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-rw-r--r--haddock-api/src/Haddock/Syb.hs16
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.