |
| // This code compiles and runs as is, but note the last line. | |
| // Q: Should this program compile without errors? If so, what should the last line print? | |
| protocol BitType {} | |
| struct Clr : BitType {} | |
| struct Set : BitType {} | |
| protocol BitAndOp { | |
| typealias A: BitType = Clr | |
| typealias B: BitType = Clr | |
| typealias Result: BitType = Clr | |
| } | |
| extension BitAndOp where A == B { | |
| typealias Result = Set // This seems to declare a new Result, (since it's ok to set it to non-BitTypes) why? | |
| } | |
| struct And<T, U where T: BitType, U: BitType> : BitAndOp { | |
| typealias A = T | |
| typealias B = U | |
| } | |
| print(And<Set, Clr>.A.self) // Prints Set | |
| print(And<Set, Clr>.B.self) // Prints Clr | |
| print(And<Set, Clr>.Result.self) // Prints Clr (As expected, since A != B) | |
| print(And<Set, Set>.A.self) // Prints Set | |
| print(And<Set, Set>.B.self) // Prints Set | |
| // print(And<Set, Set>.Result.self) // Uncommenting this line crashes compiler after about ~10 seconds of work, why? |
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