Fixed and properly implemented variant objects

doc/serialization.txt

@@ -37,8 +37,8 @@ NimYAML supports a growing number of types of Nim's ``system`` module and
 standard library, and it also supports user-defined object, tuple and enum types
 out of the box.
 
-**Important**: NimYAML currently does not support polymorphism or variant
-object types. This may be added in the future.
+**Important**: NimYAML currently does not support polymorphism. This may be
+added in the future.
 
 This also means that NimYAML is generally able to work with object, tuple and
 enum types defined in the standard library or a third-party library without
@@ -125,18 +125,43 @@ Variant Object Types
 ....................
 
 A *variant object type* is an object type that contains one or more ``case``
-clauses. NimYAML currently supports variant object types. However, this feature
-is **highly experimental**. Only the currently accessible fields of a variant
-object type are dumped, and only those may be present when loading. The
-discriminator field(s) are treated like all other fields. The value of a
-discriminator field must occur before any value of a field that depends on it.
-This violates the YAML specification and therefore will be changed in the
-future.
+clauses. NimYAML supports variant object types. Only the currently accessible
+fields of a variant object type are dumped, and only those may be present when
+loading.
 
-While dumping variant object types directly is currently not production ready,
-you can use them for processing heterogeneous data sets. For example, if you
-have a YAML document which contains differently typed values in the same list
-like this:
+The value of a discriminator field must be loaded before any value of a field
+that depends on it. Therefore, a YAML mapping cannot be used to serialize
+variant object types - the YAML specification explicitly states that the order
+of key-value pairs in a mapping must not be used to convey content information.
+So, any variant object type is serialized as a list of key-value pairs.
+
+For example, this type:
+
+.. code-block:: nim
+  type
+    AnimalKind = enum
+      akCat, akDog
+  
+    Animal = object
+      name: string
+      case kind: AnimalKind
+      of akCat:
+        purringIntensity: int
+      of akDog:
+        barkometer: int
+
+will be serialized as:
+
+.. code-block:: yaml
+  %YAML 1.2
+  --- !nim:custom:Animal
+  - name: Bastet
+  - kind: akCat
+  - purringIntensity: 7
+
+You can also use variant object types for processing heterogeneous data sets.
+For example, if you have a YAML document which contains differently typed values
+in the same list like this:
 
 .. code-block:: yaml
   %YAML 1.2

private/serialization.nim

@@ -422,7 +422,6 @@ macro constructFieldValue(t: typedesc, stream: expr, context: expr,
               newNimNode(nnkIdentDefs).add(
                   newIdentNode("value"), discType, newEmptyNode())),
           newCall("constructChild", stream, context, newIdentNode("value")),
-          newCall("reset", o),
           newAssignment(discriminant, newIdentNode("value"))))
       result.add(disOb)
       for bIndex in 1 .. len(child) - 1:
@@ -433,8 +432,10 @@ macro constructFieldValue(t: typedesc, stream: expr, context: expr,
           let field = newDotExpr(o, newIdentNode($item))
           var ifStmt = newIfStmt((cond: discTest, body: newStmtList(
               newCall("constructChild", stream, context, field))))
-          ifStmt.add(newNimNode(nnkElse).add(newNimNode(nnkDiscardStmt).add(
-              newEmptyNode()))) # todo: raise exception here
+          ifStmt.add(newNimNode(nnkElse).add(newNimNode(nnkRaiseStmt).add(
+              newCall("newException", newIdentNode("YamlConstructionError"),
+              infix(newStrLitNode("Field " & $item & " not allowed for " &
+              $child[0] & " == "), "&", prefix(discriminant, "$"))))))
           ob.add(newStmtList(ifStmt))
           result.add(ob)
     else:
@@ -444,18 +445,34 @@ macro constructFieldValue(t: typedesc, stream: expr, context: expr,
       ob.add(newStmtList(newCall("constructChild", stream, context, field)))
       result.add(ob)
 
+proc isVariantObject(t: typedesc): bool {.compileTime.} =
+  let tDesc = getType(t)
+  if tDesc.kind != nnkObjectTy: return false
+  for child in tDesc[2].children:
+    if child.kind == nnkRecCase: return true
+  return false
+
 proc constructObject*[O: object|tuple](
     s: var YamlStream, c: ConstructionContext, result: var O)
     {.raises: [YamlConstructionError, YamlStreamError].} =
   ## constructs a Nim object or tuple from a YAML mapping
   let e = s.next()
-  if e.kind != yamlStartMap:
+  const
+    startKind = when isVariantObject(O): yamlStartSeq else: yamlStartMap
+    endKind = when isVariantObject(O): yamlEndSeq else: yamlEndMap
+  if e.kind != startKind:
     raise newException(YamlConstructionError, "While constructing " & 
         typetraits.name(O) & ": Expected map start, got " & $e.kind)
-  while s.peek.kind != yamlEndMap:
+  when isVariantObject(O): reset(result) # make discriminants writeable
+  while s.peek.kind != endKind:
     # todo: check for duplicates in input and raise appropriate exception
     # also todo: check for missing items and raise appropriate exception
-    let e = s.next()
+    var e = s.next()
+    when isVariantObject(O):
+      if e.kind != yamlStartMap:
+        raise newException(YamlConstructionError,
+            "Expected single-pair map, got " & $e.kind)
+      e = s.next()
     if e.kind != yamlScalar:
       raise newException(YamlConstructionError,
           "Expected field name, got " & $e.kind)
@@ -465,7 +482,13 @@ proc constructObject*[O: object|tuple](
         if fname == name:
           constructChild(s, c, value)
           break
-    else: constructFieldValue(O, s, c, name, result)
+    else:
+      constructFieldValue(O, s, c, name, result)
+    when isVariantObject(O):
+      e = s.next()
+      if e.kind != yamlEndMap:
+        raise newException(YamlConstructionError,
+            "Expected end of single-pair map, got " & $e.kind)
   discard s.next()
 
 proc representObject*[O: object|tuple](value: O, ts: TagStyle,
@@ -473,8 +496,13 @@ proc representObject*[O: object|tuple](value: O, ts: TagStyle,
   ## represents a Nim object or tuple as YAML mapping
   result = iterator(): YamlStreamEvent =
     let childTagStyle = if ts == tsRootOnly: tsNone else: ts
-    yield startMapEvent(tag, yAnchorNone)
+    when isVariantObject(O):
+      yield startSeqEvent(tag, yAnchorNone)
+    else:
+      yield startMapEvent(tag, yAnchorNone)
     for name, value in fieldPairs(value):
+      when isVariantObject(O):
+        yield startMapEvent(yTagQuestionMark, yAnchorNone)
       yield scalarEvent(name,
           if childTagStyle == tsNone: yTagQuestionMark else:
           yTagNimField, yAnchorNone)
@@ -483,7 +511,12 @@ proc representObject*[O: object|tuple](value: O, ts: TagStyle,
         let event = events()
         if finished(events): break
         yield event
-    yield endMapEvent()
+      when isVariantObject(O):
+        yield endMapEvent()
+    when isVariantObject(O):
+      yield endSeqEvent()
+    else:
+      yield endMapEvent()
 
 proc constructObject*[O: enum](s: var YamlStream, c: ConstructionContext,
                                result: var O)

test/serializing.nim

@@ -27,6 +27,17 @@ type
     
   BetterInt = distinct int
   
+  AnimalKind = enum
+    akCat, akDog
+  
+  Animal = object
+    name: string
+    case kind: AnimalKind
+    of akCat:
+      purringIntensity: int
+    of akDog:
+      barkometer: int
+
 proc `$`(v: BetterInt): string {.borrow.}
 proc `==`(l, r: BetterInt): bool {.borrow.}
 
@@ -304,6 +315,42 @@ suite "Serialization":
         "--- !nim:custom:Person \nfirstnamechar: P\nsurname: Pan\nage: 12",
         output.data)
   
+  test "Serialization: Load custom variant object":
+    let input = newStringStream(
+      "---\n- - name: Bastet\n  - kind: akCat\n  - purringIntensity: 7\n" &
+      "- - name: Anubis\n  - kind: akDog\n  - barkometer: 13")
+    var result: seq[Animal]
+    load(input, result)
+    assert result.len == 2
+    assert result[0].name == "Bastet"
+    assert result[0].kind == akCat
+    assert result[0].purringIntensity == 7
+    assert result[1].name == "Anubis"
+    assert result[1].kind == akDog
+    assert result[1].barkometer == 13
+  
+  test "Serialization: Dump custom variant object":
+    let input = @[Animal(name: "Bastet", kind: akCat, purringIntensity: 7),
+                  Animal(name: "Anubis", kind: akDog, barkometer: 13)]
+    var output = newStringStream()
+    dump(input, output, tsNone, asTidy, blockOnly)
+    assertStringEqual """%YAML 1.2
+--- 
+- 
+  - 
+    name: Bastet
+  - 
+    kind: akCat
+  - 
+    purringIntensity: 7
+- 
+  - 
+    name: Anubis
+  - 
+    kind: akDog
+  - 
+    barkometer: 13""", output.data
+    
   test "Serialization: Dump cyclic data structure":
     var
       a = newNode("a")