For Expression

[gdotdesign]

This PR introduces the for expression which allows iteration and filtering over Array(a) Set(a) and Map(a, b) types (for now).

The syntax looks like this:

/* For arrays and sets */
for (item in array) {
  item
}

/* For maps */
for (key, value in map) {
  value
}

/* Filtering  using a condition */
for (item in array) {
  item
} when {
  item == "A"
}

The rationale behind this is that using .map, .select and functions like them is more verbose that it needs to be and also introduces multiple function calls.

So this:

["A", "B"]
|> Array.select((item : String) : Bool => { item == "A" }
|> Array.map((item : String) : String => { String.toLowerCase(item) }

Becomes this:

for (item in ["A", "B"]) {
  String.toLowerCase(item)
} when {
  item == "A"
}