add recmapZipOnto fn family

recmapZipOnto functions respect Terminal types

nt/primitives/bootstrap/parse/parse.nix

@@ -19,6 +19,12 @@
     Some
     None
     ;
+
+  inherit
+    (this.terminal)
+    isTerminal
+    unwrapTerminal
+    ;
 in rec {
   # getAttrAt :: [String] -> Attrs -> Maybe a
   # Given an attribute set path as a list of strings,
@@ -45,21 +51,33 @@ in rec {
   # recmapFrom :: [String] -> ([String] -> Attrs -> a) -> Attrs -> a | Attrs a
   # Alternative to mapAttrsRecursiveCond
   # Allows mapping directly from a child path
-  recmapFrom = path: f: T:
-    if isAttrs T
-    then mapAttrs (attr: leaf: recmapFrom (path ++ [attr]) f leaf) T
+  recmapCondFrom = path: cond: f: T: let
+    delegate = path': recmapCondFrom path' cond f;
+  in
+    if isAttrs T && cond path T
+    then T |> mapAttrs (attr: leaf: delegate (path ++ [attr]) leaf)
     else f path T;
 
   # recmap :: ([String] -> Attrs -> a) -> Attrs -> a | Attrs a
-  recmap = recmapFrom [];
+  recmapCond = recmapCondFrom [];
+
+  recmapZipOntoCondFrom = path: cond: f: dst: src: let
+    zip = f': path': dstLeaf: f' path' dstLeaf (getAttrAt path' src);
+  in
+    dst
+    |> recmapCondFrom path (zip cond) (zip f);
+
+  recmapZipOntoCond = recmapZipOntoCondFrom [];
 
   projectOnto = dst: src:
-    dst
-    |> recmap
-    (path: dstLeaf: let
-      srcLeaf = getAttrAt path src;
-    in
+    src
+    |> recmapZipOntoCond
+    (path: dstLeaf: srcLeaf: ! isTerminal dstLeaf)
+    (path: dstLeaf: srcLeaf:
       if isSome srcLeaf
       then unwrapMaybe srcLeaf
-      else dstLeaf);
+      else if isTerminal dstLeaf
+      then unwrapTerminal dstLeaf
+      else dstLeaf)
+    dst;
 }