Re: [Haskell-cafe] Conditional IO ?

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!

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