DEV Community

loading...
Cover image for Pattern Matching in Elixir

Pattern Matching in Elixir

reergymerej profile image ᴙɘɘᴙgYmɘᴙɘj ・2 min read

reposted from https://reergymerej.github.io/blog/2021/06/11/pattern-matching.html

When discussing pattern matching in Elixir (and other languages), it seems like most of what people talk about isn't the actual matching part.

Pattern Matching

Pattern matching is the ability to check if the left and right match.

iex(1)> 1 = 1
1

iex(2)> 2 = 1
** (MatchError) no match of right hand side value: 1
Enter fullscreen mode Exit fullscreen mode

It is sensitive to data types.

iex(1)> [] = []
[]

iex(2)> [] = {}
** (MatchError) no match of right hand side value: {}
Enter fullscreen mode Exit fullscreen mode

It checks the shape of the data as well.

iex(1)> {1} = {1}
{1}

iex(2)> {} = {1}
** (MatchError) no match of right hand side value: {1}
Enter fullscreen mode Exit fullscreen mode

It can be used to branch code.

case 1 do
  1 -> "one"
  2 -> "two"
end
"one"
Enter fullscreen mode Exit fullscreen mode

That's really all there is to it. Everything else is using pattern matching.

Using Pattern Matching

When matching, you can bind values on the LHS.

iex(1)> foo = 1
1

iex(2)> foo
1
Enter fullscreen mode Exit fullscreen mode

This works in cases too.

case 1 do
  foo -> foo
end
1
Enter fullscreen mode Exit fullscreen mode

You can destructure values while binding.

iex(1)> [foo, 2] = [1, 2]
[1, 2]

iex(2)> foo
1
Enter fullscreen mode Exit fullscreen mode

You can ignore parts of the data.

iex(1)> [foo] = [1, 2]
** (MatchError) no match of right hand side value: [1, 2]

iex(1)> [foo, _] = [1, 2]
[1, 2]
Enter fullscreen mode Exit fullscreen mode

You can use underscore as a wildcard when branching.

case "bitch" do
  "cops" -> "yep"
  "critics" -> "yep"
  "paparazzi" -> "yep"
  _ -> "ain't one"
end
"ain't one"
Enter fullscreen mode Exit fullscreen mode

And finally, the ever-popular splitting of head/tail of a list.

iex(1)> [head | tail] = [1, 2, 3]
[1, 2, 3]

iex(2)> head
1

iex(3)> tail
[2, 3]
Enter fullscreen mode Exit fullscreen mode

Discussion (0)

Forem Open with the Forem app