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Today's Topics:
1. Problem with Parser combinators (Baa)
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Message: 1
Date: Fri, 3 Nov 2017 13:43:40 +0200
From: Baa <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Problem with Parser combinators
Message-ID: <20171103134340.58a7b386@Pavel>
Content-Type: text/plain; charset=US-ASCII
Hello, all!
I try this parser for `Read` instance:
import qualified Text.ParserCombinators.ReadP as P
import qualified Text.ParserCombinators.ReadPrec as RP
import Data.Text as T
instance Read RV where
readPrec = parens $ do
RP.lift P.skipSpaces
s <- RP.look
guard (not $ null s)
(RP.+++) (RV1 <$> readPrec::RP.ReadPrec RV) (pure $ RV2 $ T.pack s)
And see that `+++` (or <|>) works in strange way (I don't understand
what happens). IMHO if first parser fails (i.e. `RV1 <$> readPrec::RP.ReadPrec
RV`)
then second should return `RV2 $ T.pack s`). But seems that if 1st
fails then all fail too! I tried `+++`, `<++`, `<|>` - the same result.
How is it possible?!
I need to return something if parsing fails. Best is to return another
parser but initial problem was that alternative parser must return the
same type as first one. So, I decided to return "constant" - here it is
`RV2 $ T.pack s`. Guarding from empty string is needed for this
alternative value which I'll return when 1st parser fails. But... seems
something is wrong.
===
Best regards, Paul
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