This is going to do expr -> parseAdd -> expr -> parseAdd -> ... . You need
to factor your parser to avoid this kind of left recursion.
And (+++) is going to try both parsers even though the left-hand one
succeeds, because it's symmetric. You may want to use (<++) to make it take
the left-hand parser when it succeeds without trying the right-hand.
On Wed, Sep 25, 2019 at 10:56 AM Henry Laxen
Dear Haskell gurus,
There has to be something very simple that I am misunderstanding here. I have tried sticking in trace statement, using the ghci debugger, but it just makes no sense to me. Please tell me what I am missing.
Best wishes, Henry Laxen
--------------------------------------------------------------------------------
module S where
import Text.ParserCombinators.ReadP import Data.Char
data Exp = Num Int | Add Exp Exp deriving (Eq, Show)
expr :: ReadP Exp expr = do e <- (parseNumber +++ parseAdd) return e
parseAdd :: ReadP Exp parseAdd = do e1 <- expr _ <- char '+' e2 <- expr return (Add e1 e2)
parseNumber :: ReadP Exp parseNumber = do ds <- (munch1 isDigit) return . Num . read $ ds
parse s = let parses = (readP_to_S expr) s in case parses of (p : _) -> fst (last parses) _ -> error "parse error" main = do print $ parse "1"
λ> main *** Exception: stack overflow λ> parse "1+2" *** Exception: stack overflow
--------------------------------------------------------------------------------
-- Nadine and Henry Laxen The rest is silence Villa Alta #6 Calle Gaviota #10 Never try to teach a pig to sing Chapala It wastes your time +52 (376) 765-3181 And it annoys the pig _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
-- brandon s allbery kf8nh allbery.b@gmail.com