You can't use variables introduced in "proc" anywhere except on the right side of "-<". OK, you actually can do this, but your arrow should be an instance of ArrowApply (which means that you don't really need it at all, since all such arrows are in fact monads). Your "myProblem" is desugared as something like arr (\xml -> (xml,xml)) >>> second (getAttrValue "name") >>> second (arr (\name -> (name,name))) >>> second (second (arr lookupFormatter))
arr (\(xml,(name,fmt)) -> xml) >>> fmt
On 5 Apr 2008, at 22:05, Paul Johnson wrote:
I'm using arrows for the first time, with HXT. I think I'm getting the hang of it, but one problem has me stumped. I have a function "lookupFormatter" which takes a string and returns an arrow, and I want to use that arrow. Something like this:
myProblem :: (ArrowXml a) -> a XmlTree String myProblem = proc xml do name <- getAttrValue "name" -< xml fmt <- arr lookupFormatter -< name fmt -< xml
But I can't figure out how to do it. I get "fmt: not in scope".
Can anyone help?
Thanks,
Paul.
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