How do I create a Parser data? - parsing

Question

I am trying to learn how can I do a parser for expressions in Haskell and I found this code (below), but I don't even know how to use it.
I tried with: expr (Add (Num 5) (Num 2)) , but it needs a "Parser" data type.
import Text.Parsec
import Text.Parsec.String
import Text.Parsec.Expr
import Text.Parsec.Token
import Text.Parsec.Language
data Expr = Num Int | Var String | Add Expr Expr | Sub Expr Expr | Mul Expr Expr | Div Expr Expr deriving Show
expr :: Parser Expr
expr = buildExpressionParser table factor
<?> "expression"
table = [[op "*" Mul AssocLeft, op "/" Div AssocLeft],
[op "+" Add AssocLeft, op "-" Sub AssocLeft]]
where
op s f assoc = Infix (do{ string s; return f}) assoc
factor = do{ char '('
; x <- expr
; char ')'
; return x}
<|> number
<|> variable
<?> "simple expression"
number :: Parser Expr
number = do{ ds<- many1 digit
; return (Num (read ds))}
<?> "number"
variable :: Parser Expr
variable = do{ ds<- many1 letter
; return (Var ds)}
<?> "variable"
Solution: readExpr input = parse expr "name for error messages" input
and use readExpr.

Answer 1

You can use the function parse which will run a Parser on an input string and return an Either ParseError Expr. I put a simple usage below where I turn that ParseError into a string and pass it along
readExpr :: String -> Either String Expr
readExpr input = case parse expr "name for error messages" input of
Left err -> Left $ "Oh noes parsers are failing: " ++ show err -- Handle error
Right a -> Right a -- Handle success
There are a few other functions, such as parseFromFile, which let you shorthand a few common patterns, to find them, check out the parsec haddock