src/Hakyll/Core/DirectedGraph/Internal.hs · hakyll

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1-- | Internal structure of the DirectedGraph type. Not exported outside of the

2-- library.

3--

4module Hakyll.Core.DirectedGraph.Internal

5 ( Node (..)

6 , DirectedGraph (..)

7 ) where

9import Prelude hiding (reverse, filter)

10import Data.Monoid (Monoid, mempty, mappend)

11import Data.Set (Set)

12import Data.Map (Map)

13import qualified Data.Map as M

14import qualified Data.Set as S

16-- | A node in the directed graph

18data Node a = Node

19 { nodeTag :: a -- ^ Tag identifying the node

20 , nodeNeighbours :: Set a -- ^ Edges starting at this node

21 } deriving (Show)

23-- | Append two nodes. Useful for joining graphs.

25appendNodes :: Ord a => Node a -> Node a -> Node a

26appendNodes (Node t1 n1) (Node t2 n2)

27 | t1 /= t2 = error'

28 | otherwise = Node t1 (n1 `S.union` n2)

29 where

30 error' = error $ "Hakyll.Core.DirectedGraph.Internal.appendNodes: "

31 ++ "Appending differently tagged nodes"

33-- | Type used to represent a directed graph

35newtype DirectedGraph a = DirectedGraph {unDirectedGraph :: Map a (Node a)}

36 deriving (Show)

38-- | Allow users to concatenate different graphs

40instance Ord a => Monoid (DirectedGraph a) where

41 mempty = DirectedGraph M.empty

42 mappend (DirectedGraph m1) (DirectedGraph m2) = DirectedGraph $

43 M.unionWith appendNodes m1 m2