— Expression example from chapter 13 of Programming in Haskell,
— , Cambridge University Press, 2016.
import Parsing
expr :: Parser Int
expr = do t <- term
do symbol "+"
e <- expr
return (t + e)
<|> return t
term :: Parser Int
term = do f <- factor
do symbol "*"
t <- term
return (f * t)
<|> return f
factor :: Parser Int
factor = do symbol “(”
e <- expr
symbol ")"
return e
<|> natural
eval :: String -> Int
eval xs = case (parse expr xs) of
[(n,[])] -> n
[(_,out)] -> error (“Unused input ” ++ out)
[] -> error “Invalid input”