Monoid instance for pattern

1 files changed, 7 insertions, 1 deletions

diff --git a/src/Hakyll/Core/Identifier/Pattern.hs b/src/Hakyll/Core/Identifier/Pattern.hs
index 42f67f0..28e23ad 100644
--- a/src/Hakyll/Core/Identifier/Pattern.hs
+++ b/src/Hakyll/Core/Identifier/Pattern.hs
@@ -47,7 +47,7 @@ import Data.List (isPrefixOf, inits, tails)
 import Control.Arrow ((&&&), (>>>))
 import Control.Monad (msum)
 import Data.Maybe (isJust)
-import Data.Monoid (mempty, mappend)
+import Data.Monoid (Monoid, mempty, mappend)
 
 import GHC.Exts (IsString, fromString)
 
@@ -68,6 +68,12 @@ data Pattern = Glob [GlobComponent]
 instance IsString Pattern where
     fromString = parseGlob
 
+instance Monoid Pattern where
+    mempty = Predicate (const True)
+    g@(Glob _)  `mappend` x           = Predicate (matches g) `mappend` x
+    x           `mappend` g@(Glob _)  = x `mappend` Predicate (matches g)
+    Predicate f `mappend` Predicate g = Predicate $ \i -> f i && g i
+
 -- | Parse a pattern from a string
 --
 parseGlob :: String -> Pattern