add standard attrs parsing interface

nib/parse/default.nix

@@ -0,0 +1,13 @@
+{
+  attrs,
+  result,
+}: let
+  struct = import ./struct.nix {inherit attrs result;};
+in
+  builtins.listToAttrs [
+    # submodule is included directly to this module (ie self.myFunc)
+    struct
+
+    # submodule content is accessible first by submodule name
+    # then by the name of the content (ie self.submodule.myFunc)
+  ]

nib/parse/struct.nix

@@ -0,0 +1,82 @@
+{
+  attrs,
+  result,
+}: rec {
+  cmpStructErr' = errBadKeys: errBadValues: path: S: T:
+    if builtins.isAttrs S && builtins.isAttrSet T
+    then let
+      keysS = builtins.attrNames S;
+      keysT = builtins.attrNames T;
+    in
+      # ensure all key names match, then recurse
+      if !(keysS == keysT)
+      then errBadKeys path keysS keysT
+      else
+        (result.firstErr
+          (builtins.map
+            (k: cmpStructErr' errBadKeys errBadValues (path ++ [k]) (keysS.${k}) (keysT.${k}))
+            keysS))
+    else
+      # terminating leaf in recursion tree reached
+      # ensure values' types match
+      (builtins.typeOf S == builtins.typeOf T)
+      || errBadValues path S T;
+
+  cmpStructErr = errBadKeys: errBadValues: cmpStructErr' errBadKeys errBadValues [];
+
+  cmpStruct =
+    cmpStructErr
+    (path: keysS: keysT:
+      result.Err {
+        reason = "keys";
+        inherit path;
+      })
+    (path: S: T:
+      result.Ok "ok");
+
+  cmpTypedStruct =
+    cmpStructErr
+    (path: keysS: keysT:
+      result.Err {
+        reason = "keys";
+        inherit path;
+      })
+    (path: S: T:
+      result.Err {
+        reason = "values";
+        inherit path;
+      });
+
+  # check is a function taking two structs
+  # and returning a result monad.
+  mergeStruct' = check: template: S: let
+    res = check template S;
+  in
+    result.errOr res ({...}:
+      attrs.mapAttrsRecursive (
+        path: value: let
+          valueS = attrs.attrValueAt S path;
+        in
+          if valueS != null
+          then valueS
+          else value
+      )
+      template);
+
+  mergeStruct = mergeStruct' (S: T: result.Ok "ok");
+
+  mergeTypedStruct = mergeStruct' (
+    cmpStructErr
+    (path: keysS: keysT:
+      result.Ok "ok")
+    (path: S: T:
+      result.Err {
+        reason = "values";
+        inherit path;
+      })
+  );
+
+  mergeStructStrict = mergeStruct' cmpStruct;
+
+  mergeTypedStructStrict = mergeStruct' cmpTypedStruct;
+}