import Text.Megaparsec
import Text.Megaparsec.Char
import Control.Monad.Combinators.Expr
import Data.Void (Void)

type Parser = Parsec Void String

data Expr = Var Char | Add Expr Expr | Mul Expr Expr
  deriving Show

var :: Parser Expr
var = Var <$> letterChar

parens :: Parser a -> Parser a
parens = between (char '(') (char ')')

term :: Parser Expr
term = choice [ var, parens expr ]

expr :: Parser Expr
expr = makeExprParser term 
  [ [ InfixN (Mul <$ string "")
    , InfixN (Add <$ string "+")
    ]   
  ]

main :: IO ()
main = parseTest (expr <* eof) "x+y(xz+z)"