[swift-evolution] [Proposal] Higher Kinded Types (Monads, Functors, etc.)

[Austin Zheng]

+1 on HKTs. By the way, there were a couple of people discussing the same
here (https://github.com/typelift/swift/issues/1); you may or may not want
to swap ideas with them and see if you can work together towards a formal
proposal. (However, since it doesn't seem HKTs are in scope for Swift 3, a
proposal might be deferred until next year.)

Austin

On Wed, Dec 16, 2015 at 9:08 AM, Matthew Johnson via swift-evolution <
swift-evolution at swift.org> wrote:

> IMO the keys in a dictionary are not part of the contained values and
> therefore not part of the Functor mapping for Dictionary (obviously a map
> with different semantics is possible as exhibited by the standard
> library).  Dictionary should implement Functor by mapping over the values
> while preserving the keys.
>
> A similar problem has been noted for Set.  In that case it is not possible
> to uphold the usual Functor laws.  IMO that points to the need for a
> protocol with weaker semantics than Functor.  It would preserve the
> essential structure without preserving the 1 to 1 projection.  There are
> likely a number of cases where this would make sense.  Functor could
> inherit from that protocol adding additional semantic requirements but no
> additional syntactic requirements.
>
> Matthew
>
> Sent from my iPad
>
> On Dec 16, 2015, at 8:13 AM, Al Skipp via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> let myDictionary: [String:Int] = ["test": 1]
> var otherDictionary = myDictionary.myMap { ($0, Float($1)) }
>
> // does not fail:
>
> assert(otherDictionary is [String:Float])
>
>
> The trouble with Dictionary is that peculiar things can happen if we can
> map over keys and return a Dictionary.
>
> let dict = ["a":1, "b":2, "c":3]
> let dict2 = dict.map { ("x", $1 * 2) } // [“x”:6]
>
> We start with a Dictionary with 3 keys and end up with a Dictionary with 1
> key! As the order of the keys is unknown, we have no idea what the value of
> the key will be either, it might be 6, 4 or 2.
>
> Swift currently avoids this bedlam by returning an Array of tuples when
> ‘mapping’ a Dictionary. Strictly speaking this isn’t ‘map’ in the Functor
> sense, but then again Dictionary is a rogue that refuses to obey the
> Functor laws anyway.
>
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