Adding builtin sort function about expr HOT 3 CLOSED

This is a nice idea, I'm also thinking about adding sort and reduce functions. Right now predicates takes only one arg via # symbol. I think something like #1 #2, will be cool.

sort(Tweets, #1.Len < #2.Len)

from expr.

It was more complicated than I thought.

For sort package we need to provide a comparison function Less. In Expr there are no closures and #1 > #2 predicates are not true functions. We can compile such predicates separately, but this lead to more question: how to do binding on other variables? What if in predicate someone uses a variable defined in an expression? Adding proper function to Expr is not a solution, as it will make the language turn complete and break always terminating promise.

As we can't create a function from a predicate we can implement a sorting algorithm inside the Expr language itself! I started programming this approach with the quicksort algorithm. All only utilizing opcodes from expr VM. It got complicated very quickly. I started to research the "sort" problem and here are a few conclusions I ended up with:

• Sort function should not mutate incoming array (side-effect-free promise), but always create a new copy of the array.
• Predicates actually make understanding ordering more complicated. Compare #1 > #2, #2 < #1. What order is going to be? asc and desc are much more easy to understand.
• 90% of the time, people just want to sort an array of objects (structs or maps) by a field.
  So writing #1.Value > #2.Value requires duplicating the field name twice.

I've added two functions: sort() and sortBy(field string).

Here are a few examples (using the pipe operator) of them:

values | sort()
values | sort('desc')
tweets | sortBy('Len')
tweets | sortBy('Len', 'desc')

from expr.

I tried looking into it but also got stuck in the opcodes. Thank you! I will test it.

from expr.