---

yaml
---
r: 340981
b: refs/heads/rxwei-patch-1
c: 73984ad
h: refs/heads/master
i:
  340979: b881a59

Author: rxwei

[refs]

@@ -1015,7 +1015,7 @@ refs/tags/swift-DEVELOPMENT-SNAPSHOT-2018-08-18-a: b10b1fce14385faa6d44f6b933e95
 refs/heads/rdar-43033749-fix-batch-mode-no-diags-swift-5.0-branch: a14e64eaad30de89f0f5f0b2a782eed7ecdcb255
 refs/heads/revert-19006-error-bridging-integer-type: 8a9065a3696535305ea53fe9b71f91cbe6702019
 refs/heads/revert-19050-revert-19006-error-bridging-integer-type: ecf752d54b05dd0a20f510f0bfa54a3fec3bcaca
-refs/heads/rxwei-patch-1: 2ce6f9c40acc4713189fa0a4999ac04ee6f786f8
+refs/heads/rxwei-patch-1: 73984ad36b531fdecebe9d51644e6cdb34afc82c
 refs/heads/shahmishal-patch-1: e58ec0f7488258d42bef51bc3e6d7b3dc74d7b2a
 refs/heads/typelist-existential: 4046359efd541fb5c72d69a92eefc0a784df8f5e
 refs/tags/swift-4.2-DEVELOPMENT-SNAPSHOT-2018-08-20-a: 4319ba09e4fb8650ee86061075c74a016b6baab9

branches/rxwei-patch-1/stdlib/public/core/Array.swift

@@ -1994,6 +1994,12 @@ extension Array.DifferentiableView : Equatable where Element : Equatable {
   }
 }
 
+extension Array.DifferentiableView : ExpressibleByArrayLiteral {
+  public init(arrayLiteral elements: Element...) {
+    self.init(elements)
+  }
+}
+
 extension Array.DifferentiableView : CustomStringConvertible {
   public var description: String {
     return base.description

branches/rxwei-patch-1/stdlib/public/core/AutoDiff.swift

@@ -885,3 +885,76 @@ public struct AnyDerivative : Differentiable & AdditiveArithmetic {
     _box._move(along: direction._box)
   }
 }
+
+//===----------------------------------------------------------------------===//
+// Differentiable higher order functions for collections
+//===----------------------------------------------------------------------===//
+
+public extension Array where Element: Differentiable {
+  @differentiable(wrt: (self, initialResult), vjp: _vjpDifferentiableReduce)
+  func differentiableReduce<Result: Differentiable>(
+    _ initialResult: Result,
+    _ nextPartialResult: @differentiable (Result, Element) -> Result
+  ) -> Result {
+    reduce(initialResult, nextPartialResult)
+  }
+
+  @usableFromInline
+  internal func _vjpDifferentiableReduce<Result: Differentiable>(
+    _ initialResult: Result,
+    _ nextPartialResult: @differentiable (Result, Element) -> Result
+  ) -> (value: Result,
+        pullback: (Result.TangentVector)
+          -> (Array.TangentVector, Result.TangentVector)) {
+    var pullbacks:
+      [(Result.TangentVector) -> (Result.TangentVector, Element.TangentVector)]
+        = []
+    let count = self.count
+    pullbacks.reserveCapacity(count)
+    var result = initialResult
+    for element in self {
+      let (y, pb) =
+        Swift.valueWithPullback(at: result, element, in: nextPartialResult)
+      result = y
+      pullbacks.append(pb)
+    }
+    return (value: result, pullback: { tangent in
+      var resultTangent = tangent
+      var elementTangents = TangentVector([])
+      elementTangents.base.reserveCapacity(count)
+      for pullback in pullbacks.reversed() {
+        let (newResultTangent, elementTangent) = pullback(resultTangent)
+        resultTangent = newResultTangent
+        elementTangents.base.append(elementTangent)
+      }
+      return (TangentVector(elementTangents.base.reversed()), resultTangent)
+    })
+  }
+}
+
+public extension Array where Element: Differentiable {
+  @differentiable(wrt: self, vjp: _vjpDifferentiableMap)
+  func differentiableMap<Result: Differentiable>(
+    _ body: @differentiable (Element) -> Result
+  ) -> [Result] {
+    map(body)
+  }
+
+  @usableFromInline
+  internal func _vjpDifferentiableMap<Result: Differentiable>(
+    _ body: @differentiable (Element) -> Result
+  ) -> (value: [Result],
+        pullback: (Array<Result>.TangentVector) -> Array.TangentVector) {
+    var values: [Result] = []
+    var pullbacks: [(Result.TangentVector) -> Element.TangentVector] = []
+    for x in self {
+      let (y, pb) = Swift.valueWithPullback(at: x, in: body)
+      values.append(y)
+      pullbacks.append(pb)
+    }
+    func pullback(_ tans: Array<Result>.TangentVector) -> Array.TangentVector {
+      .init(zip(tans.base, pullbacks).map { tan, pb in pb(tan) })
+    }
+    return (value: values, pullback: pullback)
+  }
+}

branches/rxwei-patch-1/test/AutoDiff/collection_hofs.swift

@@ -0,0 +1,36 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+import StdlibUnittest
+#if os(macOS)
+import Darwin.C
+#else
+import Glibc
+#endif
+
+// Test suite for differentiable higher order functions for collections
+// such as `differentiableMap(_:)` and `differentiableReduce(_:)`.
+var CollectionHOFTests = TestSuite("CollectionHOF")
+
+let xx: [Float] = [1, 2, 3, 4, 5]
+
+CollectionHOFTests.test("differentiableMap(_:)") {
+  func double(_ xx: [Float]) -> [Float] {
+    xx.differentiableMap { $0 * $0 }
+  }
+  expectEqual([], pullback(at: xx, in: double)([]))
+  expectEqual([0], pullback(at: xx, in: double)([0]))
+  expectEqual([2], pullback(at: xx, in: double)([1]))
+  expectEqual([2, 4, 6, 8, 10], pullback(at: xx, in: double)([1, 1, 1, 1, 1]))
+}
+
+CollectionHOFTests.test("differentiableReduce(_:)") {
+  func product(_ xx: [Float]) -> Float {
+    xx.differentiableReduce(1) { $0 * $1 }
+  }
+  expectEqual([1], gradient(at: [0], in: product))
+  expectEqual([1], gradient(at: [1], in: product))
+  expectEqual([120, 60, 40, 30, 24], gradient(at: xx, in: product))
+}
+
+runAllTests()