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

1 Nov 2014


      On 31/10/14 23:22, Kim-Ee Yeoh wrote:
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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?
I don't know what you're missing, but when I run the following code, it
doesn't print A.

  import Control.Monad.Except
  import Control.Applicative

  a = putStrLn "A" >> return (Left "hehe")
  b = putStrLn "B" >> return (Right 42)

  main = runExceptT $ ExceptT b <|> ExceptT a

Roman