Filling

`#` means a template, which is the start of filling process. Variables labelled with # will be filled with a certain value later.

`$` means a reference, which is the end of filling process. It searches the existing vairables in the scope and fill in a certain value.

The main reason of using #() as template is that it is actually a comment to make people understand what is going on here and reduce ambiguity!

For example, if you simply put two variables after an iteration, like R >> k, v the parser can understand what you want to do, but it is not clear with respect to readability. So you have to tell others with your code that there is a template of two variables, and you are going to fill it for each element of R.

This is a design forces you to comment, it is a strict limitation on the code writer.

Intuitive

`#`(Template) are empty pigeon holes, waiting for pigeons to move in.

`$`(Reference) are full-filled pigeon holes, there is a pigeon in each hole.

Example

Iterate over R, for each element, split it and auto fill in the template #(k, v), then execute single branch => { ... }.

R >> #(k, v) => {
    ...
}