Pass a map to External function - xquery

Question

I have the following module
xquery version "1.0-ml";
declare variable $datasets as map:map external;
...
I want to call this so I do an xdmp:invoke like this
...
let $update := xdmp:invoke("/app/lib/my-module.xqy", (xs:QName("datasets"), $map), <options xmlns="xdmp:eval">
<modules>{xdmp:modules-database()}</modules>
</options>)
...
$map is of type map:map.
Running this gives me the following error
[1.0-ml] XDMP-ARG: xdmp:invoke("/app/lib/20140527-0916-copy-y-axis-labels-from-chart-to-dataset-...", fn:QName("", "datasets"), <options xmlns="xdmp:eval"><modules>0</modules></options>) -- Invalid argument
Why is that?

Answer 1

It's hard to know for sure from the limited code samples you've posted, but I think that your $map variable is bound to the empty sequence (the rough analogue of null in XPath/XQuery).
I've created the following main module, that simply returns the external variable $datasets:
xquery version "1.0-ml";
declare variable $datasets as map:map external;
$datasets
Invoking it as follows works correctly:
let $map := map:entry("key", "value")
return
xdmp:invoke("/test.xqy",
(xs:QName("datasets"), $map),
<options xmlns="xdmp:eval">
<modules>{xdmp:modules-database()}</modules>
</options>)
This results in the "invalid argument" error:
let $map := ()
return
xdmp:invoke("/test.xqy",
(xs:QName("datasets"), $map),
<options xmlns="xdmp:eval">
<modules>{xdmp:modules-database()}</modules>
</options>)
XQuery flattens sequences (they don't nest like s-expressions), so when $map is the empty sequence, the <options/> element becomes the value of the $datasets param, which is an invalid value for the external variable.
Update: Doh! #mblakele's comments below explain the error conditions ...
TL;DR: () is not an instance of map:map.