HXT and types in Control.Arrow.ArrowTree
Hello, I'm having some trouble with HXT, the types of some functions in particular. This may well be caused by a lack of understanding for arrow programming -- I'd appreciate any hints. In short, I'm constantly running into what appear to be artificial type restrictions in Control.Arrow.ArrowTree. For example, the signature of "deep" is deep :: (Tree t, ArrowTree a) => a (t b) (t b) -> a (t b) (t b) instead of the more general deep :: (Tree t, ArrowTree a) => a (t b) c -> a (t b) c Say I have an arrow getLink :: (ArrowXml a) => a XmlTree Link that converts an HTML link (<a href="url">text</a>) into some appropriate data type (code below). To collect all links in a document, I'd like to use deep getLink instead of deep (hasName "a") >>> getLink Is this the wrong approach to converting XML into Haskell data types? Cheers Robert module Links where import Text.XML.HXT.Arrow data Link = Link { url, text :: String } deriving Show getLink :: (ArrowXml a) => a XmlTree Link getLink = isElem >>> hasName "a" >>> (getAttrValue0 "href" &&& getAllText) >>> arr (uncurry Link) getAllText :: (ArrowXml a) => a XmlTree String getAllText = listA (deep isText >>> getText) >>> arr concat
Robert Vollmert wrote:
In short, I'm constantly running into what appear to be artificial type restrictions in Control.Arrow.ArrowTree. For example, the signature of "deep" is
deep :: (Tree t, ArrowTree a) => a (t b) (t b) -> a (t b) (t b)
instead of the more general
deep :: (Tree t, ArrowTree a) => a (t b) c -> a (t b) c
You are right, there is no harm generalizing deep, since a related combinator, multi, has the more general type. Meanwhile, I don't think
deep (hasName "a") >>> getLink
looks too bad. :) Under suitable assumptions, you can use multi getLink if you want.
Hello, On Mar 23, 2008, at 18:53, Albert Y. C. Lai wrote:
You are right, there is no harm generalizing deep, since a related combinator, multi, has the more general type.
thanks for the reply! Perhaps I'll suggest the generalization to the HXT authors. Meanwhile, I've discovered examples of just the kind of thing I'm trying to do at http://www.haskell.org/haskellwiki/HXT/Practical/ .
Meanwhile, I don't think
deep (hasName "a") >>> getLink
looks too bad. :)
That's true, it was a bad example. It's just that if getLink already includes the hasName test, the above seems redundant. Cheers Robert
participants (2)
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Albert Y. C. Lai -
Robert Vollmert