simpleparser1.hs

module Main where
import System.Environment
-- a simple parser built upon the Parsec library
-- importing library function minus `spaces` function
-- that'll conflict with a function that we'll define later
import Text.ParserCombinators.Parsec hiding (spaces)

-- define a parser that recognizes one of the symbols allowed
-- in Scheme
symbol :: Parser Char --let ghc find this out
symbol = oneOf "!$%&|*+-/:<=?>#^_~#"
-- another example of monad:
-- the "extra info" being hidden is all the info about position 
-- in the input stream, backtracking record, first and follow sets,etc.
--
-- Parsec takes care of all of that for us, we just need the Parsec Library
-- function (from now on, plf) oneOf and it'll recognize a single one of any
-- of the chars in the string passed to it
--
-- Defining a function to call our parser and handle errors:
readExpr :: String -> String
readExpr input = case parse symbol "lisp" input of
    Left err -> "No match: " ++ show err
    Right val -> "Found value"
-- this
--    input  = parameter name
--    symbol = action defined above
--    "lisp" = name of the parser
-- are passed to the plf parse.
-- parse can return either the parsed value or an error:
-- Parsec returns an Either data type, using Left constructor to indicate
-- an error and the Right one for a normal value, we use case...of for the pattern
-- matching.

-- main function needed to call readExpr and print out results
-- main :: IO()
main = do args <- getArgs
          putStrLn (readExpr (args !! 0))