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PO
primarykey
Id
7031890
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0
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0
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2011-08-11T19:45:05.117
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0
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2011-08-11T19:45:05.117
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371753
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7030476
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2
Score
4
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0
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text
Body
Here's a horrible two typeclass solution. The first part (named, unhelpfully, <code>foo</code>) will take things of types like <code>Double -> Double -> CString -> IO ()</code> and turn them into things like <code>IO (Double -> IO (Double -> IO (String -> IO ())))</code>. So each conversion is forced into IO just to keep things fully uniform. The second part, (named <code>cio</code> for "collapse io) will take those things and shove all the <code>IO</code> bits to the end. <pre><code>class Foo a b | a -> b where foo :: a -> b instance Foo (IO a) (IO a) where foo = id instance Foo a (IO b) => Foo (CString -> a) (IO (String -> IO b)) where foo f = return $ \s -> withCString s $ \cs -> foo (f cs) instance Foo a (IO b) => Foo (Double -> a) (IO (Double -> IO b)) where foo f = return $ \s -> foo (f s) class CIO a b | a -> b where cio :: a -> b instance CIO (IO ()) (IO ()) where cio = id instance CIO (IO b) c => CIO (IO (a -> IO b)) (a -> c) where cio f = \a -> cio $ f >>= ($ a) {- *Main> let x = foo (undefined :: Double -> Double -> CString -> IO ()) *Main> :t x x :: IO (Double -> IO (Double -> IO (String -> IO ()))) *Main> :t cio x cio x :: Double -> Double -> String -> IO () -} </code></pre> Aside from being a generally terrible thing to do, there are two specific limitations. The first is that a catchall instance of <code>Foo</code> can't be written. So for every type you want to convert, even if the conversion is just <code>id</code>, you need an instance of <code>Foo</code>. The second limitation is that a catchall base case of <code>CIO</code> can't be written because of the <code>IO</code> wrappers around everything. So this only works for things that return <code>IO ()</code>. If you want it to work for something returning <code>IO Int</code> you need to add that instance too. I suspect that with sufficient work and some typeCast trickery these limitations can be overcome. But the code is horrible enough as is, so I wouldn't recommend it.
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1. POHaskell Polyvariadic Function With IO
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