I might be interpreting your question wrong but that is specific to Haskell (which he uses as the foundation of lambdu). It looks like sum is being composed with filter which is given the iterable and a predicate.

They mention that they modified haskell to use explicitly named parameters. You can see that in the filter call but not the sum call.

Our current thought is that single-parameter functions do not need an argument name.

We might change a lot of our decisions later as we tune it for working with more realistic code.

I hope you expose the color coding as a config file, so folks with an eye for design can contribute.

Subtext website, for the other curious http://www.subtext-lang.org/ .