[Haskell-cafe] Stack overflow with ReadP

[David Feuer]

You have "unguarded" recursion between expr and parseAdd. When you try to
parse an expression, one option is to parse an addition. When you go to do
that, you look to parse an expression followed by other stuff. Thanks to
the structure of ReadP, I *believe* this will actually work ... if you just
take the first result. But if you keep going, you will definitely find
yourself stuck.

On Wed, Sep 25, 2019, 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
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