The only avaible types are string, number, and some wiki specific types: https://www.mediawiki.org/wiki/Help:TemplateData#Description_and_parameters
There are some types of templates that have de facto boolean parameters. These are treated as strings. I.e. "nopp: Set to y, yes, or true" in https://en.wikipedia.org/wiki/Template:Citation
My own use case is: dead-url in https://en.wikipedia.org/wiki/Template:Citation; treated as string; takes "yes" or "no."
My problem is that as an API writer, this requires me to exactly copy what's in the template for this parameter, in order to have it work in TemplateData, which is not a choice I would normally make for the API. I would like template data to have a "boolean" type, and then also have the ability in 'maps' to negate it; i.e.
```
"maps": {
"citoid": {
"urlAlive": "!dead-url", // This seems a bit hacky to me though
```
This still doesn't solve my immediate problem (because the template requires 'yes' or 'no' and not 'true' or 'false' so it's still an issue because !yes isn't valid either!) but it might a) encourage template writers to use booleans in a more standard fashion b) might be usable in some other scenarios c) we might even consider interpreting 'yes' and 'no' as bools, although there is no guarantee the templates would adapt to that, and there's also the language issue ("oui" et "non")?
Even if we don't both with allowing negation at all at first, it still might be a nice thing to have the extra type, just for standards reasons.