# std/options

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This module implements types which encapsulate an optional value.

A value of type `Option[T]` either contains a value `x` (represented as `some(x)`) or is empty (`none(T)`).

This can be useful when you have a value that can be present or not. The absence of a value is often represented by `nil`, but that is not always available, nor is it always a good solution.

# Basic usage

Let's start with an example: a procedure that finds the index of a character in a string.

Example:

    import std/options
    proc find(haystack: string, needle: char): Option[int] =
      for i, c in haystack:
        if c == needle:
          return some(i)
      return none(int)  # This line is actually optional,
                        # because the default is empty

    let found = "abc".find('c')
    assert found.isSome and found.get() == 2

The `get` operation demonstrated above returns the underlying value, or raises `UnpackDefect` if there is no value. Note that `UnpackDefect` inherits from `system.Defect` and should therefore never be caught. Instead, rely on checking if the option contains a value with the isSome and isNone procs.

# Pattern matching

Note: This requires the fusion package.

fusion/matching supports pattern matching on `Option`s, with the `Some(<pattern>)` and `None()` patterns.

    {.experimental: "caseStmtMacros".}

    import fusion/matching

    case some(42)
    of Some(@a):
      assert a == 42
    of None():
      assert false

    assertMatch(some(some(none(int))), Some(Some(None())))

# Imports

typetraits

# Types

    Option[T] = object
      when T is SomePointer:
        val
      else:
          val
          has

  An optional type that may or may not contain a value of type `T`. When `T` is a a pointer type (`ptr`, `pointer`, `ref` or `proc`), `none(T)` is represented as `nil`. Source Edit

    UnpackDefect = object of Defect

  Source Edit

    UnpackError {....deprecated: "See corresponding Defect".} = UnpackDefect

  Deprecated: See corresponding Defect
  Source Edit

# Procs

    proc `$`[T](self: Option[T]): string

  Get the string representation of the `Option`.

  Example:

      assert $some(42) == "some(42)"
      assert $none(int) == "none(int)"

  Source Edit

    proc `==`[T](a, b: Option[T]): bool {.inline.}

  Returns `true` if both `Option`s are `none`, or if they are both `some` and have equal values.

  Example:

      let
        a = some(42)
        b = none(int)
        c = some(42)
        d = none(int)

      assert a == c
      assert b == d
      assert not (a == b)

  Source Edit

    proc filter[T](self: Option[T]; callback: proc (input: T): bool): Option[T] {.
        inline, effectsOf: callback.}

  Applies a `callback` to the value of the `Option`.

  If the `callback` returns `true`, the option is returned as `some`. If it returns `false`, it is returned as `none`.

  See also:

  - flatMap proc

  Example:

      proc isEven(x: int): bool =
        x mod 2 == 0

      assert some(42).filter(isEven) == some(42)
      assert none(int).filter(isEven) == none(int)
      assert some(-11).filter(isEven) == none(int)

  Source Edit

    proc flatMap[T, R](self: Option[T]; callback: proc (input: T): Option[R]): Option[
        R] {.inline, effectsOf: callback.}

  Applies a `callback` function to the value of the `Option` and returns the new value.

  If the `Option` has no value, `none(R)` will be returned.

  This is similar to `map`, with the difference that the `callback` returns an `Option`, not a raw value. This allows multiple procs with a signature of `A -> Option[B]` to be chained together.

  See also:

  - flatten proc
  - filter proc

  Example:

      proc doublePositives(x: int): Option[int] =
        if x > 0:
          some(2 * x)
        else:
          none(int)

      assert some(42).flatMap(doublePositives) == some(84)
      assert none(int).flatMap(doublePositives) == none(int)
      assert some(-11).flatMap(doublePositives) == none(int)

  Source Edit

    proc flatten[T](self: Option[Option[T]]): Option[T] {.inline.}

  Remove one level of structure in a nested `Option`.

  See also:

  - flatMap proc

  Example:

      assert flatten(some(some(42))) == some(42)
      assert flatten(none(Option[int])) == none(int)

  Source Edit

    proc get[T](self: Option[T]): lent T {.inline.}

  Returns the content of an `Option`. If it has no value, an `UnpackDefect` exception is raised.

  See also:

  - get proc with a default return value

  Example:

      assert some(42).get == 42
      doAssertRaises(UnpackDefect):
        echo none(string).get

  Source Edit

    proc get[T](self: Option[T]; otherwise: T): T {.inline.}

  Returns the content of the `Option` or `otherwise` if the `Option` has no value.

  Example:

      assert some(42).get(9999) == 42
      assert none(int).get(9999) == 9999

  Source Edit

    proc get[T](self: var Option[T]): var T {.inline.}

  Returns the content of the `var Option` mutably. If it has no value, an `UnpackDefect` exception is raised.

  Example:

      var
        a = some(42)
        b = none(string)
      inc(a.get)
      assert a.get == 43
      doAssertRaises(UnpackDefect):
        echo b.get

  Source Edit

    proc isNone[T](self: Option[T]): bool {.inline.}

  Checks if an `Option` is empty.

  See also:

  - isSome proc
  - none proc

  Example:

      assert not some(42).isNone
      assert none(string).isNone

  Source Edit

    proc isSome[T](self: Option[T]): bool {.inline.}

  Checks if an `Option` contains a value.

  See also:

  - isNone proc
  - some proc

  Example:

      assert some(42).isSome
      assert not none(string).isSome

  Source Edit

    proc map[T, R](self: Option[T]; callback: proc (input: T): R): Option[R] {.
        inline, effectsOf: callback.}

  Applies a `callback` function to the value of the `Option` and returns an `Option` containing the new value.

  If the `Option` has no value, `none(R)` will be returned.

  See also:

  - map proc
  - flatMap proc for a version with a callback that returns an `Option`

  Example:

      proc isEven(x: int): bool =
        x mod 2 == 0

      assert some(42).map(isEven) == some(true)
      assert none(int).map(isEven) == none(bool)

  Source Edit

    proc map[T](self: Option[T]; callback: proc (input: T)) {.inline,
        effectsOf: callback.}

  Applies a `callback` function to the value of the `Option`, if it has one.

  See also:

  - map proc for a version with a callback which returns a value

  Example:

      var d = 0
      proc saveDouble(x: int) =
        d = 2 * x

      none(int).map(saveDouble)
      assert d == 0
      some(42).map(saveDouble)
      assert d == 84

  Source Edit

    proc none(T: typedesc): Option[T] {.inline.}

  Returns an `Option` for this type that has no value.

  See also:

  - option proc
  - some proc
  - isNone proc

  Example:

      assert none(int).isNone

  Source Edit

    proc none[T](): Option[T] {.inline.}

  Alias for none(T). Source Edit

    proc option[T](val: sink T): Option[T] {.inline.}

  Can be used to convert a pointer type (`ptr`, `pointer`, `ref` or `proc`) to an option type. It converts `nil` to `none(T)`. When `T` is no pointer type, this is equivalent to `some(val)`.

  See also:

  - some proc
  - none proc

  Example:

      type
        Foo = ref object
          a: int
          b: string

      assert option[Foo](nil).isNone
      assert option(42).isSome

  Source Edit

    proc some[T](val: sink T): Option[T] {.inline.}

  Returns an `Option` that has the value `val`.

  See also:

  - option proc
  - none proc
  - isSome proc

  Example:

      let a = some("abc")

      assert a.isSome
      assert a.get == "abc"

  Source Edit

    proc unsafeGet[T](self: Option[T]): lent T {.inline.}

  Returns the value of a `some`. The behavior is undefined for `none`.

  Note: Use this only when you are absolutely sure the value is present (e.g. after checking with isSome). Generally, using the get proc is preferred.

  Source Edit