[swift-evolution] Checking in; more thoughts on arrays and variadic generics

[Robert Widmann]

~Robert Widmann

2017/01/26 13:37、Daryle Walker <darylew at mac.com <mailto:darylew at mac.com>> のメッセージ:

> 
>> On Jan 23, 2017, at 3:33 PM, Robert Widmann <devteam.codafi at gmail.com <mailto:devteam.codafi at gmail.com>> wrote:
>> 
>> Some thoughts inline.
>> 
>>> On Jan 21, 2017, at 11:06 AM, Daryle Walker via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> 1. Variadic generics
>>> 
>>> When I look at SwiftDoc.org <http://swiftdoc.org/>, I see some functions repeated with differing numbers of parameters. This seems like a job for variadic templates^H^H^H^H^H^H^H^H^H generics, like in C++. Fortunately, someone has already wrote about this, at <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#variadic-generics <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#variadic-generics>>. A new idea I came up with is that both homogeneous (the current support) and heterogeneous variadic parameters can appear in function declarations. Each can appear at most once in a declaration. And they can co-exist in the same declaration; there’s no problem unless the two packs are adjacent and at least the (lexically) second one doesn’t mandate a label. In that case, and when the homogenous pack appears second, count from the lexically last argument backwards until an argument cannot be part of the homogeneous type, that’ll be the border. Count the other way when the homogenous pack is first. (It’s possible for one pack to have zero elements.)
>> 
>> C++ has a simpler rule (for once): If you’re going to pack, you have to pack last.  This is roughly the rule we have as well for argument lists in functions that don’t have labels - they can have any number of variadic parameters because we can use the argument label to guide the tuple type comparison and disambiguate.  Here we lack argument labels (and I’m not sure it’s useful to have them).  
>> 
>> As for the distinction between heterogeneous and homogenous lists, I’m not sure it’s a useful thing to have unless you’re trying to roll your own Tuple (which is a thing you can do now with HLists <https://github.com/typelift/Swiftz/blob/master/Sources/HList.swift> anyway).  Any type that wishes to take a variadic number of homogeneous type variables is a type that can be parametrized by one type variable and enforce the cardinality invariant elsewhere (see std::initializer_list).
>> 
> 
> The “homogenous list” I’m talking about are the variadic parameters that are already in the language. And they can already be non-last in the list. (At least it compiled, but the parameter I had after it had a default value, so I don’t know if that made a difference.)

A warning was just introduced if you declare non-terminal variadics without argument labels to disambiguate parameters.  That's what I meant.  Being able to constrain a signature so that it must have at least a certain cardinality seems useful, but what I’m saying is I’m not sure it’s useful to constrain it in multiple directions.  Consider

struct Foo<T, U, V…, LAST> {}

which can be instantiated as Foo<String, Int, UInt, NSObject> to satisfy the requirements of the signature.  Rearranging this declaration to shuffle non-variadic parameters to the front doesn’t change that

struct Foo<T, U, LAST, V...> {}

It just changes the order in which you or the typechecker comes around to instantiate the signature.  We’re getting down to matters of practicality here: Yes, there’s nothing stopping us from allowing variadics in any position, but what’s the point?  What kinds of structures can I only model with this kind of bi-directional cardinality matching?

> The “homogenous” is to differentiate them from parameters introduced by variadic generics (which would be “heterogeneous”).

Ah, so there’s not a hard difference here just a terminology change.  The use of “homogenous” and “heterogeneous” here is superfluous, since it’s pretty much assumed that in a variadic setting you wish for a heterogeneous list of types.  After all, by linearity, Foo<T, T, T, T…> is isomorphic to Foo<T>.  

> 
> My inspiration was from C++, where a function template can have both C++ and C variadic parameters. The C-level ones have to be last, and the C++-level ones next-to-last. Last I checked, this pattern was only useful for stupid resolution tricks; you couldn’t actually use such a function. (Using C-level variadics requires using a macro on the preceding argument. But on a dual-variadic function, this would be the C++-level variadic argument (instead of a singular one), which was (is?) not compatible with said macro.) I just want to make a generic function that uses both kinds of variadic argument lists actually usable, by defining the dividing line. Again, this is only a problem if the two lists are adjacent and the second list doesn’t have an external label.
> 
> AFAIK, there is no concrete specification for variadic generic functions yet. Hopefully, that team will take my advice into account. IF you don’t think a heterogeneous list should be non-last, nor co-exist with homogenous list, you can bring it up then.
> 
>>> 2. More on Arrays
>>> 
> [SNIP]
>> 
>> This also doesn’t seem to fit with the rest of the language.  To my mind a more correct answer is, once again C++-style, integers-in-parameter-position and a catch-all homogenous `Tuple<T, .UInt>` (more than likely, magic type alias).
> 
> Right now, generics can’t use non-type parameters, so any “FixedArray<Int, 6>”-like syntax is out. I feel that anonymous arrays would have a usage model mostly like tuples; hence the close syntax. I prefer a built-in type instead of a library type since we need some magic to enforce:

As do I, but because we do not limit the cardinality of tuples (or "fixed arrays") it is, by definition, impossible to declare every possible instance of the type in stdlib without compiler magic.  The alternative is to declare bankruptcy at some arbitrary number where the compiler or sanity of stdlib authors breaks down (à la Haskell or Scala).

And as long as we're spitballing, no syntax is "out" just cosmetically appealing/unappealing ;)

> 
> 	strideof( ArrayType ) == Element-Count * strideof( ElementType )
> 
> down to the extent that there shouldn't be any padding between elements of the inner non-array type (in the case of nested arrays).

Smells like a static ContiguousArray.

> 
> Since we anonymous and named heterogenous product types (tuples vs. structs and classes), I want the same thing for homogenous product types (array extension to tuples vs. nominal “array" type). We broke away from C in the handling of enums, why not arrays too?

We do break with C by importing arrays as (occasionally massive) tuples.  Still, you're absolutely right, we can do so much better here.

> 
> — 
> Daryle Walker
> Mac, Internet, and Video Game Junkie
> darylew AT mac DOT com 
> 
> 
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