[swift-evolution] [Pitch] Extend Any.Type to allow construction of bound generic types

[Douglas Gregor]

Sent from my iPhone

> On Apr 20, 2016, at 8:36 AM, Joanna Carter <joanna at carterconsulting.org.uk> wrote:
> 
> Hi Doug
> 
>> In programming-language circles, this feature is called “dependent types” (see, e.g., https://en.wikipedia.org/wiki/Dependent_type), and it introduces significant complicates into a type system. For example, determining whether two types are equivalent becomes a run-time property rather than a compile-time property. I don’t know if Swift will end with a dependently-typed type system. To get there, we would need a number of very strongly-motivating use cases illustrating how common programming tasks can be improved with dependent types,  and we would need to solid plan for managing the complexity—both implementation complexity in the compiler’s type checker and also the language complexity seen by Swift user’s when they encounter this feature.
> 
> I must admit to being ever so slightly confused about your connection with "dependent types"; in all my years of programming, I have never come across the context of the expression as found in the article cited. Anything that needs algebraic formulae to explain is far too complicated for mere mortal programmers and I am willing to state that I certainly didn't understand more than a few words.

Programming language theory (PL) tends to be painfully formal; much of what Swift's type system can be describe in PL  formalisms as well. 

My point is that your suggestion falls into a known theoretical framework that adds significant complications to the type system (and compiler pipeline in general). That complexity needs to be understood and managed. 

> 
> All I am postulating is the ability to create instances of generic types, bound to (a) parameter type(s) in the same manner as is currently possible with non-generic types.

Yes. It's bringing runtime type information into the static type system. 

> 
> To bring it down to basics, take the example of wanting to create an array of a given type of objects:
> 
>    let aType: Any.Type = Int.self
> 
>    var arr = [aType]
> 
> … creates an array of Any.type…
> 
>    let aType: Any.Type = Int.self
> 
>    var arr = [aType]()
> 
> … results in an error "Invalid use of '()' to call a value of non-function type '[Any.Type]'"
> 
>    let aType: Any.Type = Int.self
> 
>    var arr = Array<aType>() 
> 
> … results in an error "'aType' is not a type"
> 
> 
> So, how are we meant to be able to create arrays, or any other generic type that require to be bound to a type known only at runtime?

[Any] is a heterogenous array of elements whose types are known only at runtime. 

> In C#, We have the Type class, which contains useful properties like isGenericType, isConstructedGenericType and methods like GetGenericArguments() and, most important to this use case, MakeGenericType(Type []).

This is a reflection facility. Swift should get better reflection, but that's a very different thing from dependent types. 

> 
> I would make a strong argument for completing Swift generics by including such facilities in Any.Type, so that:
> 
> 1. we do not have to use the somewhat cumbersome Mirror mechanism
> 
> 2. we can do something useful with instances of Any.Type, which, at present, does absolutely nothing.
> 
> Adding similar functionality to C#'s Type class to Any.Type would break absolutely zero code but would make life a whole load easier for those of us who are power users of generics. Otherwise, FMPOV, Swift generics are but a pale imitation of the concept and very much second class citizens 

It's a stretch to call reflection part of the generics system, but it is a feature were interested in improving upon in the future. 

  - Doug