where

src/RLP/Parse/Utils.hs

@@ -0,0 +1,30 @@
+module Rlp.Parse.Utils
+    ( withPredicate
+    , registerCustomFailure
+    , optionalList
+    )
+    where
+--------------------------------------------------------------------------------
+import Text.Megaparsec
+import Rlp.Parse.Types
+import Data.Set                     qualified as S
+import Data.Maybe
+import Control.Monad
+--------------------------------------------------------------------------------
+
+-- TODO: generalise type sig
+withPredicate :: (a -> Bool)
+              -> Parser a    -- ^ action to run should the predicate fail
+              -> Parser a
+              -> Parser a
+withPredicate f r p = do
+    o <- getOffset
+    a <- p
+    if f a then pure a else setOffset o *> r
+
+registerCustomFailure :: MonadParsec e s m => e -> m ()
+registerCustomFailure = registerFancyFailure . S.singleton . ErrorCustom
+
+optionalList :: Parser [a] -> Parser [a]
+optionalList = fmap (join . maybeToList) . optional
+