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

1 Feb 2008


      The "bit of a mess" that comes from avoiding monads is (my version):
...
import Foreign.Marshal.Array(withArray0)
import Foreign.Ptr(nullPtr,Ptr)
import Foreign.C.String(withCString,CString)
This uses withCString in order of the supplied strings, and a difference list 
([CString]->[CString]) initialized by "id" to assemble the [CString].  This is 
the laziest way to proceed.
...
acquireInOrder :: [String] -> (Ptr CString -> IO a) -> IO a
acquireInOrder strings act =
foldr (\s cs'io'a ->
        (\cs ->
          withCString s (\c -> cs'io'a (cs . (c:))
                        )
        )
      )
      (\cs ->
         withArray0 nullPtr (cs []) act
      )
      strings
      id
This uses in withCString in reversed order of the supplied strings, and normal 
list ([CString]) initialized by "[]" to assemble the [CString].  This is not as 
lazy since it needs to go to the end of the supplied list for the first IO action.
...
acquireInRerverseOrder :: [String] -> (Ptr CString -> IO a) -> IO a
acquireInRerverseOrder strings act =
  foldl (\cs'io'a s ->
          (\cs ->
            withCString s (\c -> cs'io'a (c:cs)
                          )
          )
        )
        (\cs ->
           withArray0 nullPtr cs act
        )
        strings
        []
Cale Gibbard wrote:
...
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

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

ChrisK