Re: [Haskell-cafe] <|> that short-circuits in IO ?

On Sat, Nov 01, 2014 at 04:22:28AM +0700, Kim-Ee Yeoh wrote:

On Thu, Oct 30, 2014 at 8:50 PM, Roman Cheplyaka wrote:

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liftA2 (<|>) b a

B A Right 42

(In the latter case I don't want A in the output…)

Wrap it into ExceptT (from the latest transformers), as in

runExceptT $ ExceptT a <|> ExceptT b

With the latest transformers, I still get

B A Right 42

i.e. the A hasn't been eliminated.

What am I missing?

In the actual use case ErrorT was already wrapping IO and that's what I was combining with <|>, so, for now, lacking the latest transformers, I have written a custom infix operator. I haven't had a chance to test Roman's solution and assumed it to be correct. So your observation is a valuable "data point" — thank you! Apologies, I should have tested this myself sooner, S. -- Семен Тригубенко http://trygub.com