Guard clauses

We can use pattern matching in named functions. See the Sum.to function below, which sums up the numbers from 1 to n:

defmodule Sum do
  def to(1), do: 1
  def to(n), do: n + to(n-1)
end

Unfortunately, the above implementation of Sum.to will not work for negative numbers. In fact, if you try to run Sum.to -1, we’ll be stuck in a loop.

We want to specify additional predicates regarding the value or type of the arguments that are passed into our function.

For Sum.to, we want to make sure that n is a strictly nonzero positive integer.

Time to use guard clauses:

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

We can have multiple guard clauses:

defmodule Sum do
  def to(1), do: 1
  def to(n) when n > 0 and is_integer(n), do: n + to(n-1) # only nonzero positive integers
end

Note that the expressions you can use as guard clauses are limited. You can’t use your own functions as Elixir optimizes these clauses for performance reasons. A full list of available expressions are available here.