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## std.options: Optional types.
## This module is imported by default.
use std.format
## The `Option` type.
## A type that represents either the presence or absence of a value.
pub type Option[T] = union
Some(T)
None
## Checks if a type is present within an `Option` type.
pub func is_some[T](self: Option[T]): bool =
self of Some(_)
## Checks if a type is not present within an `Option` type.
pub func is_none[T](self: Option[T]): bool =
self of None
## Converts an `Option[T]` to a `Result[T, E]` given a user-provided error.
pub func err[T, E](self: Option[T], error: E): Result[T, E] =
if self of Some(x):
Okay(x)
else:
Error(error)
## Applies a function to `T`, if it exists.
pub func map[T, U](self: Option[T], fn: T -> U): Option[U] =
if self of Some(x):
Some(fn(x))
else:
None
## Converts `T` to a `None`, if `fn` returns false and it exists.
pub func filter[T](self: Option[T], fn: T -> bool): Option[T] =
if self of Some(x) and fn(x):
Some(x)
else:
None
## Applies a function to T, if it exists. Equivalent to `self.map(fn).flatten`.
pub func flatmap[T, U](self: Option[T], fn: T -> Option[U]): Option[U] =
if self of Some(x):
fn(x)
else:
None
## Converts from Option[Option[T]] to Option[T].
pub func flatten[T](self: Option[Option[T]]): Option[T] =
match self
of Some(Some(x)):
Some(x)
of _:
None
## Returns the inner value or a default.
pub func get_or[T](self: Option[T], default: T): T =
if self of Some(x): x
else: default
## A dummy type signalling the empty value
type EmptyType = distinct unit
## Directly accesses the inner value. Throws an exception if None.
pub func : T =
if self of Some(x): x
else: raise EmptyType
## Indirect access. Propagates `None`.
## Parsed by compiler magic.
pub macro ?[T](self: Option[T]) =
quote:
match `self`
of Some(x): x
of None: return None
## Syntactic sugar for optional type declarations.
pub macro ?(T: type) =
quote:
Option[`T`]
## Overloads the `==` operation for use on Options.
pub func ==[T](a, b: Option[T]): bool =
match (a, b)
of (Some(x), Some(y)):
x == y
of _:
false
## Overloads the `str()` function for use on Options.
pub func str[T](self: Option[T]): str =
if self of Some(x):
fmt("Some({})", x.str)
else:
"None"
examples:
let x = Some(42)
if x of Some(y):
assert x! == y
# references:
# https://nim-lang.github.io/Nim/options.html
# https://doc.rust-lang.org/std/option/enum.Option.html
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