Add preprocess rule

1 files changed, 10 insertions, 0 deletions

diff --git a/src/Hakyll/Core/Rules.hs b/src/Hakyll/Core/Rules.hs
index 450df83..8037ffb 100644
--- a/src/Hakyll/Core/Rules.hs
+++ b/src/Hakyll/Core/Rules.hs
@@ -25,6 +25,8 @@ module Hakyll.Core.Rules
     , route
 
       -- * Advanced usage
+    , preprocess
+    , Dependency (..)
     , rulesExtraDependencies
     ) where
 
@@ -33,6 +35,7 @@ module Hakyll.Core.Rules
 import           Control.Applicative            ((<$>))
 import           Control.Monad.Reader           (ask, local)
 import           Control.Monad.State            (get, modify, put)
+import           Control.Monad.Trans            (liftIO)
 import           Control.Monad.Writer           (censor, tell)
 import           Data.Maybe                     (fromMaybe)
 import           Data.Monoid                    (mempty)
@@ -158,6 +161,13 @@ route route' = Rules $ modify $ \s -> s {rulesRoute = Just route'}
 
 
 --------------------------------------------------------------------------------
+-- | Execute an 'IO' action immediately while the rules are being evaluated.
+-- This should be avoided if possible, but occasionally comes in useful.
+preprocess :: IO a -> Rules a
+preprocess = Rules . liftIO
+
+
+--------------------------------------------------------------------------------
 -- | Advanced usage: add extra dependencies to compilers. Basically this is
 -- needed when you're doing unsafe tricky stuff in the rules monad, but you
 -- still want correct builds.