Elixir for Beginner - All you need to know about guard

What is Guard in Elixir

In previous post, I explain what is Pattern Matching and how to use it.

Elixir pattern matching in a nutshell

Pattern matching is so cool but some time I want to do some more complicated checking. With pattern matching I can easily do this

1def can_access?(%User{paid_user: true}), do: true

Yes, Pattern matching can do check value with exact value easily. But for example, I want to allow user with level > 25 to access.

How to do that check with Pattern matching?

Pattern matching as it’s named, it is used to match data against pattern. If you want to do more complex check, you need another guy. That is where guard shines, it is complement for Pattern Matching

1def can_access?(%User{level: level}) when level > 25, do: true

What is guard

• Guard is a complement to your pattern matching to do more complex check.

• Guard expression is invoke after pattern mattching

• In many cases, Guard and Pattern matching can produce the same result, so use which you like.

1# sum on empty list
2# pattern matching
3def sum_list([] = _input), do: 0
4
5# guard
6def sum_list(input) when input == [], do: 0

Some example

• Check primitive type

  1def sum(a, b) when is_integer(a) and is_integer(b) do
  2	a + b
  3end
  

• Check value is nil/ not nil

  1def string_length(string) when not is_nil(string) do
  2	# your code
  3end
  

• Check if input in a list of allowed values

  1def can_edit?(%User{role: role}) when role in ["admin", "moderator"] do
  2	true
  3end
  

• And many more …

Where to use guard?

Where you can use Pattern Matching, you can use Guard

• case block

  1case value do
  2	x when is_binary(x) -> String.to_integer(x)
  3	x when is_integer(x) -> x
  4	_ -> raise "Invalid value"
  5end
  

• with block

  1with user when not is_nil(user) <- find_user(id) do
  2	# your code block
  3end
  

• function clause as our example above

Why my guard not work?

Not all expression will work with guard. Only a list of built-in guard and combination of them work in guard expression.

Check this from https://hexdocs.pm/elixir/guards.html#list-of-allowed-expressions

• comparison operators (==, !=, ===, !==, >, >=, <, <=)
• strictly boolean operators (and, or, not). Note &&, ||, and ! sibling operators are not allowed as they’re not strictly boolean - meaning they don’t require arguments to be booleans
• arithmetic unary and binary operators (+, -, +, -, *, /)
• in and not in operators (as long as the right-hand side is a list or a range)
• “type-check” functions (is_list/1, is_number/1, etc.)
• functions that work on built-in datatypes (abs/1, map_size/1, etc.)

Can I define my own guard?

Yes you can define a guard with defguard/1 and defguardp/1 . But you should only define your own guard if you have a really really reasonable reason to do so.

In my experience, I have never defined a guard my own, built-in guards are too enough.

Conclustion

With Pattern matching and Guard, you have a super powerful combo in your hand. Let’s code!