Fix combination bug

src/SimpleTrie.hs

@@ -0,0 +1,26 @@
+{-# LANGUAGE TemplateHaskell #-}
+module SimpleTrie where
+import Data.Map
+import Control.Lens
+data Trie a b = Trie
+    { _trieMVal    :: Maybe b
+    , _trieChildren :: Map a (Trie a b)
+    }
+makeLenses ''Trie
+
+emptyTrie :: Trie a b
+emptyTrie = Trie Nothing empty
+
+singletonTrie :: Ord a => [a] -> b -> Trie a b
+singletonTrie (k:ks) x = Trie Nothing $ singleton k $ singletonTrie ks x
+singletonTrie [] x = Trie (Just x) empty
+
+insertInTrie :: Ord a => [a] -> b -> Trie a b -> Trie a b
+insertInTrie [] x      = trieMVal .~ Just x
+insertInTrie (k:ks) x  = trieChildren %~ insertWith f k (singletonTrie ks x)
+  where
+    f _ = insertInTrie ks x
+
+lookupTrie :: Ord a => [a] -> Trie a b -> Maybe b
+lookupTrie []     t = _trieMVal t
+lookupTrie (k:ks) t = _trieChildren t !? k >>= lookupTrie ks