We might need this too, I started taking a look at this. I made a first (not yet complete) draft based on the JavaCC grammar from FreeMarker. That grammar is pretty horrible though. Also, there's no the FreeMarker syntax, there's a couple of different syntaxes with some legacy options thrown into the mix a well (e.g. in ${...} mode [=...] is just plain text and vice versa, and there's also the legacy #{...}) And it's also got an "auto" mode where it tries to auto-detect the syntax...

Not sure yet, but I'm thinking legacy settings should be ignored, which leaves 2 expression (dollar and bracket) * 3 directive (auto, square bracket, angle bracket) syntaxes... which would result in 6 different language mode variations, hmm...

The existing code mirror highlighter I found nicely illustrates some of the complexity:

The Eclipse FreeMarker plugin implements auto mode, where you need to parse an arbitrary amount before you can establish the rules for highlighting. Note how the syntax highlighting changes, depending on the order of the lines (which don't have to be the first lines!):

And while the Eclipse plugin is definitely much better, even that plugin gets it wrong sometimes: Interpolation syntax is always set and never affects or determines the tag syntax, in auto mode the tag syntax is always determined by the first tag:

The above is wrong, if you evaluate that with FreeMarker in auto mode, you'll see that line 9 is evaluated and should thus have received syntax highlighting. Without the interpolation tag, synax highlighting seems to work:

There's also another FreeMarker feature that is completely impossible to implement (with monarch): highlighting for string interpolation. E.g. ${"a${1+2}b"} evaluates to a3b. But unlike JavaScript template literals, a ${ does not switch the tokenizer mode, instead it first and foremost parses it as a string, and then recursively parses the string content again. So the more you nest, the more escaping you need: ${"a"} evaluates to a, and so does ${"${\"a\"}"} and ${"${\"${\\\"a\\\"}\"}"} (and note that ${"${\"${\"a\"}\"}"} is a syntax error). In JavaScript

`${`${`${3}`}`}`

is valid. The solution here is to just not highlight interpolations in strings (which is what the CodeMirror and Eclipse highlighter are doing)

Apache FreeMarker Language Support #2383

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