1 files changed, 88 insertions, 0 deletions
diff --git a/src/Cron/Parser.hs b/src/Cron/Parser.hs
new file mode 100644
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--- /dev/null
+++ b/src/Cron/Parser.hs
@@ -0,0 +1,88 @@
+module Cron.Parser (parse) where
+
+import Common (fromDigits)
+import Control.Applicative (Alternative, empty, liftA2, many, some, (*>), (<$), (<|>))
+import Control.Arrow (first, (>>>))
+import qualified Cron.Expr as E
+import Data.List.NonEmpty (NonEmpty ((:|)))
+import Intro
+import Text.Read (readMaybe)
+
+type Token = Char
+
+newtype Parser a = Parser {runParser :: [Token] -> Maybe (a, [Token])}
+
+instance Functor Parser where
+  fmap f (Parser p) = Parser (p >>> fmap (first f))
+
+instance Applicative Parser where
+  pure a = Parser (\input -> Just (a, input))
+  Parser pF <*> Parser pA = Parser $ \input -> do
+    (f, rest) <- pF input
+    (a, s) <- pA rest
+    pure (f a, s)
+
+instance Alternative Parser where
+  empty = Parser (const Nothing)
+  Parser pA <|> Parser pB = Parser $ \input -> case (pA input, pB input) of
+    (Nothing, expr) -> expr
+    (expr, _) -> expr
+
+match :: (Token -> Bool) -> Parser Token
+match p = Parser $ \case
+  t : ts | p t -> Just (t, ts)
+  _ -> Nothing
+
+token :: Token -> Parser Token
+token = (==) >>> match
+
+read :: (Token -> Maybe a) -> Parser a
+read f = Parser $ \case
+  t : ts -> case f t of
+    Just x -> Just (x, ts)
+    _ -> Nothing
+  _ -> Nothing
+
+sepBy1 :: Parser a -> Parser b -> Parser (NonEmpty a)
+sepBy1 p s = liftA2 (:|) p (many (s *> p))
+
+pair :: Parser a -> Parser b -> Parser c -> Parser (a, b)
+pair p1 p2 s = liftA2 (,) p1 (s *> p2)
+
+digit :: Parser Int
+digit = read $ \case
+  x | x `elem` ['0' .. '9'] -> readMaybe [x] :: Maybe Int
+  _ -> Nothing
+
+number :: Parser Int
+number = fromDigits <$> some digit
+
+--
+-- Cron Exprs
+--
+
+cRange :: Parser E.Expr
+cRange = uncurry E.Range <$> pair number number (token '-')
+
+cStepL :: Parser E.StepLExpr
+cStepL =
+  (E.StepLEvery <$ token '*')
+    <|> (uncurry E.StepLRange <$> pair number number (token '-'))
+
+cStep :: Parser E.Expr
+cStep = uncurry E.Step <$> pair cStepL number (token '/')
+
+cMulti :: Parser E.Expr
+cMulti = E.Multi <$> sepBy1 number (token ',')
+
+cEvery :: Parser E.Expr
+cEvery = E.Every <$ token '*'
+
+cExpr :: Parser E.Expr
+cExpr = cStep <|> cEvery <|> cRange <|> cMulti
+
+parse :: [Token] -> Either String E.Expr
+parse xs = case runParser cExpr xs of
+  Nothing -> Left "Invalid cron expression"
+  Just (expr, []) -> Right expr
+  Just (_, rest) -> Left ("Invalid cron expression. Couldn't parse `" ++ rest ++ "`")