haskell - Counting and filtering Arrow for HXT -

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i'm trying parse xml, want filter , extract determinate number of children given node. example:

<root>     <node id="a" />     <node id="b" />     <node id="c" />     <node id="d" /> </root>

and if execute arrow getchildren >>> myfilter 2, nodes id "a" , "b".

intuition gives should use state arrow keep track, don't know how that.

i tried myself it's not want, doesn't elegant, , doesn't work. try run chain of arrows runsla , integer parameter initial state, , defining:

takeonly :: iosla int xmltree xmltree takeonly = changestate (\s b -> s-1)              >>> accessstate (\s b -> if s >= 0 b else nothing)

but of course can't return nothing, need return xmltree. don't want return @ all!

there's better way out there. can me?

thanks time , help!

it more idiomatic use combinators in control.arrow.arrowlist handle kind of thing.

the package provides (>>.) :: b c -> ([c] -> [d]) -> b d, "combinator converting result of list arrow list". allows use take function have lists in context.

here's quick version of how might use it:

module main  import text.xml.hxt.arrow  takeonly :: (arrowxml a) => int -> xmltree xmltree takeonly n = getchildren >>. take n   main =   let xml = "<root><node id='a' /><node id='b' />\                   \<node id='c' /><node id='d' /></root>"    print =<< runx (readstring [] xml >>> getchildren >>> takeonly 2)

this believe approximately you're looking for:

travis@sidmouth% ./arrowtake [ntree (xtag (lp node) [ntree (xattr (lp id)) [ntree (xtext "a") []]]) [],  ntree (xtag (lp node) [ntree (xattr (lp id)) [ntree (xtext "b") []]]) []]

no iosla required. note i've changed function type little—this version seems nicer me, convert more type in version.


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