Function crossMapP for fixing desugaring of comprehensions

Merge into 6.6 branch.

diff --git a/GHC/PArr.hs b/GHC/PArr.hs
index 853fc00..0b5154e 100644 (file)
--- a/GHC/PArr.hs
+++ b/GHC/PArr.hs
@@ -137,6 +137,7 @@ module GHC.PArr (
   bpermuteP,           -- :: [:Int:] -> [:e:] ->          [:e:]
   dpermuteP,           -- :: [:Int:] -> [:e:] -> [:e:] -> [:e:]
   crossP,              -- :: [:a:] -> [:b:] -> [:(a, b):]
+  crossMapP,           -- :: [:a:] -> (a -> [:b:]) -> [:(a, b):]
   indexOfP             -- :: (a -> Bool) -> [:a:] -> [:Int:]
 ) where
 
@@ -532,6 +533,25 @@ crossP a1 a2  = let
                zipP x1 x2
  -}
 
+-- |Compute a cross of an array and the arrays produced by the given function
+-- for the elements of the first array.
+--
+crossMapP :: [:a:] -> (a -> [:b:]) -> [:(a, b):]
+crossMapP a f = let
+                 bs   = mapP f a
+                 segd = mapP lengthP bs
+                 as   = zipWithP replicateP segd a
+               in
+               zipP (concatP as) (concatP bs)
+
+{- The following may seem more straight forward, but the above is very cheap
+   with segmented arrays, as `mapP lengthP', `zipP', and `concatP' are
+   constant time, and `map f' uses the lifted version of `f'.
+
+crossMapP a f = concatP $ mapP (\x -> mapP ((,) x) (f x)) a
+
+ -}
+
 -- computes an index array for all elements of the second argument for which
 -- the predicate yields `True' (EXPORTED)
 --