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 (
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 it is not always available, nor is it always a good solution.
Let's start with an example: a procedure that finds the index of a character in a string.
import 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
try: assert("abc".find('c').get() == 2) # Immediately extract the value except UnpackError: # If there is no value assert false # This will not be reached, because the value is present
get operation demonstrated above returns the underlying value, or raises
UnpackError if there is no value. There is another option for obtaining the value:
unsafeGet, but you must only use it when you are absolutely sure the value is present (e.g. after checking
isSome). If you do not care about the tiny overhead that
get causes, you should simply never use
How to deal with an absence of a value:
let result = "team".find('i') # Nothing was found, so the result is `none`. assert(result == none(int)) # It has no value: assert(result.isNone) try: echo result.get() assert(false) # This will not be reached except UnpackError: # Because an exception is raised discard
Option[T] = object val: T has: bool
UnpackError = ref object of ValueError
proc some[T](val: T): Option[T]
Optionthat has this value.
proc none(T: typedesc): Option[T]
Optionfor this type that has no value.
proc isSome[T](self: Option[T]): bool
proc isNone[T](self: Option[T]): bool
proc unsafeGet[T](self: Option[T]): T
some. Behavior is undefined for
proc get[T](self: Option[T]): T
proc get[T](self: Option[T]; otherwise: T): T
proc map[T](self: Option[T]; callback: proc (input: T))
proc map[T, R](self: Option[T]; callback: proc (input: T): R): Option[R]
proc filter[T](self: Option[T]; callback: proc (input: T): bool): Option[T]
proc `==`(a, b: Option): bool
Option``s are ``none, or if they have equal values
proc `$`[T](self: Option[T]): string
© 2006–2017 Andreas Rumpf
Licensed under the MIT License.