On Sun, Dec 15, 2013 at 3:32 PM, Nikita Karetnikov
I’m trying to write a wrapper for a C function. Here is an example of such code:
... s' <- newCString s t' <- newCString t let n = c_strcmp s' t' -- free s' -- free t'
...
It'd be better to use withCString [1] instead, to avoid a memory leak if an exception occurs between newCString and free. Also, make c_strcmp an IO function: foreign import ccall "string.h strcmp" c_strcmp :: CString -> CString -> IO CInt The way you had it, c_strcmp had an implicit unsafePerformIO, which we don't want or need here. strcmp has the side effect of reading from pointers at a given moment in time. As Brandon brought up, right now, your code might as well say: strcmp s t = unsafePerformIO $ do s' <- newCString s t' <- newCString t -- free s' -- free t' return $ case c_strcmp s' t' of _ | n == 0 -> EQ | n < 0 -> LT | otherwise -> GT Because of laziness, n = c_strcmp s' t' isn't evaluated until it is needed. What you should say instead is: ... n <- c_strcmp s' t' ... This places the strcmp in the monadic chain so it will run at the right time (after newCString and before freeCString). [1]: http://hackage.haskell.org/package/base/docs/Foreign-C-String.html#v:withCSt...