Day12 - Pattern Matching

In today's blog post I'll tell you what I've learned so far about Elixir's pattern matching concepts and which parts are really amazing in my opinion. But first let me tell you a little secret: Elixir has no assignment operator 🤯🤫

The first time I got in touch with pattern matching was with Scala. At first I thought pattern matching is a slightly better version of the switch statement I knew from other languages like Java. It took me some time to understand why pattern matching is much more powerful. I think I just got the point when I saw a roman numerals converter in Scala implemented using pattern matching.

Pattern Matching in Elixir - The good parts

In Elixir, the = operator (which looks like an assignment operator) actually is a match operator which tries to match the right side to the left side. For example, x = 5 represents a successfull match and thus x is bound to 5. In case a pattern match does not succeed, Elixir throws a MatchError. Here are some examples:

x = 5             # pattern match works fine, so x is bound to 5
5 = x             # also fine, since x is bound to 5
^x = 5            # also fine, checks if 'x' is bound to 5, x will not be rebound
42 = x            # MatchError since the right hand side value 5 does not match to 42
{x, y} = {21, 42} # works fine, x is bound to 21, y is bound to 42
{x, x} = {42, 42} # works also fine, x is bound to 42
{x, x} = {21, 42} # MatchError since we expected the tuple elements to be the same

Pattern matching however cannot only be used for value checks, but also for destructuring complex data types like tuples, lists or maps. Imagine we have a function print_point which accepts an argument called point and we want to print some information based on the point's structure and data. This could be done as follows:

def print_point(point) do
  case point do
      {x, x}    -> IO.puts("Point is in 2D space. X and Y are the same!")
      {x, y}    -> IO.puts("Point is in 2D space. X and Y are different!")
      {x, y, z} -> IO.puts("Point is in 3D space.")
      _         -> IO.puts("This is not a valid point in 2D/3D space!")
  end
end

Pattern Matching in Elixir - The great parts

Instead of pattern matching an argument within a function's body, Elixir allows us to define a function multiple times with different argument patterns. Each of these definitions is called a function clause. At runtime, Elixir checks for the corresponding clause (which is the one that first machtes the pattern in top down order) and then executes it. Function clauses allow us to split our print_point function from above into four smaller functions, with each implementing their own logic:

def print_point({x, x}) do
  IO.puts("Point is in 2D space. X and Y are the same!")
end
def print_point({x, y}) do
  IO.puts("Point is in 2D space. X and Y are different!")
end
def print_point({x, y, z}) do
  IO.puts("Point is in 3D space.")
end
def print_point(point) do:
  IO.puts("#{point} is not a valid point in 2D/3D space!")
end

Function clauses are really amazing for so much reasons:

1. Function clauses allow us to stay very close to known notations like mathematics. For example let's have a look at the mathematical definition of the Fibonacci sequence:

   fib(0) = 0

   fib(1) = 1
   fib(n) = fib(n-2) + fib(n-1)

   Function clauses help us to create code which is almost 100% identical to the original definition:

   def fib(0), do: 0
   def fib(1), do: 1
   def fib(n), do: fib(n-2) + fib(n-1)

2. We can use arguments from the clause in combination with function guards. Guards are additional checks which have to be fulfilled in order that a function clause matches. For example, we can create a guard which ensures that we can call our fib function with positive numbers only:

   def fib(n) when n > 0, do: ...

3. Function clauses can save us from creating too large functions with too much control flow logic. I think this can have real impact on the code quality with respect to readability and maintainability: rather than inflating an existing function further, a developer might be more inclined to add a new clause instead.

So folks, that's it from me for this week. I wish you happy Easter & lots of sun. 🐰🥚🍳😎

Cheers!