very simple expression parser

simplecalc.hs

expr :: String -> (Double, String)
expr s = expr1 x rest where (x, rest) = term s
expr1 lhs ('+' : s) = expr1 (lhs + x) rest where (x, rest) = term s
expr1 lhs ('-' : s) = expr1 (lhs - x) rest where (x, rest) = term s
expr1 lhs s = (lhs, s)
term s = term1 x rest where (x, rest) = factor s
term1 lhs ('*' : s) = term1 (lhs * x) rest where (x, rest) = factor s
term1 lhs ('/' : s) = term1 (lhs / x) rest where (x, rest) = factor s
term1 lhs s = (lhs, s)
factor ('(' : s) = (x, rest) where (x, ')' : rest) = expr s
factor s = head $ reads s

eval s = x where (x, "") = expr s

main = getLine >>= print . eval