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

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