02-10-comprehension_elixir.md

%{ title: "Comprehension Elixir", author: "Luiz Cattani", tags: ~w(comprehension elixir database), description: "This is my notes about Comprehenison in Elixir" }

Elixir Comprehension

Comprehension is a sugar syntax for loop over a Enumerable, filtering some result and mapping the result for a new list.

Mapping an integer list and multiplying the each integer on the list by itself

iex> for n <- [5, 6, 7, 8], do: n * n
=> [25, 36, 49, 64]

A Comprehension is made of three parts: Generators, filters and collectable

Generators

In the example below the chunk of the code n <- [5, 6, 7, 8] it's the generators We can think as "Generating values to be used in the comprehension" Any enumerable can be passed on the right-hand side of the generator expression

iex> for n <- [5, 6, 7, 8], do: n * n
=> [25, 36, 49, 64]

Multiple Generators

iex> for n <- [a:, :b, :c], j <- ["luiz", "cattani"], do: {n, j}
=> [a: "luiz", a: "cattani", b: "luiz", b: "cattani", c: "luiz", c: "cattani"]

Filters

Pattern Matching

Generators also supports Pattern Matching on their left-hand side, all non-matching patterns are ignored.

iex> classifications = [upvote: 1, downvote: 0, upvote: 1, upvote: 1, downvote: 0]

iex> for {:upvote, n} <- classifications, do: n * 5  
=> [5, 5, 5]

iex> for {:downvote, n} <- classifications, do: n * 5
=> [0, 0]

iex> for {:when_atom_does_not_matching, n} <- classifications, do: n * 5
=> []

we can use filter to select some particular elements, in the example all number multiple of 3, and discard all that does not are multiple of 3, returns false or nil.

iex> multiple_of_3? = fn(n) -> rem(n, 3) == 0 end
iex> for n <- 0..10, multiple_of_3?.(n), do: n * 10

Collectable

The variables assignments inside the comprehension, be it in generators, filters, or inside a block, are not reflected outside of the comprehension.