Hi all,
I am fairly new to Swift, so this may very well be a simple misunderstanding on
my part.
I was exploring the subtyping rules of Swift, especially regarding covariance.
I came across two examples where the outcome puzzles me and I would appreciate
if someone could explain this to me.
First I tried the covariance for the standard container types. Here is an
excerpt from a REPL session:
Michael@carpo:~ > swift
Welcome to Apple Swift version 3.0.2 (swiftlang-800.0.63 clang-800.0.42.1).
Type :help for assistance.
1> open class Super {}; class Sub: Super {}
2> print([Sub()] is [Sub])
true
3> print([Sub()] is [Super])
true
4> print(Array<Sub>(arrayLiteral: Sub()) is [Sub])
true
5> print(Array<Sub>(arrayLiteral: Sub()) is [Super])
error: repl.swift:5:39: error: 'Super' is not a subtype of 'Sub'
print(Array<Sub>(arrayLiteral: Sub()) is [Super])
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
Why does it matter for subtyping against [Super] whether I express an array as
[Sub] or Array<Sub>(arrayLiteral: Sub())?
Then I tried combining array covariance with function argument type
contravariance:
Michael@carpo:~ > swift
Welcome to Apple Swift version 3.0.2 (swiftlang-800.0.63 clang-800.0.42.1).
Type :help for assistance.
1> open class Super {}; class Sub: Super {}
2> func f(_: [Super]) {}
3> let test: ([Sub]) -> Void = f
error: repl.swift:3:29: error: cannot convert value of type '([Super]) -> ()'
to specified type '([Sub]) -> Void'
let test: ([Sub]) -> Void = f
^
Why is this assignment not possible? It works fine (as expected) when using
plain Super and Sub instead of arrays.
Michael
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