Why does this tail recursive function yield <<loop>> error?

Question

I have a strange <<loop>> error with the function below. I present it as is since any attempt to simplify eliminates the error as well.

bellmanFord :: (DistanceMap m) => m -> [Edge] -> Maybe m
bellmanFord m edges = loop m 0 (length edges)
  where
    loop dists step size =
      if step > size then
        Nothing
      else
        let newDists = foldl' proc dists edges
        in
          if identical newDists dists then
            Just dists
          else
            loop newDists (step + 1) size

    proc dists (v1, v2, w) =
      let d = get dists v2
          d' = get dists v1 + Only w
      in
        if d' < d then
          assoc dists v2 d'
        else
          dists

The whole program is available here.

Not a solution to your problem, but why are you counting up to size? Why not count down to 0 from size and use pattern matching to clean up the definition of loop? Something like loop dists 0 = Nothing; loop dists n = let newDists = ... in if ... then Just dists else loop newDists $ n - 1? You'd at least remove the parameter for what step you're on. — bheklilr
– bheklilr, Commented Jul 29, 2014 at 16:21
If removing stuff from your code makes the error disappear, you should take a closer look at the code that you removed right before the error went away. <<loop>> means that you have infinite recursion in one of your functions. — sepp2k
– sepp2k, Commented Jul 29, 2014 at 16:30

sepp2k · Accepted Answer · 2014-07-29 16:39:27Z

4

From your gist:

instance DistanceMap PotMap where
  get m v = min 0 (get m v)

Your definition of get is infinitely recursive.

answered Jul 29, 2014 at 16:39

sepp2k

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1 Comment

Eldar Over a year ago

Indeed, thank you very much and sorry for disturbing.

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