Add slice comprehensions

[themeeman]

In python, you can do [i ** 2 for i in range(10)] to iterate through each value in range and square each one and get a list of squares all in one expression. I propose a similar syntax for expr:

[v ** 2 for _, v := range 0..9]

This would return the slice of all the numbers from 0 to 9 squared.
Another thing you could do is a conditional comprehension, for example

[[i, v] for i, v := range [1, 4, 7, 2, 3] if v <= 5]

This would return a [][]int of the numbers and indexes of those numbers where the number is less than or equal to 5. In this case it would be [[0, 1], [1, 4], [3, 2], [4, 3].

This feature would make this package more powerful and greatly increase the potential of it, as currently there is no way to return a slice outside of the range operator, which is limited.

[antonmedv]

I'm thinking about adding comprehensions too. But I don't like nested comprehensions, as it difficult to read it.
Also I think before adding comprehensions will be cool to think about map/apply functionality and closures/lambdas.

On opposite side, without comprehensions expressions can stay simple. In our project we heavily rely on expr and implemented a lot of useful helpers for iterating or complex login. In expressions just combining them.

[antonmedv]

Now we have all any one etc builtins.