how to use `if case` statement as boolean in swift - ios

Question

I have this enum (with associated type)
enum CookieType {
case regular(type: Int)
case gem(type: GemType)
}
struct Cookie {
let type: CookieType
let otherStuff...
}
Now if i want to do pattern matching, i can do this with no problem:
if case .gem == cookie.type {
}
However, I want to use case .gem == cookie.type as a boolean. The following gives error
var cookies: [Cookie] {
return [cookieA, cookieB]
}
var gems: [Cookie] {
return cookies.filter { case $0.type == .gem } // this has error
}
which means case $0.type == .gem is not a boolean. How can I deal with this?

Answer 1

case .gem = cookie.type is not a boolean expression, and if statements not only accept boolean expressions. However, in the closure argument for filter, you must write an Bool expression or a block that returns Bool.
One way to do that is:
cookies.filter {
if case $0.type = .gem { return true }
else { return false }
}
Or, you can add convenient properties to CookieType that gives you Bool values, if you tend to do this a lot:
enum CookieType {
case regular(type: Int)
case gem(type: GemType)
var isRegular: Bool {
if case .regular = self { return true }
else { return false }
}
var isGem: Bool {
if case .gem = self { return true }
else { return false }
}
}
Then you can do:
cookies.filter(\.isGem)

Answer 2

Normally, if case and switch statements are abstractions based on the pattern matching operator ~=, but that's not how enums with associated types are implemented. As such, for the time being, you need to reverse-engineer how it might have been done, to allow this:
cookies.filter { CookieType.regular ~= $0.type }
I do not believe it is possible to avoid the explicit CookieType there, but it still reads better than all alternatives.
/// Match `enum` cases with associated values, while disregarding the values themselves.
/// - Parameter case: Looks like `Enum.case`.
public func ~= <Enum, AssociatedValue>(
case: (AssociatedValue) -> Enum,
instance: Enum
) -> Bool {
Mirror.associatedValue(of: instance, ifCase: `case`) != nil
}
public extension Mirror {
/// Get an `enum` case's `associatedValue`.
static func associatedValue<AssociatedValue>(
of subject: Any,
_: AssociatedValue.Type = AssociatedValue.self
) -> AssociatedValue? {
guard let childValue = Self(reflecting: subject).children.first?.value
else { return nil }
if let associatedValue = childValue as? AssociatedValue {
return associatedValue
}
let labeledAssociatedValue = Self(reflecting: childValue).children.first
return labeledAssociatedValue?.value as? AssociatedValue
}
/// Get an `enum` case's `associatedValue`.
/// - Parameter case: Looks like `Enum.case`.
static func associatedValue<Enum, AssociatedValue>(
of instance: Enum,
ifCase case: (AssociatedValue) throws -> Enum
) rethrows -> AssociatedValue? {
try associatedValue(of: instance).filter {
.equate(try `case`($0), to: instance) {
Self(reflecting: $0).children.first?.label
}
}
}
}
public extension Optional {
/// Transform `.some` into `.none`, if a condition fails.
/// - Parameters:
/// - isSome: The condition that will result in `nil`, when evaluated to `false`.
func filter(_ isSome: (Wrapped) throws -> Bool) rethrows -> Self {
try flatMap { try isSome($0) ? $0 : nil }
}
}
public extension Equatable {
/// Equate two values using a closure.
static func equate<Wrapped, Equatable: Swift.Equatable>(
_ optional0: Wrapped?, to optional1: Wrapped?,
using transform: (Wrapped) throws -> Equatable
) rethrows -> Bool {
try optional0.map(transform) == optional1.map(transform)
}
}

Answer 3

Inspired by #Jessy and SwiftLee, here is my solution:
// -----------------------
// CaseReflectable
// -----------------------
// designed for enums only
// (use it on other types not recommended)
protocol CaseReflectable {}
// default behaviors.
extension CaseReflectable {
/// case name
var caseName: String {
let mirror = Mirror(reflecting: self)
// enum cases:
// - normal case: no children
// - case with associated values: one child (with label)
guard let label = mirror.children.first?.label else {
return "\(self)" // normal case
}
// case with associated values
return label
}
/// associated values
var associatedValues: Any? {
// if no children, a normal case, no associated values.
guard let firstChild = Mirror(reflecting: self).children.first else {
return nil
}
return firstChild.value
}
}
// --------------------------
// custom operator ~=
// --------------------------
/// match enum cases with associated values, while disregarding the values themselves.
/// usage: `Enum.enumCase ~= instance`
func ~= <Enum: CaseReflectable, AssociatedValue>(
// an enum case (with associated values)
enumCase: (AssociatedValue) -> Enum, // enum case as function
// an instance of Enum
instance: Enum
) -> Bool
{
// if no associated values, `instance` can't be of `enumCase`
guard let values = instance.associatedValues else { return false }
// if associated values not of the same type, return false
guard values is AssociatedValue else { return false }
// create an instance from `enumCase` (as function)
let case2 = enumCase(values as! AssociatedValue)
// if same case name, return true
return case2.caseName == instance.caseName
}
// ------------
// Enum
// ------------
// enum with associated values
// (conforms to `CaseReflectable`)
enum Enum: CaseReflectable {
case int(Int)
case int2(Int)
case person(name: String, age: Int)
case str(String)
}
// ------------
// main
// ------------
let a: Enum = .int(3)
Enum.int ~= a // true
Enum.int2 ~= a // false
let joe = Enum.person(name: "joe", age: 8)
Enum.person ~= joe // true
Enum.int ~= joe // false
// array of enum cases
let items: [Enum] = [
.int(1), .str("hi"), .int(2)
]
// filter enum cases
let filtered = items.filter { Enum.int ~= $0 }
print(filtered) // [Enum.int(1), Enum.int(2)]