Hello!
2015-04-17 4:23 UTC+03:00, Ivan Lazar Miljenovic
### Considering dotgen vs graphviz closer
But looking into the examples, I see that `dotgen` can use "Haskell ids" to identify created nodes, whereas in graphviz's monad (see the
To bring more clear context for any readers, I put here a short excerpt from that dotgen example:
refSpec <- src "S" c1 <- box "S" refSpec .->. c1
example above) one must supply extra strings as the unique ids (by which we refer to the nodes).
Short example:
"start" --> "a0"
node "start" [shape MDiamond]
I used Strings as an example, as I was directly converting an existing piece of Dot code; the original can be found here: http://hackage.haskell.org/package/graphviz-2999.17.0.2/docs/Data-GraphViz-T...
But, you can use any type you like for the node identifiers, as long as you make them an instance of the PrintDot class. That's where the `n` in the `Dot n` type comes in.
Ok, thanks for the valuable information!
I like the first approach more ("Haskell ids").
I admittedly don't have any ability in graphviz to create new identifiers for you. I could (just add a StateT to the internal monadic stack which keeps track of the next unused node identifier)
Since the API is already monadic, adding another monad into the stack wouldn't impose big difficulties for the users of the API, because they won't need to restructure the code (as if it were a transition from some pure functional code into monadic).
but I think that would _reduce_ the flexibility of being able to use your own type (it would either only work for `Dot Int`, or even if you could apply a mapping function to use something like `GraphID`, but that has a problem if you have a `Double` with the same value - and hence same textual representation - as your Int).
I see: [GraphID](http://hackage.haskell.org/package/graphviz-2999.17.0.2/docs/Data-GraphViz-T...) can have distinct values with the same textual representation. But if we are thinking about automatically creating new IDs, then this problem can simply be treated in the code for tracking which IDs have already been used. There could be two APIs: a "flexible" one with user-supplied IDs, and an "automatic" API. The "automatic" one is implemented on top of the "flexible" one.
The way I see it, graphviz is usually used for converting existing Haskell values into Dot code and then processing with dot, neato, etc.
My preference - and hence overall design with graphviz - is that you would generate the graph first, and _then_ convert it to a Dot representation en masse.
If the Haskell representation of the graph doesn't already have unique IDs for the nodes, then such an "automatic" layer would be useful as an intermediate step in the conversion. So it seems it won't be useless even in your standard scenarios. *** You name flexibility for the user as an advantage of the existing approach. As for some advantages of the other approach (with using Haskell ids for the nodes): the compiler could catch more errors. For example, if I make a typo in an identifier when introducing an edge, then Haskell compiler would report this as an unknown identifier. Also the compiler would catch name clashes, if you accidentally give the same id to two different nodes. A potential disadvantage is then an increased verbosity: first, create the nodes, then use them for the edges. Meaning three actions instead of yours single one: "a0" --> "a1" Still, even in the "automatic ids" approach, this can be written compactly in a single line in the spirit of: bindM2 (-->) (node [textLabel "a0"]) (node [textLabel "a1"]) without explicitly giving Haskell ids to the two nodes. Perhaps, this is not important stuff, because--as you write--one is supposed to use Haskell representations of graphs and then convert them with graphviz... (I might simply not want to learn another language for representing graphs apart from dot, that's why I'd like to use the monadic API: because it closely follows the known dot format.) My last line of code already looks similar to a code constructing a Haskell representation of a graph. I'm just writing down my comments concerning the API, not that I'm confident that I know a definite way to make it better. Well, after writing this post and thinking it all over while writing, I tend to come to a conclusion resonating with your opinion stating that the monadic API turned out not as useful as you used to think: it seems that while imposing the monadic style onto the programmer, it doesn't give the advantages a monad could give (like generating unique ids automatically and catching errors with undefined or clashing ids). Without this stateful feature, much else can be done purely with dedicated graph structures. What do you think about these comments? As for dotgen: my wishes could be satisfied simply with the dotgen package, but--as you wrote--it is not safe w.r.t. to quoting/escaping user supplied values. Best regards, -- Ivan