From: simonpj@microsoft.com <unknown>
Date: Thu, 20 Dec 2007 11:19:29 +0000 (+0000)
Subject: Add groupWith, sortWith, the, to support generalised list comprehensions
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=848e9b5b80d5e25a2120011c4ac7b38ac4e557f8;p=ghc-base.git

Add groupWith, sortWith, the, to support generalised list comprehensions

  This the base-library patch to support the main compiler patch
     Implement generalised list comprehensions

  It just adds three functions to GHC.Exts.
---

diff --git a/GHC/Exts.hs b/GHC/Exts.hs
index f0c2321..7869c47 100644
--- a/GHC/Exts.hs
+++ b/GHC/Exts.hs
@@ -36,6 +36,9 @@ module GHC.Exts
 	-- * Ids with special behaviour
 	lazy, inline,
 
+        -- * Transform comprehensions
+        groupWith, sortWith, the
+
        ) where
 
 import Prelude
@@ -48,4 +51,23 @@ import GHC.Num
 import GHC.Float
 import GHC.Ptr
 import Data.String
+import Data.List
+
+-- | 'the' ensures that all the elements of the list are identical
+-- and then returns that unique element
+the :: Eq a => [a] -> a
+the (x:xs) 
+  | all (x ==) xs = x
+  | otherwise     = error "GHC.Exts.the: non-identical elements"
+the []            = error "GHC.Exts.the: empty list"
+
+-- | The 'sortWith' function sorts a list of elements using the
+-- user supplied function to project something out of each element
+sortWith :: Ord b => (a -> b) -> [a] -> [a]
+sortWith f = sortBy (\x y -> compare (f x) (f y))
 
+-- | The 'groupWith' function uses the user supplied function which
+-- projects an element out of every list element in order to to first sort the 
+-- input list and then to form groups by equality on these projected elements
+groupWith :: Ord b => (a -> b) -> [a] -> [[a]]
+groupWith f = groupBy (\x y -> f x == f y) . sortWith f