[Ur] Can one declare a type that will restrict allowable string values on tag attributes?

[Burton Samograd]

Hello Ur List,

My exposure to modern FP and type systems has been through reading
LYAHFGG and many web pages over the years, so I thought I would 
do some 'learning by doing' by expanding on the Urblog project for practice 
to learn some new concepts.

During my experimenting, one of the first things I noticed about the
Ur basis library is that not all XHTML tag attributes are handled by
the embedded XHTML language. I added the Align and Valign attributes
to the <tr> tag like this:

    val td : other ::: {Unit} -> [other ~ [Body, Tr]] => unit
-      -> tag ([Colspan = int, Rowspan = int] ++ tableAttrs)
+     -> tag ([Align = string, Valign = string , Colspan = int, Rowspan = int] ++ tableAttrs)
             ([Tr] ++ other) ([Body] ++ other) [] []

This change worked and the tags were generating correctly in the 
output XHTML, but I could see a problem. Align and Valign should 
only take a specific set of strings as parameters ("top", "left", "right", 
etc.) leaving me thinking the 'Align = string' and 'Valign = string' type 
declarations above are lacking.

What I would expect to be able to do is to declare something like a
'string enum' type that would check and restrict the allowable values
handled by the Align/Valign attributes so that a type error is given
when a given string is not within the set of valid values.

Is this level of type checking possible in Ur, and if so, could
someone give a brief example that would work with the above <td>
attribute declaration code? I would expect that Valign would be given a 
'valign' type declared to restricted it's allowable values to the strings "top", 
"middle" and "bottom".  Can this checking of string be made case 
insensitive?

--
Burton Samograd