Re: [Haskell-beginners] while loop

11 Feb 2016

      Hi,

In these cases, I write the explicitly recursive version (e.g. getList1)
and look for a pattern (e.g. getList2).

getList1 :: Ptr List -> IO [Ptr Item]
getList1 l = c_get_first_item l >>= go
   where
      go e
         | e == nullPtr = return []
         | otherwise    = do
               next <- c_get_next_item l e
               es   <- go next
               return (e:es)

getList2 :: Ptr List -> IO [Ptr Item]
getList2 l = c_get_first_item l >>= unfoldrM go
   where
      go e
         | e == nullPtr = return Nothing
         | otherwise    = do
               next <- c_get_next_item l e
               return (Just (e,next))

I hope it helps
Sylvain

2016-02-11 12:02 GMT+01:00 PICCA Frederic-Emmanuel <
frederic-emmanuel.picca@synchrotron-soleil.fr>:
...
Hello,
I am playing with FFI and I need to extract from a  C API a list of
pointers.
I have two methods available
c_get_first_item :: Ptr List -> IO (Ptr Item)
c_get_next_item :: Ptr List -> Ptr Item -> IO (Ptr Item)
I would like to obtain a [Ptr Item]
I try to used whileM but I did not find how to inject the first item in
the loop.
whileM (return . ( /= nullPtr)) (c_get_next_item list item)
the c_get_next_item return a nullPtr when there is no remaining item.
what is the haskell way in order to extract a list of pointer using these
C methods ?
thanks for your help
Frederic
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Re: [Haskell-beginners] while loop

Sylvain Henry