[swift-evolution] struct subtyping

[Colin Barrett]

> On Mar 21, 2016, at 7:58 AM, Tino Heth via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Many languages which adopt the concept of value types don't allow subclassing for those, and so does Swift.
> Inheritance for structs is more complex than inheritance for classes, but the "final" limitation isn't the only possible solution, and Dave Abrahams told me in another thread that changing this rule might be considered in the future — so I'll risk getting taunted by the cool kids who are in favor of eliminating all ancient OOP-ideas ;-) and start a discussion.
> 
> I guess most readers know about the low-level problems that arise when we switch from pointers (always the same size) to value types (size may vary), so I'll start with two possibilities for struct subtyping:
> 
> newtype (see https://www.haskell.org/tutorial/moretypes.html <https://www.haskell.org/tutorial/moretypes.html> — or just read on if you are scared by Haskell ;-)
> 
> When a subtype does not add any stored properties to its superclass (memory layout doesn't change), there is no difference at the level of object code — only the type checker may stop you from using those two types interchangeably.

As I understand it, a single argument struct ends up being as “free” as a newtype is in Haskell (modulo resiliency concerns), or close to it. For instance, my understanding is that the Int type is a single argument struct wrapper around a lower-level numeric type.

> Some use cases:
> - In Cocoa, there is no separate class for (file system) paths; instead, there are some additions to NSString. String doesn't have those abilities, and imho methods like "stringByAppendingPathExtension" deserve a separate Path-struct, so that those special methods don't pollute the method list of String (URL is the future, so that example is somewhat out-of date).
> - You could impose incompatibility on numeric types to ensure that your calculations use correct quantities. Although this can be annoying (Float vs. CGFloat), decorating numbers with quantity/unit could eliminate bugs that had really disastrous consequences in the past.
> - Increased comfort for floating-point math:
> struct CustomDouble: Double
> 
> func == (a: CustomDouble, b: CustomDouble) -> Bool {
> 	return abs(a.value - b.value) < 0.01
> }
> (no need to specify tolerance for each comparison)
> 
> Full subtyping
> 
> As long as you don't cross module borders, it wouldn't be that complicated to add inheritance without restrictions.
> imagine you have a "Customer"-type and a "Employee"-type to store personal data (name, address…).
> Those data objects are perfect candidates to be implemented as structs, but they also cry for a "Person"-superclass, so you are forced to either duplicate code, or to implement your objects as reference types.

There’s a number of wrinkles that are worth considering—for instance, are you doing nominal or structural subtyping? The value-nature of structs suggests that structural subtyping would be useul. However, structs are already nominal types (unlike tuples).

Some compelling use cases for why a class doesn’t suffice and you really need a struct would enhance a full proposal.

Cheers,
-Colin


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160324/501c4219/attachment.html>