Add induceJust

[michaelpj]

From http://hackage.haskell.org/package/witherable-0.3

Given that we can't currently have a graph with a separate key type (#141), I'm constructing a graph of keys, and then I want to map over the graph looking up the value of the key, to get a graph of values.

However, lookup is naturally a -> Maybe b, an so even with a Functor instance (#201) I'd still have a Graph (Maybe value).

Having filter/mapMaybe/catMaybes seems like an fine solution. We could either add them as simple functions or use the Filterable class.

[snowleopard]

Like Functor AdjacencyMap, this has the problem of relying on Ord constraints.

What we could do is to have a type-safe version of induce, like this:

induceJust :: Ord a => AdjacencyMap (Maybe a) -> AdjacencyMap a

Will that work for you?

[michaelpj]

That would certainly work!

[michaelpj]

Also, I've realised that I can work with Graph and then only convert to AdjacencyMap later. So instance Filterable Graph would also do for me. At this point it's mostly a question of whether you want to use the class as part of the API or not.

[michaelpj]

Er no, scratch that, I need to do my SCC computation before I map and induce, so I do care about AdjacencyMap.

[snowleopard]

@michaelpj Great, I'll try to add induceJust soon.

Regarding Filterable: I'd prefer to stay light on dependencies, so let's stick to induceJust for now.

[Avasil]

Is it still relevant? I could try contributing it if nobody is working on it

[snowleopard]

@Avasil Yes, it's still relevant! Please go ahead :)

[snowleopard]

@michaelpj Thanks to @Avasil, we now have induceJust and induceJust1 for non-empty graphs, to be released soon.

[michaelpj]

Thanks!