Add membership function and continuous set basic defs

Sources/FuzzyKit/ContinuousFuzzySet.swift

@@ -0,0 +1,27 @@
+public struct ContinuousFuzzySet: FuzzySet {
+    public typealias Element = Double
+    
+    private let membershipFunction: MembershipFunction
+    
+    public init(membershipFunction: MembershipFunction) {
+        self.membershipFunction = membershipFunction
+    }
+    
+    public init(membershipFunction: @escaping (Element) -> Grade) {
+        self.membershipFunction = MembershipFunction(membershipFunction)
+    }
+    
+    public func grade(forElement element: Element) -> Grade {
+        membershipFunction(element)
+    }
+}
+
+public extension ContinuousFuzzySet {
+    static var empty: Self {
+        .init(membershipFunction: .zero)
+    }
+    
+    static var universe: Self {
+        .init(membershipFunction: .one)
+    }
+}

Sources/FuzzyKit/MembershipFunction.swift

@@ -0,0 +1,29 @@
+public struct MembershipFunction {
+    
+    private let function: (Double) -> Double
+    
+    public init(_ function: @escaping (Double) -> Grade) {
+        self.function = function
+    }
+    
+    public func callAsFunction(_ x: Double) -> Double {
+        function(x)
+    }
+}
+
+/// Common membership functions
+public extension MembershipFunction {
+    static var zero: Self {
+        .init { _ in 0.0 }
+    }
+    
+    static var one: Self {
+        .init { _ in 1.0 }
+    }
+    
+    static func fuzzySingleton(_ a: Double) -> Self {
+        .init { $0 == a ? 1.0 : 0.0 }
+    }
+    
+    // TODO: Add more
+}

Tests/FuzzyKitTests/ContinuousFuzzySetTests.swift

@@ -0,0 +1,6 @@
+import XCTest
+import FuzzyKit
+
+final class ContinuousFuzzySetTests: XCTestCase {
+    // TODO
+}

Tests/FuzzyKitTests/MembershipFunctionTests.swift

@@ -0,0 +1,13 @@
+import XCTest
+import FuzzyKit
+
+final class MembershipFunctionTests: XCTestCase {
+    func test_isCallable() throws {
+        let sut = MembershipFunction.one
+        let x = 1.0
+        
+        let result = sut(x)
+        
+        XCTAssertEqual(result, x)
+    }
+}