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How to match against a sequence?
f#

I am completely new to F# (although not so new to programming in general). I am trying to understand whether it is possible to match against a sequence. I am writing an XML parser, and I have a node that can contain either

<instant>date</instant>

or

<startDate>date</startDate>
<endDate>date</endDate>

I need to translate this node to my choice type:

type DateRange =
{
Begin : DateTime;
End : DateTime;
}

type Period =
| RangePeriod of DateRange
| Instant of DateTime

So, it would be nice to have something like this:

let XmlPeriod (node: XmlNode) =
match node.ChildNodes with
| {(Element("instant", instant)} -> Instant( DateTime.Parse(instant.InnerText) )
|
{(Element("startDate", startDate), Element("endDate", endDate)} -> RangePeriod(...)
| _ -> failwith ...

Unofrtunately I could not figure out how to achieve that, so I resorted to matching on ChildNodes.Item(0) and ChildNodes.Item(1) which is kinda ugly.

.

Hi Ivan,

I think you need active patterns. They are a way of making .NET data structures look like F#'s union types. I wrote an article for infoQ a while ago that has an example that's surprisingly similar to what your trying to do:
[link:www.infoq.com]

Cheers,
Rob

By on 6/17/2008 12:35 AM ()

Hi, Rob!

I think I've seen your article. Note, that your list of child nodes is a sequence. It is still not clear how using "Node" and "Leaf" I would express something like "I expect a node with exactly two children: <startDate> and <endDate>".

Maybe I am missing the obvious.
It would be easier if Seq. had a "tail" operator, but it does not

By on 6/17/2008 7:51 AM ()

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