Changes between Version 5 and Version 6 of BlockObjects

Timestamp:

08/12/11 06:14:41 (23 months ago)

Author:

chak

Comment:

--

BlockObjects

@@ -10 +10 @@
 qsort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));
 }}}
-In C, we might use this function as described in Apple's introduction to block: [http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Blocks/Articles/bxGettingStarted.html#//apple_ref/doc/uid/TP40007502-CH7-SW2 Using a Block Directly].  We would like to be able to do the same in Haskell by writing:
+In C, we might use this function as described in Apple's introduction to blocks: [http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Blocks/Articles/bxGettingStarted.html#//apple_ref/doc/uid/TP40007502-CH7-SW2 Using a Block Directly].  We would like to be able to do the same in Haskell by declaring:
 {{{
 foreign import ccall qsort_b "stdlib.h" :: Ptr a -> CSize -> CSize -> (Ptr a -> Ptr a -> Int) -> IO ()
+
+myCharacters = ["TomJohn", "George", "Charles Condomine"]
+}}}
+and then executing
+{{{
+do
+    -- convert a list of strings into a C array of stable pointers to those strings in the Haskell heap
+  myCharactersArray <- newArray $ mapM newStablePtr myCharacters
+ 
+   -- get the size in bytes of a stable pointer to a Haskell string
+  let elemSize = fromInteger $ sizeof (undefined :: StablePtr String)
+
+    -- invoke C land 'qsort_b' with a Haskell comparison function passed as a block object; mutates 'myCharactersArray'
+  qsort_b myCharactersArray (length myCharacters) elemSize (\l r -> fromOrdering (l `compare` r))
+
+    -- turn the array of Haskell strings back into a list of strings
+  mySortedCharacters <- mapM deRefStablePtr myCharactersArray
+}}}
+Here we compare entire strings and not just the first characters as in the C implementation.  The marshalling function `fromOrdering` is defined as follows:
+{{{
+fromOrdering :: Ordering -> Int
+fromOrdering LT = -1
+fromOrdering EQ = 0
+fromOrdering GT = 1
 }}}