Your parseAdd parser is left recursive which causes infinite recursion. Maybe this wikipedia article can help: https://en.wikipedia.org/wiki/Left_recursion#Removing_left_recursion On 25-09-2019 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
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