use makeExprParser for parsing

haskell-experiments/haskell-experiments.cabal

@@ -82,7 +82,7 @@ test-suite haskell-exps-test
         Test.SimpleParserSpec
 
 library langfeatures
-  build-depends:   base, containers, megaparsec
+  build-depends:   base, containers, megaparsec, parser-combinators
   hs-source-dirs:  src
   exposed-modules: Ologs, SimpleParser
   ghc-options:     -Wall

haskell-experiments/src/SimpleParser.hs

@@ -15,13 +15,16 @@
 {-# LANGUAGE NoMonomorphismRestriction #-}
 {-# OPTIONS_GHC -Wno-unrecognised-pragmas #-}
 
-module SimpleParser
-  ( eval, Expr (..), BinaryOp (..), UnaryOp (..), parseExpr ) -- literalParser
+module SimpleParser (eval, Expr (..), BinaryOp (..), UnaryOp (..), parseExpr) -- literalParser
 where
-import Text.Megaparsec
+
+import Control.Monad.Combinators.Expr
+import Data.Functor (($>))
 import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Byte.Lexer (lexeme)
+import Text.Megaparsec.Char (char, space)
 import Text.Megaparsec.Char.Lexer (decimal)
-import Text.Megaparsec.Char (char)
 
 type Parser = Parsec Void String
 
@@ -29,10 +32,10 @@ data BinaryOp = Add | Subtract | Multiply | Divide deriving (Show, Eq)
 
 data UnaryOp = Negate deriving (Show, Eq)
 
-data Expr =
-  BinaryExpr BinaryOp Expr Expr |
-  UnaryExpr UnaryOp Expr |
-  Literal Int 
+data Expr
+    = BinaryExpr BinaryOp Expr Expr
+    | UnaryExpr UnaryOp Expr
+    | Literal Int
     deriving (Show, Eq)
 
 lookUpBinaryOp :: BinaryOp -> Int -> Int -> Int
@@ -50,28 +53,18 @@ eval (BinaryExpr binOp a b) = lookUpBinaryOp binOp (eval a) (eval b)
 eval (UnaryExpr unaryOp a) = lookUpUnaryOp unaryOp (eval a)
 
 parseExpr :: Parser Expr
-parseExpr = choice [
-    try parseBinaryExpr,
+parseExpr =
+    makeExprParser
         parseLiteral
-    -- bracketedExpParser,
-    -- negatedExp
+        [ [InfixL (char '/' $> BinaryExpr Divide)]
+        , [InfixL (char '*' $> BinaryExpr Multiply)]
+        , [InfixL (char '+' $> BinaryExpr Add)]
+        , [InfixL (char '-' $> BinaryExpr Subtract)]
         ]
 
 parseLiteral :: Parser Expr
-parseLiteral = Literal <$> decimal
-
-parseBinaryExpr :: Parser Expr
-parseBinaryExpr = do
-  lhs <- parseExpr
-  binOp <- parseOp
-  rhs <- parseExpr
-  pure (BinaryExpr binOp lhs rhs)
-
-parseOp :: Parser BinaryOp
-parseOp = choice [
-  char '+' >> pure Add,
-  char '-' >> pure Subtract,
-  char '/' >> pure Divide,
-  char '*' >> pure Multiply
+parseLiteral =
+    choice
+        [ Literal <$> decimal
+        , between (char '(') (char ')') parseExpr
         ]
-

haskell-experiments/test/Test/SimpleParserSpec.hs

@@ -36,5 +36,8 @@ spec = do
       checkParse "2" (Literal 2)
       checkParse "2+3" (BinaryExpr Add (Literal 2) (Literal 3))
       checkParse "2+3+5" (BinaryExpr Add (BinaryExpr Add (Literal 2) (Literal 3)) (Literal 5))
+      checkParse "2+3*5" (BinaryExpr Add (Literal 2) (BinaryExpr Multiply (Literal 3) (Literal 5)))
+      checkParse "(2+3)*5" (BinaryExpr Multiply (BinaryExpr Add (Literal 2) (Literal 3)) (Literal 5) )
+      checkParse "((2+3))*5" (BinaryExpr Multiply (BinaryExpr Add (Literal 2) (Literal 3)) (Literal 5) )