Nix.Builtins: replaceStrings: comment a bit, readability

Author: Anton-Latukha

src/Nix/Builtins.hs

@@ -929,47 +929,53 @@ replaceStrings
   -> NValue t f m
   -> NValue t f m
   -> m (NValue t f m)
+-- `ns*` goes for NixString here, which are with context - remember
 replaceStrings tfrom tto ts = fromValue (Deeper tfrom) >>= \(nsFrom :: [NixString]) ->
   fromValue (Deeper tto) >>= \(nsTo :: [NixString]) ->
     fromValue ts >>= \(ns :: NixString) -> do
-      let from = fmap stringIgnoreContext nsFrom
       when (length nsFrom /= length nsTo)
         $  throwError
         $  ErrorCall
         $  "'from' and 'to' arguments to 'replaceStrings'"
         <> " have different lengths"
       let
-        lookupPrefix s = do
+
+        from = fmap stringIgnoreContext nsFrom
+
+        lookupPrefix s = do  -- monadic context handles Maybe result here, aka if Nothing returned
           (prefix, replacement) <- find ((`Text.isPrefixOf` s) . fst)
             $ zip from nsTo
           let rest = Text.drop (Text.length prefix) s
           pure (prefix, replacement, rest)
+
         finish b =
           makeNixString (LazyText.toStrict $ Builder.toLazyText b)
+
         go orig result ctx = case lookupPrefix orig of
           Nothing -> case Text.uncons orig of
             Nothing     -> finish result ctx
             Just (h, t) -> go t (result <> Builder.singleton h) ctx
           Just (prefix, replacementNS, rest) ->
-            let replacement = stringIgnoreContext replacementNS
-                newCtx      = NixString.getContext replacementNS
-            in  case prefix of
-                  "" -> case Text.uncons rest of
-                    Nothing -> finish
+            case prefix of
+              "" -> case Text.uncons rest of
+                Nothing -> finish
+                  (result <> Builder.fromText replacement)
+                  (ctx <> newCtx)
+                Just (h, t) -> go
+                  t
+                  (mconcat
+                    [ result
+                    , Builder.fromText replacement
+                    , Builder.singleton h
+                    ]
+                  )
+                  (ctx <> newCtx)
+              _ -> go rest
                       (result <> Builder.fromText replacement)
                       (ctx <> newCtx)
-                    Just (h, t) -> go
-                      t
-                      (mconcat
-                        [ result
-                        , Builder.fromText replacement
-                        , Builder.singleton h
-                        ]
-                      )
-                      (ctx <> newCtx)
-                  _ -> go rest
-                          (result <> Builder.fromText replacement)
-                          (ctx <> newCtx)
+             where
+              replacement = stringIgnoreContext replacementNS
+              newCtx      = NixString.getContext replacementNS
       toValue
         $ go (stringIgnoreContext ns) mempty
         $ NixString.getContext ns