<|> that short-circuits in IO ? - Haskell-Cafe - Haskell.org

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Semen Trygubenko / Семен Тригубен ко

30 Oct 2014 30 Oct '14

1:42 p.m.

Dear Haskell-Cafe, Is there a way to get Control.Applicative.<|> to short-circuit when combining two IO actions? E.g.

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

Prelude> liftA2 (<|>) a b

A B Right 42

liftA2 (<|>) b a

B A Right 42 (In the latter case I don't want A in the output…) Many thanks for your help, Semen -- Семен Тригубенко http://trygub.com

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Roman Cheplyaka

30 Oct 30 Oct

1:50 p.m.

On 30/10/14 15:42, Semen Trygubenko / Семен Тригубенко wrote:

Dear Haskell-Cafe,

Is there a way to get Control.Applicative.<|> to short-circuit when combining two IO actions?

E.g.

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

...
Prelude> liftA2 (<|>) a b

A B Right 42

...
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 Roman

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Kim-Ee Yeoh

31 Oct 31 Oct

9:22 p.m.

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

...
...
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? -- Kim-Ee

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Semen Trygubenko / Семен Тригубен ко

11:43 p.m.

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:

...
...
...
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

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Chris Wong

1 Nov 1 Nov

6:09 a.m.

With the latest transformers, I still get

B A Right 42

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

What am I missing?

Can you show us your code? Looking at the source: http://hackage.haskell.org/package/transformers-0.4.1.0/docs/src/Control-Mon... http://hackage.haskell.org/package/transformers-0.4.1.0/docs/src/Control-Mon... both ExceptT and ErrorT have short-circuiting <|> and mplus. Chris

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Roman Cheplyaka

10:38 a.m.

On 31/10/14 23:22, Kim-Ee Yeoh wrote:

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

...
...
...
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

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Kim-Ee Yeoh

2 Nov 2 Nov

2:01 p.m.

Chris, thank you for the links. I now understand ExceptT a lot better. Roman, thank you for the full program. What happened was I copy-pasted runExceptT $ ExceptT a <|> ExceptT b from your email into ghci. But you said the latest transformers was required, so I did the install. Finally, I saw an "A" printed out and went "Hey, that's not right!" Returning to the mail client, I referred back to Semen's original query and copy-pasted from there. But really it was A then B that I saw and not B then A. So mea culpa. Truth be told, all that while I was figuring out how to get Semen's "liftA2 (<|>)" to work and suffered cognitively from the lazy I/O solution I pursued. -- Kim-Ee

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Chris Wong
Kim-Ee Yeoh
Roman Cheplyaka
Semen Trygubenko / Семен Тригубен ко