Re: [Haskell-cafe] All-fail case in asum

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 :

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/Da...

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) 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@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- Yuriy Syrovetskiy, http://cblp.su