Pattern Matching in Elixir

Pattern Matching in Elixir

Pattern Matching is Undoubtedely the most enticing feature of Elixir (and many other Programming Languages that supports it). In this blog I will try to explain how Elixir does Pattern Matching using example codes. There are essentially 3 different ways to do Pattern Matching in Elixir -

Pattern Matching Operator (=)

Equal (=) symbol in most programming language is considered as assigning a value to a variable. In Elixir, this is called Pattern Matching Operator instead.

In Python you can write

x = 5

This means we are assigning the value 5 to the variable x

You can write the same expression in Elixir, but the meaning is completely different. In Elixir this means we are matching the left-hand side i.e; x to the right-hand side i.e; 5. Following are the matching rules-

1. Variables & values from the left-hand side is matched against values from the right-hand side.
2. Variables can only be in the left-hand side of the expression.
3. If there is a value in the left-hand side, it expects a matching value in the right-hand side.

Because of this the following statement will also compile in Elxir

5 = x
# As x was previously assigned to 5
# This evaluates to 5 = 5, and it matches

But this will fail because y was never set

100 = y
# You will get the following error
# ** (CompileError) iex:2: undefined function y/0

Note that variables can be re-assigned to another value

x = 100
x = %{:a => "apple"}

You can also match complex data structures like Tuples, Map, List etc.

{a, b, c} = {2, 3, 4}
# a is assigned to 2
# b is assigned to 3
# c is assigned to 4

%{:a => a, :b => b} = %{:a => "apple", :b => "ball", :c => "cat"}
# a will be assigned to "apple"
# b will be assigned to "ball"

[a, b, c] = [100, "200", 300]
# a will be assigned to 100
# b will be assigned to "200"
# c will be assigned to 300

[a | b] = [1, 2, 3, 4, 5]
# a will be assigned to 1
# b will be assigned to [2, 3, 4, 5]

Notice that you can destructure a list into it’s head and tail using Pattern Matching.

The following will fail, because 2 and 3 does not match 200 and 300 respectively.

{a, 2, 3} = {100, 200, 300}
# You will get the following error if you type this in Elixir REPL
# ** (MatchError) no match of right hand side value: {100, 200, 300}

Pattern Matching in Function arguments

Pattern matching are mostly useful in the context of defining functions. Let’s say you want to define a function that takes address data as input and format that into string. Let’s say your application represents address in two different ways-

1. As a map containing :street, :postal_code and :city
2. As a tuple {street, postal_code, city_key}.

You can implement this in the following way-

defmodule Form do
  def format_address(
    %{street: street, postal_code: postal_code, city: city}) do
    format_address({street, postal_code, city})
  end
  
  def format_address({street, postal_code, city}) do
    """
    #{String.upcase(street)}
    #{String.upcase(postal_code)} #{String.upcase(city)}
    """
  end
end

You can define two functions, one taking a map and one taking tuple, and it just works.

Pattern Matching in Case Statements

So far I have discussed two ways of pattern matching in Elixir, 1. By = operator, 2. In function arguments. But there is a third way of doing pattern matching as well : The case statements. Similar to C, C++ and Javascript switch-case statement (but more powerful), one can do pattern matching in case statements

case expr do
  pattern1 -> value1
  pattern2 -> value2
  ...
end

Let’s say you want to define a function in Elixir to return the first element from the list, if non-empty, and a default value if empty You can do so using case statement

def safe_head(lst, default) do
  case lst of
    []       -> default
    [x | xs] -> x
  end
end

First, the input list lst is matched against [], if matched it returns the default value, if not then lst is matched against the next pattern [x | xs], and now it will surely match because the list lst is non-empty, it will now return the first element x and we are done.

Conclusion

Thank you very much if you are still reading, and with this I will wrap this up. Happy Coding and Keep Learning 😄