From b98f25cab9de95ec1e0095c01bcee84ec54e9fd7 Mon Sep 17 00:00:00 2001
From: Abhinav Sarkar <abhinav.sarkar@gmail.com>
Date: Tue, 19 Apr 2016 21:53:00 +0530
Subject: [PATCH] Create parser.hs

---
 2016-04-18/parser.hs | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)
 create mode 100644 2016-04-18/parser.hs

diff --git a/2016-04-18/parser.hs b/2016-04-18/parser.hs
new file mode 100644
index 0000000..854370e
--- /dev/null
+++ b/2016-04-18/parser.hs
@@ -0,0 +1,39 @@
+module Parser where
+
+import qualified Data.Char as Char
+
+newtype Parser i o = Parser (i -> Maybe (o, i))
+
+instance Functor (Parser i) where
+  fmap f (Parser p) = Parser $ \i -> fmap (\(o, i') -> (f o, i')) $ p i
+
+instance Applicative (Parser i) where
+  pure x = Parser $ \i -> Just (x, i)
+
+  Parser ff <*> Parser fv = Parser $ \i -> case ff i of
+    Just (f, i') -> case fv i' of
+      Just (v, i'') -> Just (f v, i'')
+      Nothing -> Nothing
+    Nothing -> Nothing
+
+charParser :: Char -> Parser String Char
+charParser c = Parser $ \i -> case i of
+  (x : xs) | x == c -> Just (c, xs)
+  _                 -> Nothing
+
+newtype Digit = Digit { getIntOfDigit :: Int } deriving (Show, Eq, Ord)
+
+charToDigit :: Char -> Maybe Digit
+charToDigit c =
+  if Char.isDigit c then Just $ Digit (Char.digitToInt c) else Nothing
+
+digitParser :: Parser String Digit
+digitParser = Parser $ \i -> case i of
+  (c : cs) -> fmap (\d -> (d, cs)) . charToDigit $ c
+  _        -> Nothing
+
+numberParser :: Parser String Int
+numberParser = undefined
+
+runParser :: i -> Parser i o -> Maybe o
+runParser i (Parser p) = fmap fst (p i)