#20: Guarding Your Elixir Functions

In Elixir, a guard in a function is a special clause that allows you to add additional conditions to the function definition. Guards are used to apply constraints or filters to the function arguments before the function is executed. Guards are denoted by the when keyword followed by a boolean expression.

Here are two basic examples of a function with a guard in Elixir:

# Basic Example 1
defmodule Math do
  def divide(a, b) when b != 0 do
    a / b
  end

  def divide(_, 0) do
    {:error, "Division by zero"}
  end
end

#Basic Example 2
number = 121

case number do
  n when n < 100 -> :low
  n when n >= 100 and n < 1000 -> :medium
  n when n >= 1000 -> :high
  _ -> :unknown
end

In the divide/2 example, the guard when b != 0 ensures that the division operation is only performed when the divisor b is not equal to zero. If b is zero, the function pattern matching falls through to the second definition, which handles the case of division by zero.

In the basic example 2, the case statement evaluates the value of number and matches it against different patterns with guards. If number is less than 100, it returns :low, if it is between 100 and 1000 (inclusive), it returns :medium, if it is 1000 or greater, it returns :high. The _ wildcard pattern is used as a catch-all for any other value, returning :unknown.

Guards can include various expressions such as comparison operators, logical operators, type checks, and function calls. Guards provide a way to add additional conditions to function clauses, making the function definitions more robust and flexible.