Handling Errors

[Simon Marlow]

> > The question is, if newCString fails to get allocation to s1 or s2,
> > how must i handle this error?. 
> 
> Using the normal Haskell98 IOError mechanisms:
> 
>   http://www.haskell.org/onlinereport/io-13.html
> 
> More usefully, you can use the GHC extensions (also supported 
> by Hugs):
> 
>   http://www.haskell.org/ghc/docs/latest/html/base/GHC.Exception.html

Slight correction: the right library to use is Control.Exception:

http://www.haskell.org/ghc/docs/latest/html/base/Control.Exception.html

GHC.Exception is not indended for external consumption, as is the case with all the GHC.* libraries except for GHC.Exts - the clue
is "Stability: internal" in the header.  We should really omit these from the docs.

> of which the most useful are bracket and bracket_ which provide
> functionality like Java/C++'s try/finally syntax.
> 
>   bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
>   bracket_ :: IO a -> (a -> IO b) -> IO c -> IO c
> 
> e.g., 
> 
> main = bracket (newCString s1) free $ \p1 ->
>        bracket (newCString s2) free $ \p2 ->
>        let p3 = hstrL p2 p1
>        in peekCString p3 >>= \s ->
>           print s
> 
> This code will take care to free the two strings even if an 
> exception is raised.

In cases like the above, you almost certainly want to use withCString which also guarantees to free the string if an exception is
raised.

Cheers,
	Simon