diff --git a/default.nix b/default.nix
index 08dad5fe2..b0585e1a8 100644
--- a/default.nix
+++ b/default.nix
@@ -111,19 +111,19 @@
 
 let
 
- getDefaultGHC = "ghc${
-    (
-      # Remove '.' from the string 8.8.4 -> 884
-      pkgs.lib.stringAsChars (c: if c == "." then "" else c)
-        # Get default GHC version,
-        (pkgs.lib.getVersion pkgs.haskellPackages.ghc)
-    )
-  }";
-
- compilerPackage =
-   if ((compiler == "") || (compiler == "default"))
-     then getDefaultGHC
-     else compiler;
+  getDefaultGHC = "ghc${
+      (
+        # Remove '.' from the string 8.8.4 -> 884
+        pkgs.lib.stringAsChars (c: if c == "." then "" else c)
+          # Get default GHC version,
+          (pkgs.lib.getVersion pkgs.haskellPackages.ghc)
+      )
+    }";
+
+  compilerPackage =
+    if ((compiler == "") || (compiler == "default"))
+      then getDefaultGHC
+      else compiler;
 
   #  2020-12-31: NOTE: Remove after `hnix-store 0.4` arrives into Nixpkgs
   hnix-store-src = pkgs.fetchFromGitHub {
@@ -226,13 +226,14 @@ let
       # 2020-12-07 We really want cryptohash-sha512, but it conflicts with
       # recent versions of base, for seemingly no valid reason.
       # As the update is slow to happen, just jailbreak here
-      # See https://github.com/haskell-hvr/cryptohash-sha512/pull/3
-      # and https://github.com/haskell-hvr/cryptohash-sha512/pull/5
+      # See https://github.com/haskell-hvr/cryptohash-sha512 PRs 3, 5 and issue 4
+      # See also https://github.com/NixOS/nixpkgs/pull/106333 for a temporary fix.
       cryptohash-sha512 = pkgs.haskell.lib.unmarkBroken ( pkgs.haskell.lib.doJailbreak super.cryptohash-sha512 );
 
       # 2020-12-07 hnix-store-remote fails when trying to connect to a real hnix daemon.
       # probably due to nix sandbox restrictions.
-      hnix-store-remote = pkgs.haskell.lib.dontCheck super.hnix-store-remote;
+      # Upstream issue @ https://github.com/haskell-nix/hnix-store/issues/80
+      hnix-store-remote = pkgs.haskell.lib.removeConfigureFlag super.hnix-store-remote "-fio-testsuite";
 
       # 2020-08-04 hnix uses custom LayoutOptions and therefore is
       # likely to be affected by the change in the ribbon width
diff --git a/src/Nix/Effects.hs b/src/Nix/Effects.hs
index c1df93300..0ce82e677 100644
--- a/src/Nix/Effects.hs
+++ b/src/Nix/Effects.hs
@@ -244,14 +244,15 @@ type StorePathSet = HS.HashSet StorePath
 
 class Monad m => MonadStore m where
 
-    -- | Add a path to the store, with bells and whistles
+    -- | Copy the contents of a local path to the store.  The resulting store
+    -- path is returned.  Note: This does not support yet support the expected
+    -- `filter` function that allows excluding some files.
     addToStore :: StorePathName -> FilePath -> RecursiveFlag -> RepairFlag -> m (Either ErrorCall StorePath)
     default addToStore :: (MonadTrans t, MonadStore m', m ~ t m') => StorePathName -> FilePath -> RecursiveFlag -> RepairFlag -> m (Either ErrorCall StorePath)
     addToStore a b c d = lift $ addToStore a b c d
 
-    -- | Add a nar (action) to the store
-    -- addToStore' :: StorePathName -> IO Nar -> RecursiveFlag -> RepairFlag -> m (Either ErrorCall StorePath)
-
+    -- | Like addToStore, but the contents written to the output path is a
+    -- regular file containing the given string.
     addTextToStore' :: StorePathName -> Text -> Store.StorePathSet -> RepairFlag -> m (Either ErrorCall StorePath)
     default addTextToStore' :: (MonadTrans t, MonadStore m', m ~ t m') => StorePathName -> Text -> Store.StorePathSet -> RepairFlag -> m (Either ErrorCall StorePath)
     addTextToStore' a b c d = lift $ addTextToStore' a b c d
diff --git a/src/Nix/Effects/Derivation.hs b/src/Nix/Effects/Derivation.hs
index 47e32bffc..51a0f3b16 100644
--- a/src/Nix/Effects/Derivation.hs
+++ b/src/Nix/Effects/Derivation.hs
@@ -219,10 +219,10 @@ derivationParser = do
       let (hashType, hashMode) = case Text.splitOn ":" rht of
             ["r", ht] -> (ht, Recursive)
             [ht] ->      (ht, Flat)
-            _ -> undefined -- What ?! -- TODO: Throw a proper error
+            _ -> error $ "Unsupported hash type for output of fixed-output derivation in .drv file: " ++ show fullOutputs
       in case Store.mkNamedDigest hashType hash of
         Right digest -> (Just digest, hashMode)
-        Left _err -> undefined -- TODO: Raise a proper parse error.
+        Left err -> error $ "Unsupported hash " ++ show (hashType <> ":" <> hash) ++ "in .drv file: " ++ err
     _ -> (Nothing, Flat)
 
 
@@ -282,7 +282,7 @@ defaultDerivationStrict = fromValue @(AttrSet (NValue t f m)) >=> \s -> do
       AllOutputs ->
         -- TODO: recursive lookup. See prim_derivationStrict
         -- XXX: When is this really used ?
-        undefined
+        error "Not implemented: derivations depending on a .drv file are not yet supported."
 
 
 -- | Build a derivation in a context collecting string contexts.