You're pretty much there. The following form...
<form action="/foo">
<ul>
<li>Row 1: <input type="checkbox" name="a" value="1"/></li>
<li>Row 2: <input type="checkbox" name="a" value="2"/></li>
<li>Row 3: <input type="checkbox" name="a" value="3"/></li>
<li>Row 4: <input type="checkbox" name="a" value="4"/></li>
<li>Row 5: <input type="checkbox" name="a" value="5"/></li>
</ul>
<input type="submit" name="submit" value="Submit"/>
</form>
...gets submitted like this.
http://localhost:8000/foo?a=2&a=3&a=5&submit=Submit
Then, inside your handler, this will get you a list of ByteStrings.
fooHandler = do
as <- getsRequest (rqParam "a")
So this doesn't require JavaScript at all. But it works with JavaScript as well. If you use jQuery to submit a list like this...
var fieldData = { rows: [0,1,4], cols: [2,3,5] };
$.getJSON('http://localhost:8000/foo', fieldData, ...);
...then you'll have to make an adjustment for the brackets
rs <- getsRequest (rqParam "rows[]")
cs <- getsRequest (rqParam "cols[]")