[Haskell-cafe] All-fail case in asum

[Oleg Grenrus]

It depends on definition of equivalence for your Functor. When proving Alternative laws for artoparsec's Parser all errors are treated as equivalent: so no problem. Also attoparsec concentrates on the speed, not error reporting:

Parsec parsers can produce more helpful error messages than attoparsec parsers. This is a matter of focus: attoparsec avoids the extra book-keeping in favour of higher performance.



Sent from my iPhone
> On 11 Jun 2016, at 10:15, Юрий Сыровецкий (Yuriy Syrovetskiy) <cblp at cblp.su> wrote:
> 
> 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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