[Haskell-cafe] Stack overflow with ReadP

Henry Laxen nadine.and.henry at pobox.com
Wed Sep 25 14:56:00 UTC 2019

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
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