Added toOptional function.

source/handy_http_primitives/optional.d

@@ -100,14 +100,21 @@ struct Optional(T) {
             * Returns: An exception if one is thrown.
             */
             SerdeException deserializeFromAsdf(Asdf data) {
-                if (data == null) {
+                static if (!__traits(compiles, {T t; t = T.init;})) {
-                    this.isNull = true;
+                    return new SerdeException(
-                    this.value = T.init;
+                        "Cannot deserialize Optional that contains immutable or otherwise unassignable value. " ~
+                        "Ensure that the following code compiles: T t; t = T.init; for the given type T."
+                    );
                 } else {
-                    this.isNull = false;
+                    if (data == null) {
-                    this.value = deserialize!T(data);
+                        this.isNull = true;
+                        this.value = T.init;
+                    } else {
+                        this.isNull = false;
+                        this.value = deserialize!T(data);
+                    }
+                    return null;
                 }
-                return null;
             }
 
             /**
@@ -116,11 +123,11 @@ struct Optional(T) {
             * Params:
             *   serializer = The serializer to use (provided by ASDF).
             */
-            void serialize(S)(ref S serializer) {
+            void serialize(S)(ref S serializer) const {
                 if (this.isNull) {
                     serializer.putValue(null);
                 } else {
-                    serializer.putValue(this.value);
+                    serializeValue(serializer, this.value);
                 }
             }
         }
@@ -140,6 +147,18 @@ auto mapIfPresent(alias fn, T)(Optional!T opt) {
     return Optional!U.of(fn(opt.value));
 }
 
+/**
+ * Helper function to get an Optional value for an existing value. Due to D's
+ * type inference, you can simply write `auto opt = toOptional(x);` to avoid
+ * having to write out types when constructing optionals.
+ * Params:
+ *   t = The value to construct an optional from.
+ * Returns: The optional with the given value.
+ */
+Optional!T toOptional(T)(T t) {
+    return Optional!T.of(t);
+}
+
 unittest {
     Optional!string s = Optional!string.of("hello");
     assert(!s.isNull);
@@ -183,5 +202,29 @@ unittest {
         assert(sa.b && sa.b.value == "hello world!");
         assert(sa.c && sa.c.value == 3.14f);
         assert(sa.d && sa.d.value == Sd(42, 67));
+
+        // Check that optionals containing immutable data aren't supported for serialization & deserialization:
+        struct Invalid {
+            immutable int x;
+        }
+        Optional!Invalid opt = Optional!(Invalid).of(Invalid(42));
+        static assert(__traits(compiles, serializeToJson(opt)));
+        try {
+            deserialize!(Optional!(Invalid))(`{"x": 123}`);
+            assert(
+                false,
+                "Failed to ensure that an exception is thrown when attempting to deserialize an incompatible Optional."
+            );
+        } catch (SerdeException exc) {
+            // pass.
+        }
     }
 }
+
+// Tests for toOptional.
+unittest {
+    int x = 5;
+    auto optX = x.toOptional;
+    assert(!optX.isNull);
+    assert(optX.value == 5);
+}