[Haskell-cafe] All-fail case in asum

[Юрий Сыровецкий (Yuriy Syrovetskiy)]

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