Describe guard() sequences in Elixir.

In Elixir, guard() sequences derive from when() clauses in Erlang. Their main function is pattern matching augmentation. Guards allow developers to specify predicates for a given argument type, as seen in the example below:

defmodule Sum do

  def to(1), do: 1

  def to(n) when n > 0, do: n + to(n-1) # only nonzero positive numbers

It’s worth noting that most expressions don’t support guard testing.

Developers mainly use guards when working in Kernel. In and not in boolean-only, and comparison operators, as well as datatype and type-check functions, support guard expressions.