[Haskell-cafe] Stack overflow with ReadP

[Brandon Allbery]

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 <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
> _______________________________________________
> 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 at gmail.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20190925/f0faea71/attachment.html>