Parsec

{-# LANGUAGE NoMonomorphismRestriction #-}
import Text.Parsec
import Text.Parsec.Expr
import Control.Applicative hiding ((<|>), many)

main = case parse p "" "$c = $a[$b]" of
          Left err -> putStrLn . show $ err
          Right r  -> putStrLn . show $ r
    where p = (try assignment <|> command) <* eof


data Command = Command [Variable]
             | Assignment Variable Expr
             deriving Show

-- parses command of form: $var = $var[$index]
command = do
    v1 <- var
    lexeme $ char '='
    v2 <- var
    lexeme $ char '['
    v3 <- var
    lexeme $ char ']'
    return $ Command [v1, v2, v3]

assignment = do
    v1 <- var
    lexeme $ char '='
    e <- expr
    return $ Assignment v1 e

data Variable = Variable String deriving Show
data Expr = Add Expr Expr
          | Var Variable
          deriving Show

--expr = fail ""
expr = buildExpressionParser table term <?> "expression"

term = Var <$> var <?> "term"

table = [ [ binary "+" Add AssocLeft ] ]
    where binary name fun assoc = Infix (do lexeme . string $ name; return fun) assoc

var = do char '$'
         name <- lexeme $ many letter
         return $ Variable name
       <?> "variable"

lexeme p = p <* spaces