[Haskell] Single Source All Paths

[Lazy]

Hi all,

This is a little program I put together to practice Haskell. The specs can be found here: http://www.doc.ic.ac.uk/~ajf/Teaching/Haskell/sampleTest2.pdf, although I deviated a little from the assignment.

It's far from perfect. The instance declaration of Finite is incomplete, and I feel that some things can be done more elegantly. Time permiting, I'll try to rewrite it as a State monad, to carry around the graph.

All comments and advice are appreciated.

code:

import List type Id = Int type Weight = Int type Edge  = ( Id, Id ) type Graph = [ ( Edge, Weight ) ] data Cost = Finite Weight | Infinity                 deriving ( Eq, Show ) edge     = fst cost     = snd fromId = ( fst . fst ) toId   = ( snd . fst ) instance Num Cost where         Infinity + _ = Infinity         _ + Infinity = Infinity         Finite a + Finite b = Finite ( a + b )         instance Ord Cost where         Infinity <= Infinity = True         Infinity <= Finite _ = False         Finite _ <= Infinity = True         Finite a <= Finite b = a <= b solve :: Id -> Graph -> [ ( Edge, Cost ) ] solve s graph = solve' s ( costs s graph ) [] where         solve' :: Id -> [ ( Edge, Cost ) ] -> [ ( Edge, Cost ) ] -> [ ( Edge, Cost ) ]         solve' _ [] n = n         solve' s c acc = solve' s c' acc' where                  minp  = cheapest c                  restp = delete minp c                  c' = map ( \ x -> relax x minp graph ) restp                  acc' = minp : acc   allIds :: Graph -> [ Id ] allIds graph = nub $ concat [ ( [ fromId g ] ++ [ toId g ] ) | g <- graph ] lookUpCost :: Edge -> Graph -> Cost lookUpCost edge graph = let found = lookup edge graph in                   case found of                   Nothing -> Infinity                   Just a  -> Finite a costs :: Id -> Graph -> [ ( Edge , Cost ) ] costs s graph = [  ( ( s, x ), lookUpCost ( s, x ) graph ) | x <- allIds graph ] cheapest :: [ ( Edge, Cost ) ] -> ( Edge, Cost ) cheapest n = minimumBy ( \ x y -> compare ( cost x ) ( cost y ) ) n relax :: ( Edge, Cost ) -> ( Edge, Cost ) -> Graph -> ( Edge, Cost ) relax p minp graph = ( edge p, min ( cost p ) transit  ) where         transit = lookUpCost (fromId p, toId minp) graph + lookUpCost (toId minp, toId p) graph                        -- test data test :: Graph test= [ ( ( 0, 1 ), 1 ),         ( ( 0, 2 ), 3 ),         ( ( 0, 4 ), 6 ),         ( ( 1, 2 ), 1 ),         ( ( 1, 3 ), 3 ),         ( ( 2, 0 ), 1 ),         ( ( 2, 1 ), 2 ),         ( ( 2, 3 ), 1 ),         ( ( 3, 0 ), 3 ),         ( ( 3, 4 ), 2 ),         ( ( 4, 3 ), 1 ),         ( ( 5, 2 ), 9 ) ]

[Tyr_God_Of_War]

The pdf is gone, could you state the problem here?

[apython1992]

Um...that post is about five years old.