Pattern Matching Function (Without Equal Sign)

tobleron

I was going through elixirschool.com and I am stuck to understand what the author means by specifically this kind of pattern matching. The other kinds I get it, but this one doesn’t make sense.

Functions and Pattern Matching

Behind the scenes, functions are pattern-matching the arguments that they’re called with.

Say we needed a function to accept a map but we’re only interested in using a particular key. We can pattern-match the argument on the presence of that key like this:

defmodule Greeter1 do
  def hello(%{name: person_name}) do
    IO.puts "Hello, " <> person_name
  end
end

This was the output from iex. A bunch of numbers?

{:module, Greeter1,
 <<70, 79, 82, 49, 0, 0, 4, 52, 66, 69, 65, 77, 65, 116, 85,
   56, 0, 0, 0, 133, 0, 0, 0, 14, 15, 69, 108, 105, 120,
   105, 114, 46, 71, 114, 101, 101, 116, 101, 114, 50, 8,
   95, 95, 105, 110, 102, 111, ...>>, {:hello, 1}}

So can anyone elaborate what’s happening here?

7 comments

#pattern-matching #functions

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2019-03-15 19:23:38 UTC

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asummers

That’s the compiled module byte code. You need to call Greeter1.hello(%{name: "something"}) to actually invoke the function.

Post #1

NobbZ

Basically its the binary representation of the compiled module, you can ignore it when you see it in IEx.

Post #2

tobleron

Ok, but I do not see any pattern matching here. We just send an argument “something” and it returns “Hello, something”. Where is the pattern matching? Maybe this term is a bit over used?

Post #3

NobbZ

Thats where the pattern match happens.

Post #4

tobleron

But this means that this pattern matching will never fail or give an error, because it will accept all kinds of arguments.

Post #5

NobbZ

No, that match will fail as soon as you pass something into the function that is not a map, or if it is a map and does not have a key :name.

iex(1)> defmodule Greeter1 do
...(1)>   def hello(%{name: person_name}) do
...(1)>     IO.puts "Hello, " <> person_name
...(1)>   end
...(1)> end
{:module, Greeter1,
 <<70, 79, 82, 49, 0, 0, 4, 248, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 162,
   0, 0, 0, 18, 15, 69, 108, 105, 120, 105, 114, 46, 71, 114, 101, 101, 116,
   101, 114, 49, 8, 95, 95, 105, 110, 102, 111, ...>>, {:hello, 1}}
iex(2)> Greeter1.hello(1)
** (FunctionClauseError) no function clause matching in Greeter1.hello/1

    The following arguments were given to Greeter1.hello/1:

        # 1
        1

    iex:2: Greeter1.hello/1
iex(2)> Greeter1.hello(%{})
** (FunctionClauseError) no function clause matching in Greeter1.hello/1

    The following arguments were given to Greeter1.hello/1:

        # 1
        %{}

    iex:2: Greeter1.hello/1
iex(2)> Greeter1.hello(%{sur_name: "Random"})
** (FunctionClauseError) no function clause matching in Greeter1.hello/1

    The following arguments were given to Greeter1.hello/1:

        # 1
        %{sur_name: "Random"}

    iex:2: Greeter1.hello/1
iex(2)> Greeter1.hello(%{name: "Random"})
Hello, Random
:ok
iex(3)> Greeter1.hello(%{name: "Random", sur_name: "Guy"})
Hello, Random
:ok

Post #6