# graphs: A simple monadic graph library

A "not-very-Haskelly" API for calculating traversals of graphs that may be too large to fit into memory. The algorithms included are inspired by the visitor concept of the Boost Graph Library.

Here is a very simple example of how we might execute a depth-first-search. In this case the visitor simply collects the edges and vertices in the order that the corresponding functions get called. After the necessary imports,

import Data.Array import Data.Monoid import Data.Graph.AdjacencyList import Data.Graph.Algorithm import Data.Graph.Algorithm.DepthFirstSearch

create an adjacency list where the vertices are labeled with integers.

graph :: Array Int [Int] graph = array (0, 3) [(0, [1,2]), (1, [3]), (2, [3]), (3, [])]

We need a data structure that instantiates `Monoid`

to combine the results of
our visitor functions.

data Orderings = Orderings { enterV :: [Int] , enterE :: [(Int, Int)] , gray :: [(Int, Int)] , exitV :: [Int] , black :: [(Int, Int)] } deriving Show instance Monoid Orderings where mempty = Orderings [] [] [] [] [] mappend (Orderings a1 a2 a3 a4 a5)(Orderings b1 b2 b3 b4 b5) = Orderings (a1 ++ b1) (a2 ++ b2) (a3 ++ b3) (a4 ++ b4) (a5 ++ b5)

The `dfs`

function's first argument is of type `GraphSearch`

which is
a visitor containing the functions to be run at various times during the search.
The second argument is the starting vertex for the search.

orderings :: GraphSearch (AdjacencyList Int) Orderings orderings = GraphSearch (\v -> return $ mempty {enterV = [v]}) (\e -> return $ mempty {enterE = [e]}) (\e -> return $ mempty {gray = [e]}) (\v -> return $ mempty {exitV = [v]}) (\e -> return $ mempty {black = [e]})

Finally `runAdjacencylist`

unwraps the function in the `Adjacencylist`

newtype and runs
it on `graph`

.

dfsTest :: Orderings dfsTest = runAdjacencyList (dfs orderings 0) graph

Running `dfsTest`

in ghci will yield:

Orderings {enterV = [0,2,3,1], enterE = [(0,2),(2,3),(0,1)], gray = [], exitV = [3,2,1,0], black = [(1,3)]}

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Versions [RSS] | 0.1, 0.2, 0.3, 0.3.1, 0.3.2, 0.3.2.1, 0.3.2.2, 0.3.2.3, 0.4, 0.4.0.1, 0.4.0.3, 0.4.1, 0.5, 0.5.0.1, 0.6, 0.6.0.1, 0.7, 0.7.1, 0.7.2 |
---|---|

Change log | CHANGELOG.markdown |

Dependencies | array (>=0.3 && <0.7), base (>=4 && <5), containers (>=0.3 && <0.7), semigroups (>=0.16 && <1), transformers (>=0.2.2 && <0.7), transformers-compat (>=0.3 && <1), void (>=0.5.5.1 && <1) [details] |

License | BSD-3-Clause |

Copyright | Copyright (C) 2011-2013 Edward A. Kmett |

Author | Edward A. Kmett |

Maintainer | Edward A. Kmett <ekmett@gmail.com> |

Category | Algorithms, Data Structures, Graphs |

Home page | http://github.com/ekmett/graphs |

Bug tracker | http://github.com/ekmett/graphs/issues |

Source repo | head: git clone git://github.com/ekmett/graphs.git |

Uploaded | by ryanglscott at 2022-05-07T23:42:40Z |

Distributions | LTSHaskell:0.7.2, NixOS:0.7.2, Stackage:0.7.2 |

Reverse Dependencies | 1 direct, 0 indirect [details] |

Downloads | 14307 total (22 in the last 30 days) |

Rating | (no votes yet) [estimated by Bayesian average] |

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Status | Docs available [build log] Last success reported on 2022-05-08 [all 1 reports] |