Putting into practice lessons learned from "Write Yourself a Scheme in 48 hours" book https://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours/Parsing

parser.hs

module Main (main) where

import Prelude
import System.Environment
import Control.Monad
import Text.ParserCombinators.Parsec

main :: IO ()
main = do
  (text:_) <- getArgs
  putStrLn $ show $ parseExpr text

data BinOp
  = Add Lang Lang
  deriving (Show)

data Lang
  = BinOp BinOp
  | Val Int
  deriving (Show)

numeric :: Parser Int
numeric = liftM (read) $ many1 digit

addOp :: Parser BinOp
addOp = do
  a <- numeric
  _ <- char '+'
  b <- numeric
  return $ Add (Val a) (Val b)

parseBinOp :: Parser Lang
parseBinOp = liftM BinOp addOp

parseNumeric :: Parser Lang
parseNumeric = liftM Val numeric

parser :: Parser Lang
parser = try parseBinOp <|> parseNumeric 
  
parseExpr :: String -> Either ParseError Lang
parseExpr text = parse parser "Lang" text 

eval :: Lang -> Int
eval (BinOp (Add a b)) = (eval a) + (eval b)
eval (Val a) = a
 
-- >>> parseExpr "123"
-- Right (Val 123)

-- >>> parseExpr "a12"
-- Left "Lang" (line 1, column 1):
-- unexpected "a"
-- expecting digit

-- >>> parseExpr "2+2"
-- Right (BinOp (Add (Val 2) (Val 2)))

-- >>> fmap eval $ parseExpr "2+2"-- <interactive>:1286:9: error:
-- Right 4