Conditional IO ?

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Dmitri O.Kondratiev

20 Jun 2011 20 Jun '11

8 a.m.

Hi, What is right way to do conditional IO? For example, I need to write to file errors only in case they exist, otherwise my function should do nothing: handleParseErrors errors | (not . null) errors = writeFile "parse-errors.txt" (show errors) | otherwise = ? What should be an 'otherwise' case ? Thanks!

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Arlen Cuss

20 Jun 20 Jun

8:08 a.m.

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 20/06/11 18:00, Dmitri O.Kondratiev wrote:

...

Hi, What is right way to do conditional IO? For example, I need to write to file errors only in case they exist, otherwise my function should do nothing:

handleParseErrors errors | (not . null) errors = writeFile "parse-errors.txt" (show errors) | otherwise = ?

What should be an 'otherwise' case ?

How about `return ()'? -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQIcBAEBAgAGBQJN/v/hAAoJEDiWqExGnQ/Q7VQP/js4XZc92bHeM8f/ErQeQ4LT UwINtt8WFtTWevQtwfO6wNc96CVcjnybb2SwPqnQMISnb4EPc9RvfFz24FTBneQ9 d7MD44uNnW1wAgY8VKaQQJ2v5VVrUd8Gq/m0toUKuefaT9yKMVIG/EeEPaLEUe3y tJUyykCXcUpTzllDyk9kjb/1Og9CNWD+y4rdrWSldWCwgIPMHE2nwNOvs6RquPDQ Om5eFdk2AFwAgDl1FIR2EiuWdQ3jUVPaDsHC0L24fvCeC/TA6AoHmC0panCYCchc 8nEruqwmiA/nU+Uh1Z6SWEicBeuWcXSk6iPuRYM7xU9dDLKFNEHAvCTLYdg2+zwR sovOOYrOEnQ8FDwyZGXC7atxrNGl/nk09Q2RNkXryGo/nNdkUG0AuvoXI5gn8Q6v 7mZiXJr2AbUFqcDA4qXOj8IwddtU5hvwqkGj7xSqCck9sHWWXjZLrJqFH+mW8oMU 8rHVkSpMEFH0vbfvvgRMqzdcpnuDKa4/t+o8wvJy/lr9pdhP2o2azpxvb6NFlDpP dgDwnOe/v8c9Mz7tqs41k94SYTVk2lNpsoXShVoB9tg0EttHKjDNM2ykGJl90Hcf 6vJUGEyNqE78s6qMxiAauxatlQdkw40j2ifcriElj5U7cogpZUGxnb/RODsl0rqn BHPJVkHC/iEQ+t/4lfaT =HGbC -----END PGP SIGNATURE-----

Eugene Kirpichov

8:08 a.m.

| otherwise = return () or: handleParseErrors errors = when (not . null $ errors) $ writeFile ..... 2011/6/20 Dmitri O.Kondratiev :

...

Hi, What is right way to do conditional IO? For example, I need to write to file errors only in case they exist, otherwise my function should do nothing:

handleParseErrors errors | (not . null) errors = writeFile "parse-errors.txt" (show errors) | otherwise = ?

What should be an 'otherwise' case ?

Thanks!

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

Lyndon Maydwell

8:14 a.m.

Your errors branch has the type writeFile "parse-errors.txt" (show errors) :: IO () This means that your otherwise branch should have the same type. You can use the return function that has the type return :: Monad m => a -> m a specialised to m = IO in conjunction with the value () :: () giving return () :: IO () There is also the when function that eliminates the else case for conditional IO: http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Mon... On Mon, Jun 20, 2011 at 4:00 PM, Dmitri O.Kondratiev wrote:

...

Hi, What is right way to do conditional IO? For example, I need to write to file errors only in case they exist, otherwise my function should do nothing:

handleParseErrors errors | (not . null) errors = writeFile "parse-errors.txt" (show errors) | otherwise = ?

What should be an 'otherwise' case ?

Thanks!

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Dmitri O.Kondratiev

8:33 a.m.

Thanks! Everything works, and 'when' is really nice. ( I still have only basic monad knowledge, need more time to spend on existing libraries) On Mon, Jun 20, 2011 at 12:14 PM, Lyndon Maydwell wrote:

...

Your errors branch has the type

writeFile "parse-errors.txt" (show errors) :: IO ()

This means that your otherwise branch should have the same type.

You can use the return function that has the type

return :: Monad m => a -> m a

specialised to m = IO

in conjunction with the value

() :: ()

giving

return () :: IO ()

There is also the when function that eliminates the else case for conditional IO:

http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Mon...

On Mon, Jun 20, 2011 at 4:00 PM, Dmitri O.Kondratiev wrote:

...
Hi, What is right way to do conditional IO? For example, I need to write to file errors only in case they exist, otherwise my function should do nothing:

handleParseErrors errors | (not . null) errors = writeFile "parse-errors.txt" (show errors) | otherwise = ?

What should be an 'otherwise' case ?

Thanks!

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe