From 92834a8968f2f2e68b49d3191ea6ff056ef4855b Mon Sep 17 00:00:00 2001
From: Emery Hemingway <ehmry@posteo.net>
Date: Wed, 7 Dec 2022 03:23:10 -0600
Subject: [PATCH] Add expand, contract for embedded conversions

---
 src/preserves.nim | 50 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/src/preserves.nim b/src/preserves.nim
index 0df0f39..27313d1 100644
--- a/src/preserves.nim
+++ b/src/preserves.nim
@@ -1213,6 +1213,56 @@ proc mapEmbeds*[A, B](pr: sink Preserve[A]; op: proc (v: A): B): Preserve[B] =
     of pkEmbedded:
       result = embed op(pr.embed)
 
+proc contract*[E](pr: sink Preserve[E]; op: proc (v: E): Preserve[void]): Preserve[void] =
+  ## Convert `Preserve[E]` to `Preserve[void]` using an `E → Preserve[void]` procedure.
+  if not pr.embedded:
+    case pr.kind
+    of pkBoolean, pkFloat, pkDouble, pkSignedInteger, pkString, pkByteString, pkSymbol:
+      result = cast[Preserve[void]](pr)
+    of pkRecord:
+      result = Preserve[void](kind: pr.kind)
+      result.record = map(pr.record) do (x: Preserve[E]) -> Preserve[void]:
+        contract(x, op)
+    of pkSequence:
+      result = Preserve[void](kind: pr.kind)
+      result.sequence = map(pr.sequence) do (x: Preserve[E]) -> Preserve[void]:
+        contract(x, op)
+    of pkSet:
+      result = Preserve[void](kind: pr.kind)
+      result.set = map(pr.set) do (x: Preserve[E]) -> Preserve[void]:
+        contract(x, op)
+    of pkDictionary:
+      result = Preserve[void](kind: pr.kind)
+      result.dict = map(pr.dict) do (e: DictEntry[E]) -> DictEntry[void]:
+        (contract(e.key, op), contract(e.val, op))
+    of pkEmbedded:
+      result = embed op(pr.embed)
+
+proc expand*[E](pr: sink Preserve[void]; op: proc (v: Preserve[void]): Preserve[E]): Preserve[E] =
+  ## Convert `Preserve[void]` to `Preserve[E]` using an `Preserve[void] → Preserve[E]` procedure.
+  if not pr.embedded:
+    case pr.kind
+    of pkBoolean, pkFloat, pkDouble, pkSignedInteger, pkString, pkByteString, pkSymbol:
+      result = cast[Preserve[E]](pr)
+    of pkRecord:
+      result = Preserve[E](kind: pr.kind)
+      result.record = map(pr.record) do (x: Preserve[void]) -> Preserve[E]:
+        expand(x, op)
+    of pkSequence:
+      result = Preserve[E](kind: pr.kind)
+      result.sequence = map(pr.sequence) do (x: Preserve[void]) -> Preserve[E]:
+        expand(x, op)
+    of pkSet:
+      result = Preserve[E](kind: pr.kind)
+      result.set = map(pr.set) do (x: Preserve[void]) -> Preserve[E]:
+        expand(x, op)
+    of pkDictionary:
+      result = Preserve[E](kind: pr.kind)
+      result.dict = map(pr.dict) do (e: DictEntry[void]) -> DictEntry[E]:
+        (expand(e.key, op), expand(e.val, op))
+    of pkEmbedded:
+      result = op(pr.embed)
+
 proc getOrDefault*[T, V](pr: Preserve[T]; key: string; default: V): V =
   ## Retrieves the value of `pr[key]` if `pr` is a dictionary containing `key`
   ## or returns the `default` value.