[Haskell-cafe] A handy little consequence of the Cont monad

[Cale Gibbard]

Hello,

Today on #haskell, resiak was asking about a clean way to write the
function which allocates an array of CStrings using withCString and
withArray0 to produce a new with* style function. I came up with the
following:

nest :: [(r -> a) -> a] -> ([r] -> a) -> a
nest xs = runCont (sequence (map Cont xs))

withCStringArray0 :: [String] -> (Ptr CString -> IO a) -> IO a
withCStringArray0 strings act = nest (map withCString strings)
                                     (\rs -> withArray0 nullPtr rs act)

Originally, I'd written nest without using the Cont monad, which was a
bit of a mess by comparison, then noticed that its type was quite
suggestive.

Clearly, it would be more generally useful whenever you have a bunch
of with-style functions for managing the allocation of resources, and
would like to turn them into a single with-style function providing a
list of the acquired resources.

 - Cale