Re: [Haskell-cafe] Stack overflow with ReadP

Dear Nadine, apparently you are new to parser combinators. I am attaching some code I have used in the past for our first year students as part of our lecture notes on FP. They use the applicatiev style, which I think is to be preferred. I hope you find them instructive. We build a pocket calculator in a numer of steps. In daily life I use my uu-parsinglib, since it comes with fewer surprises for beginning users. If you have any questions do not hesitate to ask me. Succes, Doaitse Swierstra

On 25 Sep 2019, at 16:56, Henry Laxen wrote:

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.