[Haskell-cafe] Re: What do _you_ want to see in FGL?

[Heinrich Apfelmus]

Ivan Lazar Miljenovic wrote:
> Heinrich Apfelmus writes:
>> Ivan Lazar Miljenovic wrote:
>>>
>>> Well, we'll provide a Map-based one that lets you specify the vertex
>>> type as a type parameter; this functionality (type parameter being ued
>>> for the vertex type) won't be required since not all graphs will be able
>>> to chop and choose which vertex type to use.
>>
>> Ah, that would indeed make it easy. Maybe make the map based graph
>> generic, so that it can be used with any primitive graph implementation?
>>
>>    data VertexGraph node gr a b = VG (gr a b) (BiMap node (Vertex gr))
>>
>>    instance (Ord node, Graph gr) => Graph VertexGraph node gr where
> 
> Not sure I understand what the point of the `gr' is there; what I meant
> was something like:
> 
>     newtype MapGraph node a b = MG (Map node (Map node b, a, Map node b))

Yes; what I mean is that you can retrofit a custom vertex type to any
graph implementation that uses a fixed vertex type. So, let's say that

   data Gr a b = .. -- graph with vertex type  Vertex Gr = Int

then

   type Gr' node a b = CustomVertex node Gr a b

   data CustomVertex node gr a b = CV (gr a b) (Map node (Vertex gr))

is a graph with custom vertex type  node .


Regards,
Heinrich Apfelmus

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