diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 75ef6bd..74d27e2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,18 +13,23 @@ jobs:
     name: MacOS
     runs-on: macOS-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Run tests
       run: make test-swift
     - name: Run tests
       run: make build-release-swift
 
   ubuntu:
-    name: Ubuntu
+    strategy: 
+      matrix:
+        swift:
+          - '5.9'
+    name: Ubuntu (Swift ${{ matrix.swift }})
     runs-on: ubuntu-latest
+    container: swift:${{ matrix.swift }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Run tests
-      run: make test-linux
-    - name: Build for release
-      run: make build-release-linux
+      run: swift test --parallel
+    - name: Run tests (release)
+      run: swift test -c release --parallel
diff --git a/Package.swift b/Package.swift
index 55aa546..c727a75 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:5.0
+// swift-tools-version:5.9
 import Foundation
 import PackageDescription
 
@@ -8,7 +8,11 @@ let package = Package(
     .library(name: "Gen", targets: ["Gen"])
   ],
   targets: [
-    .target(name: "Gen", dependencies: []),
+    .target(
+      name: "Gen",
+      dependencies: [],
+      swiftSettings: [.enableExperimentalFeature("StrictConcurrency")]
+    ),
     .testTarget(name: "GenTests", dependencies: ["Gen"]),
   ]
 )
diff --git a/Sources/Gen/Gen.swift b/Sources/Gen/Gen.swift
index 04c072e..57b44c8 100644
--- a/Sources/Gen/Gen.swift
+++ b/Sources/Gen/Gen.swift
@@ -1,10 +1,10 @@
 /// A composable, transformable context for generating random values.
-public struct Gen<Value> {
+public struct Gen<Value: Sendable>: Sendable {
   @usableFromInline
-  internal var _run: (inout AnyRandomNumberGenerator) -> Value
+  internal var _run: @Sendable (inout AnyRandomNumberGenerator) -> Value
 
   @inlinable
-  public init(run: @escaping (inout AnyRandomNumberGenerator) -> Value) {
+  public init(run: @escaping @Sendable (inout AnyRandomNumberGenerator) -> Value) {
     self._run = run
   }
 
@@ -48,31 +48,22 @@ extension Gen {
   /// - Parameter transform: A function that transforms `Value`s into `NewValue`s.
   /// - Returns: A generator of `NewValue`s.
   @inlinable
-  public func map<NewValue>(_ transform: @escaping (Value) -> NewValue) -> Gen<NewValue> {
+  public func map<NewValue>(
+    _ transform: @escaping @Sendable (Value) -> NewValue
+  ) -> Gen<NewValue> {
     return Gen<NewValue> { rng in transform(self._run(&rng)) }
   }
 }
 
-/// Combines two generators into a single one.
-///
-/// - Parameters:
-///   - a: A generator of `A`s.
-///   - b: A generator of `B`s.
-/// - Returns: A generator of `(A, B)` pairs.
-@inlinable
-public func zip<A, B>(_ a: Gen<A>, _ b: Gen<B>) -> Gen<(A, B)> {
-  return Gen<(A, B)> { rng in
-    (a._run(&rng), b._run(&rng))
-  }
-}
-
 extension Gen {
   /// Transforms a generator of `Value`s into a generator of `NewValue`s by transforming a value into a generator of `NewValue`s.
   ///
   /// - Parameter transform: A function that transforms `Value`s into a generator of `NewValue`s.
   /// - Returns: A generator of `NewValue`s.
   @inlinable
-  public func flatMap<NewValue>(_ transform: @escaping (Value) -> Gen<NewValue>) -> Gen<NewValue> {
+  public func flatMap<NewValue>(
+    _ transform: @escaping @Sendable (Value) -> Gen<NewValue>
+  ) -> Gen<NewValue> {
     return Gen<NewValue> { rng in
       transform(self._run(&rng))._run(&rng)
     }
@@ -83,7 +74,9 @@ extension Gen {
   /// - Parameter transform: A closure that accepts an element of this sequence as its argument and returns an optional value.
   /// - Returns: A generator of the non-nil results of calling the given transformation with a value of the generator.
   @inlinable
-  public func compactMap<NewValue>(_ transform: @escaping (Value) -> NewValue?) -> Gen<NewValue> {
+  public func compactMap<NewValue>(
+    _ transform: @escaping @Sendable (Value) -> NewValue?
+  ) -> Gen<NewValue> {
     return Gen<NewValue> { rng in
       while true {
         if let value = transform(self._run(&rng)) {
@@ -98,7 +91,7 @@ extension Gen {
   /// - Parameter predicate: A predicate.
   /// - Returns: A generator of values that match the predicate.
   @inlinable
-  public func filter(_ predicate: @escaping (Value) -> Bool) -> Gen<Value> {
+  public func filter(_ predicate: @escaping @Sendable (Value) -> Bool) -> Gen<Value> {
     return self.compactMap { predicate($0) ? $0 : nil }
   }
 
@@ -143,7 +136,7 @@ extension Gen where Value: BinaryFloatingPoint, Value.RawSignificand: FixedWidth
   }
 }
 
-extension Gen where Value == Int {
+extension Gen<Int> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -154,7 +147,7 @@ extension Gen where Value == Int {
   }
 }
 
-extension Gen where Value == Int8 {
+extension Gen<Int8> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -165,7 +158,7 @@ extension Gen where Value == Int8 {
   }
 }
 
-extension Gen where Value == Int16 {
+extension Gen<Int16> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -176,7 +169,7 @@ extension Gen where Value == Int16 {
   }
 }
 
-extension Gen where Value == Int32 {
+extension Gen<Int32> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -187,7 +180,7 @@ extension Gen where Value == Int32 {
   }
 }
 
-extension Gen where Value == Int64 {
+extension Gen<Int64> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -198,7 +191,7 @@ extension Gen where Value == Int64 {
   }
 }
 
-extension Gen where Value == UInt {
+extension Gen<UInt> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -209,7 +202,7 @@ extension Gen where Value == UInt {
   }
 }
 
-extension Gen where Value == UInt8 {
+extension Gen<UInt8> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -220,7 +213,7 @@ extension Gen where Value == UInt8 {
   }
 }
 
-extension Gen where Value == UInt16 {
+extension Gen<UInt16> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -231,7 +224,7 @@ extension Gen where Value == UInt16 {
   }
 }
 
-extension Gen where Value == UInt32 {
+extension Gen<UInt32> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -242,7 +235,7 @@ extension Gen where Value == UInt32 {
   }
 }
 
-extension Gen where Value == UInt64 {
+extension Gen<UInt64> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -253,7 +246,7 @@ extension Gen where Value == UInt64 {
   }
 }
 
-extension Gen where Value == Double {
+extension Gen<Double> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -264,7 +257,7 @@ extension Gen where Value == Double {
   }
 }
 
-extension Gen where Value == Float {
+extension Gen<Float> {
   /// Returns a generator of random values within the specified range.
   ///
   /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -276,7 +269,7 @@ extension Gen where Value == Float {
 }
 
 #if !(os(Windows) || os(Android)) && (arch(i386) || arch(x86_64))
-  extension Gen where Value == Float80 {
+  extension Gen<Float80> {
     /// Returns a generator of random values within the specified range.
     ///
     /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -291,7 +284,7 @@ extension Gen where Value == Float {
 #if canImport(CoreGraphics)
   import CoreGraphics
 
-  extension Gen where Value == CGFloat {
+  extension Gen<CGFloat> {
     /// Returns a generator of random values within the specified range.
     ///
     /// - Parameter range: The range in which to create a random value. `range` must be finite.
@@ -303,7 +296,7 @@ extension Gen where Value == Float {
   }
 #endif
 
-extension Gen where Value == Bool {
+extension Gen<Bool> {
   /// A generator of random boolean values.
   public static let bool = Gen { rng in Bool.random(using: &rng) }
 }
@@ -313,7 +306,8 @@ extension Gen {
   ///
   /// - Parameter collection: A collection.
   @inlinable
-  public static func element<C>(of collection: C) -> Gen where C: Collection, Value == C.Element? {
+  public static func element<C>(of collection: C) -> Gen
+  where C: Collection & Sendable, Value == C.Element? {
     return Gen { rng in collection.randomElement(using: &rng) }
   }
 
@@ -321,26 +315,27 @@ extension Gen {
   ///
   /// - Parameter collection: A collection.
   @inlinable
-  public static func shuffled<C>(_ collection: C) -> Gen where C: Collection, Value == [C.Element] {
+  public static func shuffled<C>(_ collection: C) -> Gen
+  where C: Collection & Sendable, Value == [C.Element] {
     return Gen { rng in collection.shuffled(using: &rng) }
   }
 }
 
-extension Gen where Value: Collection {
+extension Gen where Value: Collection, Value.Element: Sendable {
   /// Produces a generator of random elements of this generator's collection.
   @inlinable
   public var element: Gen<Value.Element?> {
-    return self.flatMap(Gen<Value.Element?>.element)
+    self.flatMap { Gen<Value.Element?>.element(of: $0) }
   }
 
   /// Produces a generator of shuffled arrays of this generator's collection.
   @inlinable
   public var shuffled: Gen<[Value.Element]> {
-    return self.flatMap(Gen<[Value.Element]>.shuffled)
+    self.flatMap { Gen<[Value.Element]>.shuffled($0) }
   }
 }
 
-extension Gen where Value: CaseIterable {
+extension Gen where Value: CaseIterable, Value.AllCases: Sendable {
   /// Produces a generator of all case-iterable cases.
   @inlinable
   public static var allCases: Gen<Value> {
@@ -369,7 +364,7 @@ extension Gen {
       }
     }
   }
-
+  
   /// Produces a new generator of arrays of this generator's values.
   ///
   /// - Parameter count: The size of the random array.
@@ -388,7 +383,7 @@ extension Gen {
       }
     }
   }
-
+  
   /// Produces a new generator of dictionaries of this generator's pairs.
   ///
   /// - Parameter count: The size of the random dictionary.
@@ -408,7 +403,7 @@ extension Gen {
       }
     }
   }
-
+  
   /// Produces a new generator of sets of this generator's values.
   ///
   /// - Parameter count: The size of the random set.
@@ -426,7 +421,9 @@ extension Gen {
       }
     }
   }
+}
 
+extension Gen {
   /// Produces a new generator of optional values.
   ///
   /// - Returns: A generator of optional values.
@@ -434,7 +431,7 @@ extension Gen {
   public var optional: Gen<Value?> {
     return Gen<Value?>.frequency(
       (1, Gen<Value?>.always(Value?.none)),
-      (3, self.map(Value?.some))  // TODO: Change to use `size` with resizable generators?
+      (3, self.map { Value?.some($0) })  // TODO: Change to use `size` with resizable generators?
     )
   }
 
@@ -444,8 +441,8 @@ extension Gen {
   @inlinable
   public func asResult<Failure>(withFailure gen: Gen<Failure>) -> Gen<Result<Value, Failure>> {
     return Gen<Result<Value, Failure>>.frequency(
-      (1, gen.map(Result.failure)),
-      (3, self.map(Result.success))  // TODO: Change to use `size` with resizable generators?
+      (1, gen.map { Result.failure($0) }),
+      (3, self.map { Result.success($0) })  // TODO: Change to use `size` with resizable generators?
     )
   }
 }
@@ -470,7 +467,7 @@ extension Gen where Value: Hashable {
   }
 }
 
-extension Gen where Value == UnicodeScalar {
+extension Gen<UnicodeScalar> {
   /// Returns a generator of random unicode scalars within the specified range.
   ///
   /// - Parameter range: The range in which to create a random unicode scalar. `range` must be finite.
@@ -483,7 +480,7 @@ extension Gen where Value == UnicodeScalar {
   }
 }
 
-extension Gen where Value == Character {
+extension Gen<Character> {
   // FIXME: Make safe for characters with multiple scalars.
   /// Returns a generator of random characters within the specified range.
   ///
@@ -495,7 +492,7 @@ extension Gen where Value == Character {
       .unicodeScalar(
         in: range.lowerBound.unicodeScalars.first!...range.upperBound.unicodeScalars.last!
       )
-      .map(Character.init)
+      .map { Character($0) }
   }
 
   /// A generator of random numeric digits.
@@ -533,17 +530,22 @@ extension Gen where Value == Character {
   /// - Returns: A generator of strings.
   @inlinable
   public func string(of count: Gen<Int>) -> Gen<String> {
-    return self.map(String.init).array(of: count).map { $0.joined() }
+    return self
+      .map { String($0) }
+      .array(of: count)
+      .map { $0.joined() }
   }
 }
 
-extension Sequence {
+extension Sequence where Self: Sendable, Element: Sendable {
   /// Transforms each value of an array of generators before rewrapping the array in an array generator.
   ///
   /// - Parameter transform: A transform function to apply to the value of each generator.
   /// - Returns: A generator of arrays.
   @inlinable
-  public func traverse<A, B>(_ transform: @escaping (A) -> B) -> Gen<[B]> where Element == Gen<A> {
+  public func traverse<A, B>(
+    _ transform: @escaping @Sendable (A) -> B
+  ) -> Gen<[B]> where Element == Gen<A> {
     return Gen<[B]> { rng in
       self.map { transform($0.run(using: &rng)) }
     }
diff --git a/Sources/Gen/Zip.swift b/Sources/Gen/Zip.swift
index 9fa3095..3c6839c 100644
--- a/Sources/Gen/Zip.swift
+++ b/Sources/Gen/Zip.swift
@@ -1,3 +1,16 @@
+/// Combines two generators into a single one.
+///
+/// - Parameters:
+///   - a: A generator of `A`s.
+///   - b: A generator of `B`s.
+/// - Returns: A generator of `(A, B)` pairs.
+@inlinable
+public func zip<A, B>(_ a: Gen<A>, _ b: Gen<B>) -> Gen<(A, B)> {
+  return Gen<(A, B)> { rng in
+    (a._run(&rng), b._run(&rng))
+  }
+}
+
 @inlinable
 public func zip<A, B, C>(
   _ a: Gen<A>,
diff --git a/Tests/GenTests/GenTests.swift b/Tests/GenTests/GenTests.swift
index 757aa38..c0fe07e 100644
--- a/Tests/GenTests/GenTests.swift
+++ b/Tests/GenTests/GenTests.swift
@@ -5,7 +5,7 @@ final class GenTests: XCTestCase {
   var xoshiro = Xoshiro(seed: 0)
 
   func testMap() {
-    let gen = Gen.bool.map(String.init)
+    let gen = Gen.bool.map { String($0) }
     XCTAssertEqual("false", gen.run(using: &xoshiro))
   }
 
@@ -101,7 +101,7 @@ final class GenTests: XCTestCase {
   }
 
   func testTraverse() {
-    let gen = [Gen.int(in: 1...100), Gen.int(in: 1_000...1_000_000)].traverse(String.init)
+    let gen = [Gen.int(in: 1...100), Gen.int(in: 1_000...1_000_000)].traverse { String($0) }
     XCTAssertEqual(["15", "18473"], gen.run(using: &xoshiro))
   }