From 3deaeee24a9a56f8368f52e40d8c1e6c045cbd4e Mon Sep 17 00:00:00 2001
From: Laborratte5 <laborratte5@gmail.com>
Date: Wed, 17 Jul 2024 00:22:38 +0200
Subject: [PATCH] Implement Applicative Parser

---
 app/Parser.hs | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/app/Parser.hs b/app/Parser.hs
index 3e981e8..5dee86d 100644
--- a/app/Parser.hs
+++ b/app/Parser.hs
@@ -1,6 +1,7 @@
 module Parser where
 
-import qualified Lexer
+import qualified Lexer as L
+import Control.Applicative as Applicative
 
 type AST = Expr
 
@@ -18,3 +19,21 @@ data Factor = Integer Int
             | Parantheses Expr
             | NegFactor Factor
             deriving Show
+
+type Tokens = [L.Token]
+newtype Parser a = Parser {
+    runParser :: Tokens -> Maybe (Tokens, a)
+}
+
+instance Functor Parser where
+    fmap f (Parser p) = Parser $ \input -> case p input of
+        Nothing -> Nothing
+        Just (rest, a) -> Just (rest, f a)
+
+instance Applicative Parser where
+    pure x = Parser $ \input -> Just (input, x)
+    (<*>) (Parser fParser) (Parser a) = Parser $ \input -> case fParser input of
+        Nothing -> Nothing
+        Just (rest, f) -> case a rest of
+            Nothing -> Nothing
+            Just (rest', x) -> Just (rest', f x)