[Haskell-cafe] All-fail case in asum

[Юрий Сыровецкий (Yuriy Syrovetskiy)]

So should we consider it a bug in attoparsec? And should somebody fix it?
On 11 Jun 2016 10:04, "Erik Hesselink" <hesselink at gmail.com> wrote:

> OK, so you're using attoparsec (through aeson). That's valuable
> information would have been useful to include in your initial email.
>
> After a bit of experimenting and reading the docs, I think the problem
> is that attoparsec's Alternative instance isn't completely correct.
> From the docs, Alternative is supposed to be "a monoid on applicative
> functors". The Monoid laws say, among other things, that `mappend x
> mempty = x`. For Applicative, this would mean `x <|> empty = x`.
> However, in attoparsec:
>
>     λ parse (fail "boom" <|> empty) "hello"
>     Fail "hello" [] "Failed reading: empty"
>
> In parsec, on the other hand:
>
>     λ parse (fail "boom" <|> empty) "" "hello"
>     Left (line 1, column 1):
>     boom
>
> So I don't think changing Applicative or asum is the way to go here.
>
> Erik
>
> On 11 June 2016 at 07:15, Юрий Сыровецкий (Yuriy Syrovetskiy)
> <cblp at cblp.su> wrote:
> > λ> :show imports
> > import Prelude -- implicit
> > import Data.Aeson.Types
> > import Data.Foldable
> > import Control.Applicative
> > λ> parse (\v -> withObject "Object" (\_ -> pure "OK") v <|> fail
> > "Expected object") Null
> > Error "Expected object"
> > λ> parse (\v -> asum [withObject "Object" (\_ -> pure "OK") v, fail
> > "Expected object"]) Null
> > Error "empty"
> >
> > Maybe instance Alternative Parser should be changed to ignore "empty"?
> >
> > 2016-06-10 20:01 GMT+03:00 Oleg Grenrus <oleg.grenrus at iki.fi>:
> >> If you aren’t scared of the dependencies: semigroupoids has asum1 (yet
> it’s easy to write yourself)
> >>
> >>
> https://s3.amazonaws.com/haddock.stackage.org/lts-6.2/semigroupoids-5.0.1/Data-Semigroup-Foldable.html#v:asum1
> >>
> >> λ Prelude Data.Semigroup.Foldable Data.List.NonEmpty > asum1 ([Right 1,
> Right 2, Left "error"] :: NonEmpty (Either String Int))
> >> Right 1
> >> λ Prelude Data.Semigroup.Foldable Data.List.NonEmpty > asum1 ([Left
> "error"] :: NonEmpty (Either String Int))
> >> Left "error"
> >>
> >> - Oleg
> >>
> >>> On 10 Jun 2016, at 17:37, Юрий Сыровецкий (Yuriy Syrovetskiy) <
> cblp at cblp.su> wrote:
> >>>
> >>> Hello
> >>>
> >>> I want to define some parser such way:
> >>>
> >>>    myParser = tryA <|> tryB <|> fail "input must be either A or B"
> >>>
> >>> It works. But then I want to rewrite it with asum:
> >>>
> >>>    myParser = asum [tryA, tryB, fail "must be A or B"]
> >>>
> >>> It works, but the wrong way. Instead of my error it writes "empty".
> >>> Just "empty".
> >>>
> >>> It is so because in base library
> >>>
> >>>    asum = foldr (<|>) empty
> >>>
> >>> What if it was defined
> >>>
> >>>    asum [] = empty
> >>>    asum [x:xs] = x <|> asum xs
> >>>
> >>> It would help me with my parser. But what can this break? Why isn't
> >>> this done yet?
> >>>
> >>> --
> >>> Yuriy Syrovetskiy, http://cblp.su
> >>> _______________________________________________
> >>> Haskell-Cafe mailing list
> >>> Haskell-Cafe at haskell.org
> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> >>>
> >>
> >
> >
> >
> > --
> > Yuriy Syrovetskiy, http://cblp.su
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>
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