sane-lib: path: fix from bug; tidy

modules/lib/path.nix

@@ -8,30 +8,34 @@ let path = rec {
   # split "a//b/" => ["a" "b"]
   # split "/a/b" =>  ["a" "b"]
   split = str: builtins.filter (seg: seg != "") (lib.splitString "/" str);
+
   # given an array of components, returns the equivalent string path
   join = comps: "/" + (builtins.concatStringsSep "/" comps);
+
   # given an a sequence of string paths, concatenates them into one long string path
   concat = paths: path.join (builtins.concatLists (builtins.map path.split paths));
+
   # normalize the given path
+  # canonical form is an *absolute* path;
+  # always starting with '/', never trailing with '/' unless it's the empty (root) path
   norm = str: path.join (path.split str);
+
   # return the parent directory. doesn't care about leading/trailing slashes.
   # the parent of "/" is "/".
   parent = str: path.norm (builtins.dirOf (path.norm str));
   hasParent = str: (path.parent str) != (path.norm str);
-  # return the path from `from` to `to`, but keeping absolute form
-  # e.g. `pathFrom "/home/colin" "/home/colin/foo/bar"` -> "/foo/bar"
 
   # return the last path component; error on the empty path
   leaf = str: lib.last (split str);
 
-  # XXX: this is bugged in that
-  # from "/foo/bar" "/foo/barbag" => "/bag"
+  # return the path from `from` to `to`, but keeping absolute form
+  # e.g. `pathFrom "/home/colin" "/home/colin/foo/bar"` -> "/foo/bar"
   from = start: end: let
     s = path.norm start;
     e = path.norm end;
   in (
-    assert lib.hasPrefix s e;
-    "/" +  (lib.removePrefix s e)
+    assert isChild start end;
+    "/" + lib.removePrefix s e
   );
 
   isChild = parent: child:
@@ -40,7 +44,7 @@ let path = rec {
       (walk "/" child)
   ;
 
-  # yield every node between start and end, including each the endpoints
+  # yield every node between start and end, including each of the endpoints
   # e.g. walk "/foo" "/foo/bar/baz" => [ "/foo" "/foo/bar" "/foo/bar/baz" ]
   # XXX: assumes input paths are normalized
   walk = start: end: if start == end then