Today I Learned

A Hashrocket project

Pattern matching with `Kernel.match`

Pattern matching is powerful, but when iterating over a list with an Enum function you must allow all variants of the list to be processed. So this fails:

sample = [{1, "A"}, {2, "B"}]
 > Enum.filter(sample, fn ({_, "B"}) -> true end)
** (FunctionClauseError) no function clause matching in :erl_eval."-inside-an-interpreted-fun-"/1

    The following arguments were given to :erl_eval."-inside-an-interpreted-fun-"/1:

        # 1
        {1, "A"}

    (stdlib) :erl_eval."-inside-an-interpreted-fun-"/1
    (stdlib) erl_eval.erl:826: :erl_eval.eval_fun/6
    (elixir) lib/enum.ex:2898: Enum.filter_list/2

Instead of using pattern matching here we can just use an anonymous function that takes all args and makes a comparison.

sample = [{1, "A"}, {2, "B"}]
Enum.filter(sample, fn ({_, letter}) -> letter == "B" end)
# [{2, "B"}]

But the cool way to do it is with Kernel.match?. Which according to the docs is:

A convenience macro that checks if the right side (an expression) matches the left side (a pattern).

What that looks like:

sample = [{1, "A"}, {2, "B"}]
Enum.filter(sample, &match?({_, "B"}, &1))
# [{2, "B"}]

H/T Taylor Mock

Looking for help? Elixir is quickly gaining momentum for web applications that need concurrency, performance, and the ability to connect to many different clients. The developers at Hashrocket are learning along with the rest of the development community that Elixir and Phoenix are viable Rails alternatives for the right application. Check out the source code for Today I Learned, written in Elixir, and contact us if you need help with your Elixir project.