fix: always isolate modules

Previous to this fix, if `dir` was passed as a string instead of a path
literal, then the isolation mechanisms provided by the implicit copy
behavior of a path in string interpolation was broken, and modules could
see the files in their directory.

Now both cases are treated identically and isolation is preserved.
`strToPath` takes a path like string and converts it back to a path
literal to accomplish this.

default.nix

@@ -2,6 +2,7 @@ let
   fix = import ./std/fix.nix;
   filterMap = scopedImport { std = builtins; } ./std/set/filterMap.nix;
   parse = scopedImport { std = builtins; } ./std/file/parse.nix;
+  strToPath = scopedImport { std = builtins; } ./std/path/strToPath.nix;
   cond = import ./std/set/cond.nix;
   compose = import ./.;
 
@@ -42,8 +43,13 @@ let
   );
 
   atom = fix (
-    f: pre: dir:
+    f: pre: dir':
     let
+      # It is crucial that the directory is a path literal, not a string
+      # since the implicit copy to the /nix/store, which provides isolation,
+      # only happens for path literals.
+      dir = strToPath dir';
+
       contents = builtins.readDir dir;
 
       hasMod = contents."mod.nix" or null == "regular";

std/mod.nix

@@ -2,4 +2,5 @@
   pub_fix = mod.fix;
   pub_file = mod.file;
   pub_set = mod.set;
+  pub_path = mod.path;
 }

std/path/mod.nix

@@ -0,0 +1 @@
+{ pub_strToPath = mod.strToPath; }

std/path/strToPath.nix

@@ -0,0 +1,13 @@
+str:
+let
+  # strip off the first `/` of an absolute path
+  frag = std.substring 1 (-1) str;
+  # will fail if the string does not represent an absolute path
+  # which is what we want since this function makes little sense otherwise
+  validate = std.toPath str;
+  # recombine the fragment string with an absolute path starting at the root
+  # the result with be a path literal instead of a string
+  path = /. + frag;
+in
+# avoid the extra work if it is already a path literal
+if std.isPath str then str else std.seq validate path

test/bld.nix

@@ -1,4 +1,7 @@
 let
-  mod = import ../. { } ./bld;
+  mod =
+    import ../. { }
+      # added to test implicit path conversion when path is a string
+      (builtins.toPath ./bld);
 in
 builtins.deepSeq mod mod