Don't write empty optional fields (#47)

json_serialization/std/options.nim

@@ -1,6 +1,13 @@
 import std/options, ../../json_serialization/[reader, writer, lexer]
 export options
 
+template writeField*(w: var JsonWriter,
+                     fieldName: static string,
+                     field: Option,
+                     record: auto) =
+  if field.isSome:
+    writeField(w, fieldName, field.get, record)
+
 proc writeValue*(writer: var JsonWriter, value: Option) =
   if value.isSome:
     writer.writeValue value.get
@@ -14,4 +21,3 @@ proc readValue*[T](reader: var JsonReader, value: var Option[T]) =
     reader.lexer.next()
   else:
     value = some reader.readValue(T)
-

json_serialization/stew/results.nim

@@ -0,0 +1,26 @@
+import
+  stew/results, ../../json_serialization/[reader, writer, lexer]
+
+export
+  results
+
+template writeField*[T](w: var JsonWriter,
+                        fieldName: static string,
+                        field: Result[T, void],
+                        record: auto) =
+  if field.isOk:
+    writeField(w, fieldName, field.get, record)
+
+proc writeValue*[T](writer: var JsonWriter, value: Result[T, void]) =
+  if value.isOk:
+    writer.writeValue value.get
+  else:
+    writer.writeValue JsonString("null")
+
+proc readValue*[T](reader: var JsonReader, value: var Result[T, void]) =
+  let tok = reader.lexer.lazyTok
+  if tok == tkNull:
+    reset value
+    reader.lexer.next()
+  else:
+    value.ok reader.readValue(T)

json_serialization/writer.nim

@@ -142,8 +142,21 @@ template isStringLike(v: string|cstring|openArray[char]|seq[char]): bool = true
 template isStringLike[N](v: array[N, char]): bool = true
 template isStringLike(v: auto): bool = false
 
+template writeField*[FieldType, RecordType](w: var JsonWriter,
+                                            fieldName: static string,
+                                            field: FieldType,
+                                            record: RecordType) =
+  mixin writeFieldIMPL
+
+  type
+    F = type field
+    R = type record
+
+  w.writeFieldName(fieldName)
+  w.writeFieldIMPL(FieldTag[R, fieldName, F], field, record)
+
 proc writeValue*(w: var JsonWriter, value: auto) =
-  mixin enumInstanceSerializedFields, writeValue, writeFieldIMPL
+  mixin enumInstanceSerializedFields, writeValue
 
   when value is JsonNode:
     append if w.hasPrettyOutput: value.pretty
@@ -216,12 +229,11 @@ proc writeValue*(w: var JsonWriter, value: auto) =
     w.writeArray(value)
 
   elif value is (object or tuple):
-    w.beginRecord(type(value))
     type RecordType = type value
+    w.beginRecord RecordType
     value.enumInstanceSerializedFields(fieldName, field):
-      type FieldType = type field
-      w.writeFieldName(fieldName)
-      w.writeFieldIMPL(FieldTag[RecordType, fieldName, FieldType], field, value)
+      mixin writeField
+      writeField(w, fieldName, field, value)
       w.state = AfterField
     w.endRecord()
 

tests/test_serialization.nim

@@ -4,7 +4,8 @@ import
   serialization/testing/generic_suite,
   ../json_serialization, ./utils,
   ../json_serialization/lexer,
-  ../json_serialization/std/[options, sets, tables]
+  ../json_serialization/std/[options, sets, tables],
+  ../json_serialization/stew/results
 
 type
   Foo = object
@@ -72,6 +73,10 @@ type
     entry, exit: TokKind
     dup: bool
 
+  HoldsResultOpt* = object
+    r*: ref Simple
+    o*: Opt[Simple]
+
 var
   customVisit: TokenRegistry
 
@@ -327,7 +332,15 @@ suite "toJson tests":
       h2 = HoldsOption(r: newSimple(1, "2", Meter(3)))
 
     Json.roundtripTest h1, """{"r":null,"o":{"distance":3,"x":1,"y":"2"}}"""
-    Json.roundtripTest h2, """{"r":{"distance":3,"x":1,"y":"2"},"o":null}"""
+    Json.roundtripTest h2, """{"r":{"distance":3,"x":1,"y":"2"}}"""
+
+  test "Result Opt types":
+    let
+      h1 = HoldsResultOpt(o: Opt[Simple].ok Simple(x: 1, y: "2", distance: Meter(3)))
+      h2 = HoldsResultOpt(r: newSimple(1, "2", Meter(3)))
+
+    Json.roundtripTest h1, """{"r":null,"o":{"distance":3,"x":1,"y":"2"}}"""
+    Json.roundtripTest h2, """{"r":{"distance":3,"x":1,"y":"2"}}"""
 
   test "Case object as field":
     let