[Haskell-cafe] Beginners arrow question

Miguel Mitrofanov miguelimo38 at yandex.ru
Sat Apr 5 14:19:24 EDT 2008

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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