Fix builtins.substring for negative lengths

src/Nix/Builtins.hs

@@ -269,7 +269,7 @@ builtinsList = sequence
  -}
  , add' Normal "stringLength" (arity1 $ Text.length . principledStringIgnoreContext)
  , add' Normal "sub" (arity2 ((-) @Integer))
- , add' Normal "substring" (substring @e @t @f @m)
+ , add' Normal "substring" substring
  , add Normal "tail" tail_
  , add0 Normal "true" (pure $ nvConstant $ NBool True)
  , add TopLevel "throw" throw_
@@ -668,13 +668,13 @@ splitMatches numDropped (((_, (start, len)) : captures) : mts) haystack =
 thunkStr s = nvStr (hackyMakeNixStringWithoutContext (decodeUtf8 s))
 
 substring :: forall e t f m. MonadNix e t f m => Int -> Int -> NixString -> Prim m NixString
-substring start len str = Prim $ if start < 0 --NOTE: negative values of 'len' are OK
- then
-  throwError
-   $  ErrorCall
-  $ "builtins.substring: negative start position: "
-  ++ show start
- else pure $ principledModifyNixContents (Text.take len . Text.drop start) str
+substring start len str = Prim $
+ if start < 0
+ then throwError $ ErrorCall $ "builtins.substring: negative start position: " ++ show start
+ else pure $ principledModifyNixContents (take . Text.drop start) str
+ where
+ --NOTE: negative values of 'len' are OK, and mean "take everything"
+ take = if len < 0 then id else Text.take len
 
 attrNames
  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)