# Comprehensions

In Elixir, it is common to loop over an `Enumerable`, often filtering out some results and mapping values into another list. Comprehensions are syntactic sugar for such constructs: they group those common tasks into the `for` special form.

For example, we can map a list of integers into their squared values:

```elixir
iex> for n <- [1, 2, 3, 4], do: n * n
[1, 4, 9, 16]
```

A comprehension is made of three parts: generators, filters, and collectables.

## Generators and filters

In the expression above, `n <- [1, 2, 3, 4]` is the **generator**. It is literally generating values to be used in the comprehension. Any enumerable can be passed on the right-hand side of the generator expression:

```elixir
iex> for n <- 1..4, do: n * n
[1, 4, 9, 16]
```

Generator expressions also support pattern matching on their left-hand side; all non-matching patterns are *ignored*. Imagine that, instead of a range, we have a keyword list where the key is the atom `:good` or `:bad` and we only want to compute the square of the `:good` values:

```elixir
iex> values = [good: 1, good: 2, bad: 3, good: 4]
iex> for {:good, n} <- values, do: n * n
[1, 4, 16]
```

Alternatively to pattern matching, filters can be used to select some particular elements. For example, we can select the multiples of 3 and discard all others:

```elixir
iex> for n <- 0..5, rem(n, 3) == 0, do: n * n
[0, 9]
```

Comprehensions discard all elements for which the filter expression returns `false` or `nil`; all other values are selected.

Comprehensions generally provide a much more concise representation than using the equivalent functions from the `Enum` and `Stream` modules. Furthermore, comprehensions also allow multiple generators and filters to be given. Here is an example that receives a list of directories and gets the size of each file in those directories:

```elixir
dirs = ["/home/mikey", "/home/james"]

for dir <- dirs,
    file <- File.ls!(dir),
    path = Path.join(dir, file),
    File.regular?(path) do
  File.stat!(path).size
end
```

Multiple generators can also be used to calculate the Cartesian product of two lists:

```elixir
iex> for i <- [:a, :b, :c], j <- [1, 2], do:  {i, j}
[a: 1, a: 2, b: 1, b: 2, c: 1, c: 2]
```

Finally, keep in mind that variable assignments inside the comprehension, be it in generators, filters or inside the block, are not reflected outside of the comprehension.

## Bitstring generators

Bitstring generators are also supported and are very useful when you need to comprehend over bitstring streams. The example below receives a list of pixels from a binary with their respective red, green and blue values and converts them into tuples of three elements each:

```elixir
iex> pixels = <<213, 45, 132, 64, 76, 32, 76, 0, 0, 234, 32, 15>>
iex> for <<r::8, g::8, b::8 <- pixels>>, do: {r, g, b}
[{213, 45, 132}, {64, 76, 32}, {76, 0, 0}, {234, 32, 15}]
```

A bitstring generator can be mixed with "regular" enumerable generators, and supports filters as well.

## The `:into` option

In the examples above, all the comprehensions returned lists as their result. However, the result of a comprehension can be inserted into different data structures by passing the `:into` option to the comprehension.

For example, a bitstring generator can be used with the `:into` option in order to easily remove all spaces in a string:

```elixir
iex> for <<c <- " hello world ">>, c != ?\s, into: "", do: <<c>>
"helloworld"
```

Sets, maps, and other dictionaries can also be given to the `:into` option. In general, `:into` accepts any structure that implements the `Collectable` protocol.

A common use case of `:into` can be transforming values in a map:

```elixir
iex> for {key, val} <- %{"a" => 1, "b" => 2}, into: %{}, do: {key, val * val}
%{"a" => 1, "b" => 4}
```

Let's make another example using streams. Since the `IO` module provides streams (that are both `Enumerable`s and `Collectable`s), an echo terminal that echoes back the upcased version of whatever is typed can be implemented using comprehensions:

```elixir
iex> stream = IO.stream(:stdio, :line)
iex> for line <- stream, into: stream do
...>   String.upcase(line) <> "\n"
...> end
```

Now type any string into the terminal and you will see that the same value will be printed in upper-case. Unfortunately, this example also got your IEx shell stuck in the comprehension, so you will need to hit `Ctrl+C` twice to get out of it. :)

## Other options

Comprehensions support other options, such as `:reduce` and `:uniq`. Here are additional resources to learn more about comprehensions:

  * [`for` official reference in Elixir documentation](`for/1`)
  * [Mitchell Hanberg's comprehensive guide to Elixir's comprehensions](https://www.mitchellhanberg.com/the-comprehensive-guide-to-elixirs-for-comprehension/)
