Trouble constraining a generic type in an extension


(Jens Alfke) #1

I’ve got a simple generic struct that wraps an instance of its parameter type.

public struct check<T> {
    let actual: T
    public init(_ a: T) {actual = a}
    ...

Now I want to add a method that only works with a specific type, Bool:
    public func isTrue() {XCTAssertTrue(actual)}

As I’d expect, the compiler doesn’t allow this: “Cannot convert value of type ’T’ to expected argument type ‘Bool’”.
So I’m trying to put this method in an extension that constrains T:

public extension check<Bool> {
    public func isTrue() {XCTAssertTrue(actual)}
}

This fails with "Constrained extension must be declared on the unspecialized generic type 'check' with constraints specified by a 'where’ clause”. OK, so I add a ‘where’ clause:

public extension check where T: Bool {
    public func isTrue() {XCTAssertTrue(actual)}
}

This produces the error "type 'T' constrained to non-protocol type ‘Bool’”. This confuses me — why is constraining to a non-protocol type an error? The Swift Programming Language says “the ‘where’ clause … can express the constraints that a generic type T inherits from a class C”.

—Jens


(David Sweeris) #2

Bool isn’t a class (or protocol), so nothing can inherit from it.

Until Swift gains the ability to extend generic types where their parameters are *equal* to concrete types, as opposed to where they *inherit from or conform to* something, I think the best you can do is this:
public extension check where T: BooleanType {
    public func isTrue() { XCTAssertTrue(actual.boolValue) }
}

I *think* the feature you’re trying to use is on the todo list for Swift 3, but the last time I said that, I was completely wrong. The swift-evolution mailing list probably has more information.

Anyway, I hope that helps.

- Dave Sweeris

···

On Apr 12, 2016, at 11:11 AM, Jens Alfke via swift-users <swift-users@swift.org> wrote:

I’ve got a simple generic struct that wraps an instance of its parameter type.

public struct check<T> {
    let actual: T
    public init(_ a: T) {actual = a}
    ...

Now I want to add a method that only works with a specific type, Bool:
    public func isTrue() {XCTAssertTrue(actual)}

As I’d expect, the compiler doesn’t allow this: “Cannot convert value of type ’T’ to expected argument type ‘Bool’”.
So I’m trying to put this method in an extension that constrains T:

public extension check<Bool> {
    public func isTrue() {XCTAssertTrue(actual)}
}

This fails with "Constrained extension must be declared on the unspecialized generic type 'check' with constraints specified by a 'where’ clause”. OK, so I add a ‘where’ clause:

public extension check where T: Bool {
    public func isTrue() {XCTAssertTrue(actual)}
}

This produces the error "type 'T' constrained to non-protocol type ‘Bool’”. This confuses me — why is constraining to a non-protocol type an error? The Swift Programming Language says “the ‘where’ clause … can express the constraints that a generic type T inherits from a class C”.

—Jens
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(Brent Royal-Gordon) #3

I *think* the feature you’re trying to use is on the todo list for Swift 3, but the last time I said that, I was completely wrong. The swift-evolution mailing list probably has more information.

I think it's on the list of things they'd like to do to generics in the future, but it's a very long list and they might not get to that particular one.

In short, though, my understanding is that there is no principled reason for this limitation; it simply isn't supported yet.

···

--
Brent Royal-Gordon
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