diff --git a/prototype/Doubt.xcodeproj/project.pbxproj b/prototype/Doubt.xcodeproj/project.pbxproj
index 35c301881..f713e1ec6 100644
--- a/prototype/Doubt.xcodeproj/project.pbxproj
+++ b/prototype/Doubt.xcodeproj/project.pbxproj
@@ -9,6 +9,7 @@
 /* Begin PBXBuildFile section */
 		D432D4711BA9AC0B00F3FABC /* DiffTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = D432D4701BA9AC0B00F3FABC /* DiffTests.swift */; };
 		D432D4731BA9C55300F3FABC /* Stream.swift in Sources */ = {isa = PBXBuildFile; fileRef = D432D4721BA9C55300F3FABC /* Stream.swift */; };
+		D432D4751BA9D6A400F3FABC /* Memo.swift in Sources */ = {isa = PBXBuildFile; fileRef = D432D4741BA9D6A400F3FABC /* Memo.swift */; };
 		D4AAE50E1B5AE22E004E581F /* Doubt.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = D4AAE4FD1B5AE22E004E581F /* Doubt.framework */; };
 		D4AAE5131B5AE22E004E581F /* DoubtTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = D4AAE5121B5AE22E004E581F /* DoubtTests.swift */; };
 		D4AAE5401B5AE2D0004E581F /* Array.swift in Sources */ = {isa = PBXBuildFile; fileRef = D4AAE5351B5AE2D0004E581F /* Array.swift */; };
@@ -37,6 +38,7 @@
 /* Begin PBXFileReference section */
 		D432D4701BA9AC0B00F3FABC /* DiffTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DiffTests.swift; sourceTree = "<group>"; };
 		D432D4721BA9C55300F3FABC /* Stream.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Stream.swift; sourceTree = "<group>"; };
+		D432D4741BA9D6A400F3FABC /* Memo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Memo.swift; sourceTree = "<group>"; };
 		D4AAE4FD1B5AE22E004E581F /* Doubt.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = Doubt.framework; sourceTree = BUILT_PRODUCTS_DIR; };
 		D4AAE5011B5AE22E004E581F /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		D4AAE50D1B5AE22E004E581F /* DoubtTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = DoubtTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
@@ -107,6 +109,7 @@
 				D4AAE53F1B5AE2D0004E581F /* Syntax.swift */,
 				D4AAE54B1B5AE42D004E581F /* Equatable.swift */,
 				D432D4721BA9C55300F3FABC /* Stream.swift */,
+				D432D4741BA9D6A400F3FABC /* Memo.swift */,
 				D4AAE5001B5AE22E004E581F /* Supporting Files */,
 			);
 			path = Doubt;
@@ -247,6 +250,7 @@
 				D432D4731BA9C55300F3FABC /* Stream.swift in Sources */,
 				D4AAE5421B5AE2D0004E581F /* Doc.swift in Sources */,
 				D4AAE5461B5AE2D0004E581F /* Parse.swift in Sources */,
+				D432D4751BA9D6A400F3FABC /* Memo.swift in Sources */,
 				D4AAE5491B5AE2D0004E581F /* StringLiteralConvertible.swift in Sources */,
 				D4AAE5451B5AE2D0004E581F /* Operators.swift in Sources */,
 			);
diff --git a/prototype/Doubt/Memo.swift b/prototype/Doubt/Memo.swift
new file mode 100644
index 000000000..c3df13e0e
--- /dev/null
+++ b/prototype/Doubt/Memo.swift
@@ -0,0 +1,31 @@
+public struct Memo<A> {
+	public var value: A {
+		return _value.value.value()
+	}
+
+	private var _value: MutableBox<Thunk<A>>
+}
+
+private final class MutableBox<A> {
+	init(_ value: A) {
+		self.value = value
+	}
+
+	var value: A
+}
+
+private enum Thunk<A> {
+	case Evaluated(A)
+	case Unevaluated(() -> A)
+
+	mutating func value() -> A {
+		switch self {
+		case let .Evaluated(a):
+			return a
+		case let .Unevaluated(f):
+			let a = f()
+			self = .Evaluated(a)
+			return a
+		}
+	}
+}