Make `projectPaths` fail without side-effects

Fix ehmry/xdg_open_ng#2

src/syndicate/skeletons.nim

@@ -9,14 +9,17 @@ type
   Value = Preserve[Ref]
   Path = seq[Value]
 
-proc projectPath(v: Value; path: Path): Value =
-  result = v
+func projectPath(v: Value; path: Path): Option[Value] =
+  result = some(v)
   for index in path:
-    result = result[index]
+    result = preserves.step(result.get, index)
+    if result.isNone: break
 
-proc projectPaths(v: Value; paths: seq[Path]): seq[Value] =
-  result.setLen(paths.len)
-  for i, path in paths: result[i] = projectPath(v, path)
+func projectPaths(v: Value; paths: seq[Path]): seq[Value] =
+  result = newSeq[Value](paths.len)
+  for i, path in paths:
+    var vv = projectPath(v, path)
+    if vv.isSome: result[i] = get(vv)
 
 type Class = distinct string
 
@@ -148,7 +151,8 @@ proc extend(node: Node; popCount: Natural; stepIndex: Value; pat: Pattern; path:
       new result.nextNode.continuation
       table[class] = result.nextNode
       for a in node.continuation.cachedAssertions:
-        if class == classOf projectPath(a, path):
+        var v = projectPath(a, path)
+        if v.isSome and class == classOf(get(v)):
           result.nextNode.continuation.cachedAssertions.incl a
     result.popCount = 0
     template walkKey(pat: Pattern; stepIndex: Value) =

syndicate.nimble

@@ -9,4 +9,4 @@ srcDir        = "src"
 
 # Dependencies
 
-requires "nim >= 1.4.8", "nimSHA2 >= 0.1.1", "preserves"
+requires "nim >= 1.4.8", "nimSHA2 >= 0.1.1", "preserves > 3.2.0"