#Option

The Option type is a built-in type in MoonBit that represents an optional value. The type annotation Option[A] can also be written as A?.

It is an enum with two variants: Some(T), which represents a value of type T, and None, representing no value.

Note that some methods of the Option are defined in the core/builtin package.

#Usage

#Create

You can create an Option value using the Some and None constructors, remember to give proper type annotations.

///|
test {
  let some : Int? = Some(42)
  let none : String? = None
  debug_inspect(some, content="Some(42)")
  debug_inspect(none, content="None")
}

#Extracting values

You can extract the value from an Option using the match expression (Pattern Matching).

///|
test {
  let i = Some(42)
  let j = match i {
    Some(value) => value
    None => abort("unreachable")
  }
  @test.assert_eq(j, 42)
}

Or using the unwrap method, which will panic if the result is None and return the value if it is Some.

///|
test {
  let some : Int? = Some(42)
  let value = some.unwrap() // 42
  @test.assert_eq(value, 42)
}

A safer alternative to unwrap is the unwrap_or method, which returns the value if it is Some, otherwise, it returns the default value.

///|
test {
  let none : Int? = None
  let value = none.unwrap_or(0) // 0
  @test.assert_eq(value, 0)
}

There is also the unwrap_or_else method, which returns the value if it is Some, otherwise, it returns the result of the provided function.

///|
test {
  let none : Int? = None
  let value = none.unwrap_or_else(() => 0) // 0
  @test.assert_eq(value, 0)
}

#Transforming values

You can transform the value of an Option using the map method. It applies the provided function to the value if it is Some, otherwise, it returns None.

///|
test {
  let some : Int? = Some(42)
  let new_some = some.map((value : Int) => value + 1) // Some(43)
  assert_true(new_some == Some(43))
}

There is a filter method that applies a predicate to the value if it is Some, otherwise, it returns None.

///|
test {
  let some : Int? = Some(42)
  let new_some = some.filter((value : Int) => value > 40) // Some(42)
  let none = some.filter((value : Int) => value > 50) // None
  assert_true(new_some == Some(42))
  assert_true(none == None)
}

#Monadic operations

You can chain multiple operations that return Option using the bind method, which applies a function to the value if it is Some, otherwise, it returns None. Different from map, the function in argument returns an Option.

///|
test {
  let some : Int? = Some(42)
  let new_some = some.bind((value : Int) => Some(value + 1)) // Some(43)
  assert_true(new_some == Some(43))
}

Sometimes we want to reduce the nested Option values into a single Option, you can use bind(x => x) to achieve this. It transforms Some(Some(value)) into Some(value), and None otherwise.

///|
test {
  let some : Int?? = Some(Some(42))
  let new_some = some.bind(fn(x) { x }) // Some(42)
  assert_true(new_some == Some(42))
  let none : Int?? = Some(None)
  let new_none = none.bind(fn(x) { x }) // None
  assert_true(new_none == None)
}

empty

source

#deprecated("Use `None` instead")
fn[T] empty() -> T?

Return an empty value.

some

source

#deprecated("Use `Some(value)` instead")
fn[T] some(value : T) -> T?

Function some.

unless

source

#deprecated("Use `if !condition { Some(value()) } else { None }` instead")
fn[T] unless(condition : Bool, value : () -> T) -> T?

Conditional option helper unless.

when

source

#deprecated("Use `if condition { Some(value()) } else { None }` instead")
fn[T] when(condition : Bool, value : () -> T) -> T?

Conditional option helper when.

Source Files

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#Option

#Usage

#Create

#Extracting values

#Transforming values

#Monadic operations

#empty

#some

#unless

#when

Source Files

empty

some

unless

when