Add SYB-like utility function for performing stateful queries.

1 files changed, 15 insertions, 1 deletions

diff --git a/haddock-api/src/Haddock/Syb.hs b/haddock-api/src/Haddock/Syb.hs
index 2016b74c..4847e486 100644
--- a/haddock-api/src/Haddock/Syb.hs
+++ b/haddock-api/src/Haddock/Syb.hs
@@ -2,7 +2,7 @@
 
 
 module Haddock.Syb
-    ( everything, everywhere
+    ( everything, everythingWithState, everywhere
     , mkT
     , combine
     ) where
@@ -20,6 +20,20 @@ everything :: (r -> r -> r) -> (forall a. Data a => a -> r)
            -> (forall a. Data a => a -> r)
 everything k f x = foldl k (f x) (gmapQ (everything k f) x)
 
+
+-- | Perform a query with state on each level of a tree.
+--
+-- This is the same as 'everything' but allows for stateful computations. In
+-- SYB it is called @everythingWithContext@ but I find this name somewhat
+-- nicer.
+everythingWithState :: s -> (r -> r -> r)
+                    -> (forall a. Data a => a -> s -> (r, s))
+                    -> (forall a. Data a => a -> r)
+everythingWithState s k f x =
+    let (r, s') = f x s
+    in foldl k r (gmapQ (everythingWithState s' k f) x)
+
+
 -- | Apply transformation on each level of a tree.
 --
 -- Just like 'everything', this is stolen from SYB package.