add expected helper class (#1321)

Co-authored-by: Matthew Kennedy <makenne@microsoft.com>

firmware/controllers/sensors/converters/func_chain.h

@@ -22,7 +22,7 @@ class FuncChain<> {
 protected:
 	SensorResult convert(float input) const {
 		// Base case is the identity function
-		return {true, input};
+		return input;
 	}
 
 	void showInfo(Logging* logger, float testInputValue) const {
@@ -48,7 +48,7 @@ public:
 		if (currentStep.Valid) {
 			return TBase::convert(currentStep.Value);
 		} else {
-			return {false, 0};
+			return unexpected;
 		}
 	}
 

firmware/controllers/sensors/converters/resistance_func.cpp

@@ -13,18 +13,18 @@ void ResistanceFunc::configure(float supplyVoltage, float pullupResistor) {
 SensorResult ResistanceFunc::convert(float raw) const {
 	// If the voltage is very low, the sensor is a dead short.
 	if (raw < 0.05f) {
-		return {false, 0.0f};
+		return unexpected;
 	}
 
 	// If the voltage is very high (98% VCC), the sensor is open circuit.
 	if (raw > (m_supplyVoltage * 0.98f)) {
-		return {false, 1e6};
+		return unexpected;
 	}
 
 	// Voltage is in a sensible range - convert
 	float resistance = m_pullupResistor / (m_supplyVoltage / raw - 1);
 
-	return {true, resistance};
+	return resistance;
 }
 
 void ResistanceFunc::showInfo(Logging* logger, float testInputValue) const {

firmware/controllers/sensors/converters/thermistor_func.cpp

@@ -13,7 +13,7 @@ SensorResult ThermistorFunc::convert(float ohms) const {
 	// This resistance should have already been validated - only
 	// thing we can check is that it's non-negative
 	if (ohms <= 0) {
-		return {false, 0};
+		return unexpected;
 	}
 
 	float lnR = logf(ohms);
@@ -26,7 +26,7 @@ SensorResult ThermistorFunc::convert(float ohms) const {
 
 	float celsius = convertKelvinToCelcius(kelvin);
 
-	return {true, celsius};
+	return celsius;
 }
 
 void ThermistorFunc::configure(thermistor_conf_s &cfg) {

firmware/controllers/sensors/sensor.cpp

@@ -92,7 +92,7 @@ bool Sensor::Register() {
 
 	// Next check for mock
 	if (entry->useMock) {
-		return {true, entry->mockValue};
+		return entry->mockValue;
 	}
 
 	// Get the sensor out of the entry
@@ -103,7 +103,7 @@ bool Sensor::Register() {
 	}
 
 	// We've exhausted all valid ways to return something - sensor not found.
-	return {false, 0};
+	return unexpected;
 }
 
 /*static*/ float Sensor::getRaw(SensorType type) {

firmware/controllers/sensors/sensor.h

@@ -48,23 +48,11 @@
 #pragma once
 
 #include "sensor_type.h"
+#include "expected.h"
 
 #include <cstddef>
 
-struct SensorResult {
+using SensorResult = expected<float>;
-	const bool Valid;
-	const float Value;
-
-	// Implicit conversion operator to bool, so you can do things like if (myResult) { ... }
-	constexpr explicit operator bool() const {
-		return Valid;
-	}
-
-	// Easy default value handling
-	constexpr float value_or(float valueIfInvalid) const {
-		return Valid ? Value : valueIfInvalid;
-	}
-};
 
 // Fwd declare - nobody outside of Sensor.cpp needs to see inside this type
 struct SensorRegistryEntry;
@@ -156,7 +144,7 @@ private:
 		return 0;
 	}
 
-	SensorType m_type;
+	const SensorType m_type;
 
 	// Get this sensor's index in the list
 	constexpr size_t getIndex() {

firmware/controllers/sensors/stored_value_sensor.h

@@ -41,7 +41,7 @@ public:
 			return {false, value};
 		}
 
-		return {true, value};
+		return value;
 	}
 
 protected:

firmware/util/expected.h

@@ -0,0 +1,43 @@
+/**
+ * @file    expected.h
+ * @brief This utility class provides a way for a function to accept or return a value that may be invalid.
+ * 
+ * For example, suppose there needs to be a solution for prevention of divide by zero.  One could write this function:
+ * 
+ * expected<int> my_divide(int num, int denom) {
+ *     if (denom == 0) return unexpected;
+ *     return num / denom;
+ * }
+ *
+ * @date April 18, 2020
+ * @author Matthew Kennedy, (c) 2020
+ */
+
+#pragma once
+
+template <class TValue>
+struct expected {
+	const bool Valid;
+	const TValue Value;
+
+	constexpr expected(bool valid, TValue value) : Valid(valid), Value(value) {}
+
+	// Implicit constructor to convert from TValue (for valid values, so an expected<T> behaves like a T)
+	constexpr expected(TValue validValue)
+		: Valid(true)
+		, Value(validValue)
+	{
+	}
+
+	// Implicit conversion operator to bool, so you can do things like if (myResult) { ... }
+	constexpr explicit operator bool() const {
+		return Valid;
+	}
+
+	// Easy default value handling
+	constexpr float value_or(TValue valueIfInvalid) const {
+		return Valid ? Value : valueIfInvalid;
+	}
+};
+
+constexpr expected<float> unexpected = {false, 0.0f};

unit_tests/tests/sensor/func_chain.cpp

@@ -4,19 +4,19 @@
 
 struct AddOne final : public SensorConverter {
     SensorResult convert(float input) const {
-        return {true, input + 1};
+        return input + 1;
     }
 };
 
 struct SubOne final : public SensorConverter {
     SensorResult convert(float input) const {
-        return {true, input - 1};
+        return input - 1;
     }
 };
 
 struct Doubler final : public SensorConverter {
     SensorResult convert(float input) const {
-        return {true, input * 2};
+        return input * 2;
     }
 };