Allow specifying a custom serialization procedure for a particular field

rlp.nim

@@ -3,7 +3,7 @@
 ## https://ethereum.github.io/yellowpaper/paper.pdf
 
 import
-  strutils, parseutils,
+  macros, strutils, parseutils,
   rlp/[types, writer, object_serialization],
   rlp/priv/defs
 
@@ -357,6 +357,10 @@ proc readImpl(rlp: var Rlp, T: typedesc[object|tuple],
     rlp.position += payloadOffset
 
   template op(field) =
+    when hasCustomPragma(field, rlpCustomSerialization):
+      field = rlp.read(type(field),
+                       getCustomPragmaVal(field, rlpCustomSerialization))
+    else:
       field = rlp.read(type(field))
 
   enumerateRlpFields(result, op)

rlp/object_serialization.nim

@@ -20,3 +20,9 @@ template rlpInline* {.pragma.}
   ## This can be specified on a record field in order to avoid the
   ## default behavior of wrapping the record in a RLP list.
 
+template rlpCustomSerialization*(T: typedesc) {.pragma.}
+  ## This can be specified on a record field to trigger the use of a
+  ## custom `read` and `append` serialization functions.
+  ## The custom functions will be passed an additional `type` argument
+  ## specefied in the pragma (it is used as an overload tag)
+

rlp/writer.nim

@@ -217,7 +217,12 @@ proc appendTupleOrObject(self; data: object|tuple, wrapInList: bool) =
     enumerateRlpFields(data, countFields)
     self.startList(fieldsCount)
 
-  template op(x) = append(self, x)
+  template op(field) =
+    when hasCustomPragma(field, rlpCustomSerialization):
+      append(self, field, getCustomPragmaVal(field, rlpCustomSerialization))
+    else:
+      append(self, field)
+
   enumerateRlpFields(data, op)
 
 proc appendImpl(self; data: object, wrapInList = wrapObjectsInList) {.inline.} =

tests/test_object_serialization.nim

@@ -17,6 +17,11 @@ type
     b: string
     f: Foo
 
+  CompressedFoo = object
+
+  Bar2 = object
+    f {.rlpCustomSerialization: CompressedFoo}: Foo
+
 rlpFields Foo,
   x, y, z
 
@@ -25,6 +30,14 @@ rlpFields Transaction,
 
 proc default(T: typedesc): T = discard
 
+proc append*(rlpWriter: var RlpWriter, f: Foo, tag: type CompressedFoo) =
+  rlpWriter.append(f.x)
+  rlpWriter.append(f.y.len)
+
+proc read*(rlp: var Rlp, T: type Foo, tag: type CompressedFoo): Foo =
+  result.x = rlp.read(uint64)
+  result.y = newString(rlp.read(int))
+
 test "encoding and decoding an object":
   var originalBar = Bar(b: "abracadabra",
                         f: Foo(x: 5'u64, y: "hocus pocus", z: @[100, 200, 300]))
@@ -47,3 +60,13 @@ test "encoding and decoding an object":
     t2.receiver == "Bob"
     t2.amount == 1000
 
+test "custom field serialization":
+  var origVal = Bar2(f: Foo(x: 10'u64, y: "y", z: @[]))
+  var bytes = encode(origVal)
+  var r = rlpFromBytes(bytes)
+  var restored = r.read(Bar2)
+
+  check:
+    origVal.f.x == restored.f.x
+    origVal.f.y.len == restored.f.y.len
+