Implement generic transformation constructor.

1 files changed, 9 insertions, 0 deletions

diff --git a/haddock-api/src/Haddock/Syb.hs b/haddock-api/src/Haddock/Syb.hs
index 3cec724e..2016b74c 100644
--- a/haddock-api/src/Haddock/Syb.hs
+++ b/haddock-api/src/Haddock/Syb.hs
@@ -3,6 +3,7 @@
 
 module Haddock.Syb
     ( everything, everywhere
+    , mkT
     , combine
     ) where
 
@@ -25,6 +26,14 @@ everything k f x = foldl k (f x) (gmapQ (everything k f) x)
 everywhere :: (forall a. Data a => a -> a) -> (forall a. Data a => a -> a)
 everywhere f = f . gmapT (everywhere f)
 
+-- | Create generic transformation.
+--
+-- Another function stolen from SYB package.
+mkT :: (Typeable a, Typeable b) => (b -> b) -> (a -> a)
+mkT f = case cast f of
+    Just f' -> f'
+    Nothing -> id
+
 -- | Combine two queries into one using alternative combinator.
 combine :: Alternative f => (forall a. Data a => a -> f r)
                          -> (forall a. Data a => a -> f r)