Background

For the longest time I have been estranged to the magic lands of comprehensions in FP languages.
Now I am trying to pick it up.

Comprehensions use the for expression in Elixir:

• https://elixir-lang.org/getting-started/comprehensions.html#generators-and-filters

How Scala does it

Most FP languages these days have comprehensions in one way or another. For the purposes of this discussion I am picking Scala because I think it looks somewhat familiar (syntax is fairly similar to that of elixir).

See the following comprehension (Scala):

for {
  a <- List(1, 2)
  b <- Set(2, 1)
} yield a * b

> List(2, 1, 4, 2)

This comprehension is the equivalent of (Elixir):

for a <- [1, 2], 
    b <- MapSet.new([2, 1]) do 
  a * b 
end

> [1, 2, 2, 4]

The Scala example returns a List because in Scala, the comprehension’s return type will be the type of the first enumerator (in this case a <- [1, 2]).

The Elixir example returns also a List. I have no idea why. Maybe they work in reverse? Let’s find out:

for {
  a <- Set(1, 2)
  b <- List(2, 1)
} yield a * b

> Set(2, 1, 4)

In this Scala example I switched things around. Lets see what Elixir returns:

for a <- MapSet.new([1, 2]), 
    b <- [2, 1] do 
  a * b 
end

> [2, 1, 4, 2]

It also returns a List …

Questions

So, I am rather confused here. So here is my question:

• In an Elixir comprehension with enumerators of different types, how do I know what the resulting type of the expression will be?