[Haskell-cafe] Stack overflow with ReadP

[Doaitse Swierstra]

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

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> On 25 Sep 2019, at 16:56, Henry Laxen <nadine.and.henry at pobox.com> 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
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