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+{-# LANGUAGE QuasiQuotes #-}
+
+module Parse where
+
+import Control.Category qualified
+import Control.Selective (Selective)
+import Data.Error.Tree
+import Data.Functor.Compose
+import Data.List qualified as List
+import Data.Monoid (First (..))
+import Data.Semigroup.Traversable
+import Data.Semigroupoid qualified as Semigroupoid
+import Data.Text qualified as Text
+import FieldParser (FieldParser)
+import FieldParser qualified as Field
+import PossehlAnalyticsPrelude
+import Validation (partitionValidations)
+import Prelude hiding (init, maybe)
+import Prelude qualified
+
+-- | A generic applicative “vertical” parser.
+-- Similar to `FieldParser`, but made for parsing whole structures and collect all errors in an `ErrorTree`.
+newtype Parse from to = Parse ((Context, from) -> Validation (NonEmpty ErrorTree) (Context, to))
+  deriving
+    (Functor, Applicative, Selective)
+    via ( Compose
+            ( Compose
+                ((->) (Context, from))
+                (Validation (NonEmpty ErrorTree))
+            )
+            ((,) Context)
+        )
+
+-- | Every parser can add to the context, like e.g. an element parser will add the name of the element it should be parsing.
+-- This should be added to the error message of each parser, with `showContext`.
+newtype Context = Context (Maybe [Text])
+  deriving stock (Show)
+  deriving (Semigroup, Monoid) via (First [Text])
+
+instance Semigroupoid Parse where
+  o p2 p1 = Parse $ \from -> case runParse' p1 from of
+    Failure err -> Failure err
+    Success to1 -> runParse' p2 to1
+
+instance Category Parse where
+  (.) = Semigroupoid.o
+  id = Parse $ \t -> Success t
+
+instance Profunctor Parse where
+  lmap f (Parse p) = Parse $ lmap (second f) p
+  rmap = (<$>)
+
+runParse :: Error -> Parse from to -> from -> Either ErrorTree to
+runParse errMsg parser t =
+  (Context (Just ["$"]), t)
+    & runParse' parser
+    <&> snd
+    & first (nestedMultiError errMsg)
+    & validationToEither
+
+runParse' :: Parse from to -> (Context, from) -> Validation (NonEmpty ErrorTree) (Context, to)
+runParse' (Parse f) from = f from
+
+showContext :: Context -> Text
+showContext (Context context) = context & fromMaybe [] & List.reverse & Text.intercalate "."
+
+addContext :: Text -> Context -> Context
+addContext x (Context mxs) = Context (Just $ x : (mxs & fromMaybe []))
+
+mkParsePushContext :: Text -> ((Context, from) -> Either ErrorTree to) -> Parse from to
+mkParsePushContext toPush f = Parse $ \(ctx, from) -> case f (ctx, from) of
+  Right to -> Success (addContext toPush ctx, to)
+  Left err -> Failure $ singleton err
+
+mkParseNoContext :: (from -> Either ErrorTree to) -> Parse from to
+mkParseNoContext f = Parse $ \(ctx, from) -> case f from of
+  Right to -> Success (ctx, to)
+  Left err -> Failure $ singleton err
+
+-- | Accept only exactly the given value
+exactly :: (Eq from) => (from -> Text) -> from -> Parse from from
+exactly errDisplay from = Parse $ \(ctx, from') ->
+  if from == from'
+    then Success (ctx, from')
+    else Failure $ singleton [fmt|Field has to be exactly {errDisplay from}, was: {errDisplay from'} at {showContext ctx}|]
+
+-- | Make a parser to parse the whole list
+multiple :: Parse a1 a2 -> Parse [a1] [a2]
+multiple inner = dimap nonEmpty (Prelude.maybe [] toList) (maybe $ multipleNE inner)
+
+-- | Make a parser to parse the whole non-empty list
+multipleNE :: Parse from to -> Parse (NonEmpty from) (NonEmpty to)
+multipleNE inner = Parse $ \(ctx, from) ->
+  from
+    & zipIndex
+    & traverse (\(idx, f) -> runParse' inner (ctx, f) & first (singleton . nestedMultiError [fmt|{idx}|]))
+    -- we assume that, since the same parser is used everywhere, the context will be the same as well (TODO: correct?)
+    & second (\((ctx', y) :| ys) -> (ctx', y :| (snd <$> ys)))
+
+-- | Like '(>>>)', but returns the intermediate result alongside the final parse result.
+andParse :: Parse to to2 -> Parse from to -> Parse from (to, to2)
+andParse outer inner = Parse $ \from -> case runParse' inner from of
+  Failure err -> Failure err
+  Success (ctx, to) -> runParse' outer (ctx, to) <&> (second (to,))
+
+-- | Lift a parser into an optional value
+maybe :: Parse from to -> Parse (Maybe from) (Maybe to)
+maybe inner = Parse $ \(ctx, m) -> case m of
+  Nothing -> Success (ctx, Nothing)
+  Just a -> runParse' inner (ctx, a) & second (fmap Just)
+
+-- | Assert that there is exactly one element in the list
+exactlyOne :: Parse [from] from
+exactlyOne = Parse $ \(ctx, xs) -> case xs of
+  [] -> Failure $ singleton [fmt|Expected exactly 1 element, but got 0, at {ctx & showContext}|]
+  [one] -> Success (ctx, one)
+  _more -> Failure $ singleton [fmt|Expected exactly 1 element, but got 2, at {ctx & showContext}|]
+
+-- | Assert that there is exactly zero or one element in the list
+zeroOrOne :: Parse [from] (Maybe from)
+zeroOrOne = Parse $ \(ctx, xs) -> case xs of
+  [] -> Success (ctx, Nothing)
+  [one] -> Success (ctx, Just one)
+  _more -> Failure $ singleton [fmt|Expected exactly 1 element, but got 2, at {ctx & showContext}|]
+
+-- | Find the first element on which the sub-parser succeeds; if there was no match, return all error messages.
+find :: Parse from to -> Parse [from] to
+find inner = Parse $ \(ctx, xs) -> case xs of
+  [] -> failure [fmt|Wanted to get the first sub-parser that succeeds, but there were no elements in the list, at {ctx & showContext}|]
+  (y : ys) -> runParse' (findNE' inner) (ctx, y :| ys)
+
+-- | Find the first element on which the sub-parser succeeds; if there was no match, return all error messages.
+findNE' :: Parse from to -> Parse (NonEmpty from) to
+findNE' inner = Parse $ \(ctx, xs) ->
+  xs
+    <&> (\x -> runParse' inner (ctx, x))
+    & traverse1
+      ( \case
+          Success a -> Left a
+          Failure e -> Right e
+      )
+    & \case
+      Left a -> Success a
+      Right errs ->
+        errs
+          & zipIndex
+          <&> (\(idx, errs') -> nestedMultiError [fmt|{idx}|] errs')
+          & nestedMultiError [fmt|None of these sub-parsers succeeded|]
+          & singleton
+          & Failure
+
+-- | Find all elements on which the sub-parser succeeds; if there was no match, return an empty list
+findAll :: Parse from to -> Parse [from] [to]
+findAll inner = Parse $ \(ctx, xs) ->
+  xs
+    <&> (\x -> runParse' inner (ctx, x))
+    & partitionValidations
+    & \case
+      (_miss, []) ->
+        -- in this case we just arbitrarily forward the original context …
+        Success (ctx, [])
+      (_miss, (hitCtx, hit) : hits) -> Success (hitCtx, hit : (hits <&> snd))
+
+-- | convert a 'FieldParser' into a 'Parse'.
+fieldParser :: FieldParser from to -> Parse from to
+fieldParser fp = Parse $ \(ctx, from) -> case Field.runFieldParser fp from of
+  Right a -> Success (ctx, a)
+  Left err -> Failure $ singleton (singleError err)
+
+zipNonEmpty :: NonEmpty a -> NonEmpty b -> NonEmpty (a, b)
+zipNonEmpty (x :| xs) (y :| ys) = (x, y) :| zip xs ys
+
+zipIndex :: NonEmpty b -> NonEmpty (Natural, b)
+zipIndex = zipNonEmpty (1 :| [2 :: Natural ..])