From 376c0e04c586b3cafecae85576da034114ce721f Mon Sep 17 00:00:00 2001
From: Emery Hemingway <ehmry@posteo.net>
Date: Mon, 28 Jun 2021 12:30:16 +0200
Subject: [PATCH] Add a {.record: "label".} pragma

A pragma for serializing tuples and objects to or from records.
---
 src/preserves.nim | 72 ++++++++++++++++++++++++++++++++---------------
 1 file changed, 50 insertions(+), 22 deletions(-)

diff --git a/src/preserves.nim b/src/preserves.nim
index 12ae622..d4573b5 100644
--- a/src/preserves.nim
+++ b/src/preserves.nim
@@ -1,7 +1,8 @@
 # SPDX-License-Identifier: ISC
 
-import base64, endians, json, hashes, sets, streams, tables
 import bigints
+import std/[base64, endians, json, hashes, macros, sets, streams, tables]
+
 
 type
   PreserveKind* = enum
@@ -228,14 +229,6 @@ func isRecord*(prs: Preserve): bool =
     result = true
     assert(prs.record.len > 0)
 
-proc isNamedTuple(T: typedesc): bool {.magic: "TypeTrait".}
-proc distinctBase(T: typedesc): typedesc {.magic: "TypeTrait".}
-template distinctBase[T](a: T): untyped = distinctBase(type(a))(a)
-
-proc symbol*(s: string): Preserve {.inline.} =
-  ## Symbol constructor.
-  Preserve(kind: pkSymbol, symbol: s)
-
 proc label*(prs: Preserve): Preserve {.inline.} =
   ## Return the label of a record value.
   prs.record[prs.record.high]
@@ -252,6 +245,13 @@ iterator fields*(prs: Preserve): Preserve =
   ## Iterate the fields of a record value.
   for i in 0..<prs.record.high: yield prs.record[i]
 
+proc distinctBase(T: typedesc): typedesc {.magic: "TypeTrait".}
+template distinctBase[T](a: T): untyped = distinctBase(type(a))(a)
+
+proc symbol*(s: string): Preserve {.inline.} =
+  ## Symbol constructor.
+  Preserve(kind: pkSymbol, symbol: s)
+
 proc writeVarint(s: Stream; n: int) =
   var n = n
   while true:
@@ -453,6 +453,16 @@ proc parsePreserve*(s: Stream): Preserve =
     else:
       assertStream(false)
 
+template record*(label: string) {.pragma.}
+  ## Serialize this object or tuple as a record.
+  ## ```
+  ##  type Foo {.record: "foobar".} = tuple
+  ##    a, b: int
+  ##  let r: Foo = (1, 2)
+  ##  echo $(toPreserve(r))
+  ##  # <foobar 1 2>
+  ## ```
+
 proc toPreserve*[T](x: T): Preserve =
   ## Serializes `x` to Preserves; uses `toPreserveHook(x: A)` if it's in scope to
   ## customize serialization.
@@ -473,8 +483,13 @@ proc toPreserve*[T](x: T): Preserve =
   elif T is float64:
     result = Preserve(kind: pkDouble, double: x)
   elif T is object | tuple:
-    result = Preserve(kind: pkDictionary)
-    for k, v in x.fieldPairs: result.dict[symbol(k)] = toPreserve(v)
+    when T.hasCustomPragma(record):
+      result = Preserve(kind: pkRecord)
+      for _, f in x.fieldPairs: result.record.add(toPreserve(f))
+      result.record.add(symbol(T.getCustomPragmaVal(record)))
+    else:
+      result = Preserve(kind: pkDictionary)
+      for k, v in x.fieldPairs: result.dict[symbol(k)] = toPreserve(v)
   elif T is Ordinal:
     result = Preserve(kind: pkSignedInteger, int: x.ord.BiggestInt)
   elif T is ptr | ref:
@@ -557,6 +572,14 @@ proc toJsonHook*(prs: Preserve): JsonNode =
   of pkEmbedded:
     raise newException(ValueError, "cannot convert embedded value to JSON")
 
+proc checkRecordLabel(T: typedesc; prs: Preserve) =
+  when T.hasCustomPragma(record):
+    const label = symbol(T.getCustomPragmaVal(record))
+    if prs.label != label:
+      raise newException(ValueError, $prs & " is not a record of class " & label.string)
+  else:
+      raise newException(Defect, $T & "  lacks a {.record: \"…\".} annotation")
+
 proc fromPreserve*[T](result: var T; prs: Preserve) =
   # Inplace version of `preserveTo`.
   when compiles(fromPreserveHook(result, prs)):
@@ -577,21 +600,26 @@ proc fromPreserve*[T](result: var T; prs: Preserve) =
       fromPreserve(result[i], val)
   elif T is float64:
     result = prs.double
-  elif T is object:
-    result = Preserve(kind: pkDictionary)
-    for k, v in x.fieldPairs: result.dict[symbol(k)] = toPreserve(v)
-  elif T is Ordinal:
-    result = Preserve(kind: pkSignedInteger, int: x.ord.BiggestInt)
+  elif T is object | tuple:
+    case prs.kind
+    of pkRecord:
+      checkRecordLabel(T, prs)
+      var i: int
+      for k, v in result.fieldPairs:
+        fromPreserve(v, prs.record[i])
+        inc(i)
+    of pkDictionary:
+      for k, v in result.fieldPairs:
+        fromPreserve(v, prs.dict[symbol(k)])
+    else:
+      raise newException(ValueError, "cannot convert to Preserves value to type " & $T)
+  elif T is Ordinal | SomeInteger:
+    result = (T)prs.int
   elif T is ptr | ref:
     if system.`==`(x, nil): result = symbol"null"
     else: result = toPreserve(x[])
   elif T is string:
-    result = Preserve(kind: pkString, string: x)
-  elif T is SomeInteger:
-    result = Preserve(kind: pkSignedInteger, int: x.BiggestInt)
-  elif T is tuple:
-    for i in 0..<prs.record.high:
-      fromPreserve(result[i], prs.record[i])
+    result = prs.string
   else:
     raiseAssert("cannot convert from Preserves: " & $T)