Case Expressions in

About Case Expressions

In Gleam a case expression can be used to conditionally execute code. With a case expression a value can be tested against one or more patterns. An example of such a pattern is the literal pattern, which matches a value against a literal value (e.g. 1 or "hello").

Case expressions are written with the case keyword:

pub fn describe(number: Int) -> String {
  case number {
    0 -> "Zero"
    1 -> "One"
  }
}

While this may look like switch statements in other languages, pattern matching starts to shine when also using other patterns. One such pattern is the variable pattern, which allows one to assign a value to a variable. In this example, the variable i will be assigned the value of number if it is not 0:

pub fn describe(number: Int) -> String {
  case number {
    0 -> "Zero"
    i -> "Non zero"
  }
}

In some cases, you may want to add an additional condition to a pattern. This is known as a guard (clause), which can be added using the if keyword:

pub fn describe(number: Int) -> String {
  case number {
    0 -> "Zero"
    i if i < 0 -> "Negative number"
  }
}

For performance reasons only basic mathematical and boolean operators are allowed in guards, other functions cannot be used in guards.

In the above example, not all possible input will have a matching pattern. The compiler will detect this and output an error. This is known as exhaustive checking. To solve the warning, one has to handle all cases. For this, the discard pattern can be used, which is a pattern that matches on any value:

pub fn describe(number: Int) -> String {
  case number {
    0 -> "Zero"
    i if i < 0 -> "Negative number"
    _ -> "Positive number"
  }
}

// No compiler error

Case expressions will test a value against each pattern from top to bottom, until it finds a matching pattern and executes the logic associated with that pattern. The order of patterns matters!