I'm considering porting a very simple text-templating library to scala, mostly as an exercise in learning the language. The library is currently implemented in both Python and Javascript, and its basic operation more or less boils down to this (in python):

```
template = CompiledTemplate('Text {spam} blah {eggs[1]}')
data = { 'spam': 1, 'eggs': [ 'first', 'second', { 'key': 'value' }, true ] }
output = template.render(data)

```

None of this is terribly difficult to do in Scala, but the thing I'm unclear about is how to best express the static type of the `data` parameter.

Basically this parameter should be able to contain the sorts of things you'd find in JSON: a few primitives (strings, ints, booleans, null), or lists of zero or more items, or maps of zero or more items. (For the purposes of this question the maps can be constrained to having string keys, which seems to be how Scala likes things anyways.)

My initial thought was just to use a `Map[string, Any]` as a top-level object, but that's doesn't seem entirely correct to me. In fact I don't want to add arbitrary objects of any sort of class in there; I want only the elements I outlined above. At the same time, I think in Java the closest I'd really be able to get would be `Map<String, ?>`, and I know one of the Scala authors designed Java's generics.

One thing I'm particularly curious about is how other functional languages with similar type systems handle this sort of problem. I have a feeling that what I really want to do here is come up with a set of case classes that I can pattern-match on, but I'm not quite able to envision how that would look.

I have *Programming in Scala*, but to be honest my eyes started glazing over a bit at the covariance / contravariance stuff and I'm hoping somebody can explain this to me a bit more clearly and succinctly.