I think you can't make this work without the parens; without them the postfix call will cause ambiguity. I would just accept the fact that most of the time you can't achieve full English like sentence structure in most non-trivial cases and use a workaround such as onThe
or skipping the the
part and making on
infix.
'the' is unneeded anyway because it's statically deduceable from context whether you mean a pre-existing name (the) or a new name i.e. free variable (a).
Also, I don't even find it very important or useful to achive 100% English like sentences unless it's a competition. In practice it might actually make things less readable and much more convoluted under the hood. If you really need something like natural language like DSL, define a new language grammar using Scala's powerful parser combinators. You should even be able to write, say, an Eclipse plugin to syntax highlight your custom DSL.