Adding state to HXT parsing

Question

I'm trying to write an arrow that will generate numbered elements from an XML parse with HXT. It seems like it should be done with a state arrow, like the State monad, but all the examples I can find in the HXT package and the methods to access the state (XmlState) use the IOStateArrow and I would much rather keep it pure to simplify the testing process, with the StateArrow. Do such variants exist? If not, how would I add state to an HXT parse without having to resort to running the Arrow in IO?

Answer 1

You can use ArrowState instance of SLA for that. An example: let assume we need to concatenate all the second-level text elements.

Prelude Text.XML.HXT.Core> fst $ runSLA (xread >>> getChildren >>> getChildren >>> isText >>> getText >>> changeState (++)) "" "<xml><item>a</item><item>b</item></xml>" 
"ab"

Note, that you can't use IO inside the SLA since in doesn't have ArrowIO instance.

Hope, it is what you need.