better approach via mixin

lib/pure/json.nim

@@ -1269,11 +1269,8 @@ proc isNamedTuple(T: typedesc): bool {.magic: "TypeTrait".}
 
 proc toJson*[T](a: T): JsonNode {.since: (1,3,5).} =
   ## like `%` but allows custom serialization hook if `serialize(a: T)` is in scope
-  when compiles(serialize(a)):
-    proc funAdd(t: JsonNode, key: string, val: string) = t[key] = %val
-    proc funAdd2(t: JsonNode, key: string, val: JsonNode) = t[key] = val
-    proc funObj(): JsonNode = newJObject()
-    result = serialize(a, funObj, funAdd, funAdd2)
+  when compiles(toJsonHook(a)):
+    result = toJsonHook(a)
   elif T is object | tuple:
     const isNamed = T is object or isNamedTuple(T)
     when isNamed:

lib/pure/strtabs.nim

@@ -419,14 +419,16 @@ proc `%`*(f: string, t: StringTableRef, flags: set[FormatFlag] = {}): string {.
       add(result, f[i])
       inc(i)
 
-proc serialize*(a: StringTableRef) = discard
-proc serialize*[T1, T2, T3](a: StringTableRef, funObj: T1, funAdd: T2, funAdd2: T3): auto =
+proc toJsonHook*[](a: StringTableRef): auto =
   ## allows a custom serializer (eg json) to serialize this as we want.
-  result = funObj()
-  funAdd(result, "mode", $a.mode)
-  let t = funObj()
-  for k,v in a: funAdd(t, k, v)
-  funAdd2(result, "table", t)
+  mixin newJObject
+  mixin toJson
+  result = newJObject()
+  result["mode"] = toJson($a.mode)
+  let t = newJObject()
+  for k,v in a:
+    t[k] = toJson(v)
+  result["table"] = t
 
 when isMainModule:
   var x = {"k": "v", "11": "22", "565": "67"}.newStringTable

tests/stdlib/tjsonmacro.nim

@@ -642,7 +642,7 @@ import strtabs
 proc testCustom()=
   var t = {"name": "John", "city": "Monaco"}.newStringTable
   let s = toJson(t)
-  doAssert $s == """{"mode":"modeCaseSensitive","table":{"city":"Monaco","name":"John"}}"""
+  doAssert $s == """{"mode":"modeCaseSensitive","table":{"city":"Monaco","name":"John"}}""", $s
 
 proc testToJson() =
   var t = {"z": "Z", "y": "Y"}.newStringTable