Including an overwriteDemand GET param with TYPO3 Fluid and tx_news

List.html

{namespace n=Tx_News_ViewHelpers}
<section>
    <f:for each="{news}" as="newsItem" iteration="iterator">
        <n:link 
            newsItem="{newsItem}" 
            settings="{settings}" 
            configuration="{additionalParams: '&tx_news_pi1[overwriteDemand][categories]={f:for each=\"{newsItem.categories}\" as=\"category\" iteration=\"categoryIteration\">{category.uid}<f:if condition=\"{categoryIteration.isLast}\"><f:then></f:then><f:else>,</f:else></f:if></f:for>'}">
        {newsItem.title}</n:link>
    </f:for>
</section>

<f:comment>The expected result of the above is a link to a detail view with the &tx_news_pi1[overwriteDemand][categories]=a,b,..n GET param attached. However, right now all I get is an exception telling me my template has the wrong number of opening tags to match my closing tags.</f:comment>

Yes, I finally found a solution: Use ext:vhs and use v:iterator to create the right csv-string for the GET param.

Well, no wonder that you got an exeption about tag mismatch. There is no opening section tag in your template, but a closing one.

I updated the snippet to include a section start tag.