[Haskell-cafe] All-fail case in asum

[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/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
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