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Questionable 🤔

Option and Result are two powerful abstractions that can be used instead of raising errors. They can be a bit unwieldy though. This library is an attempt at making their use a bit more elegant.

Installation

Use the Nimble package manager to add questionable to an existing project. Add the following to its .nimble file:

requires "questionable >= 0.10.15 & < 0.11.0"

If you want to make use of Result types, then you also have to add either the result package, or the stew package:

requires "results" # either this
requires "stew"   # or this

Options

You can use ? to make a type optional. For example, the type ?int is just short for Option[int].

import questionable

var x: ?int

Assigning values is done using the some and none procs from the standard library:

x = 42.some   # Option x now holds the value 42
x = int.none  # Option x no longer holds a value

Option binding

The =? operator lets you bind the value inside an Option to a new variable. It can be used inside of a conditional expression, for instance in an if statement:

x = 42.some

if y =? x:
  # y equals 42 here
else:
  # this is never reached

x = int.none

if y =? x:
  # this is never reached
else:
  # this is reached, and y is not defined

The without statement can be used to place guards that ensure that an Option contains a value:

proc someProc(option: ?int) =
  without value =? option:
    # option did not contain a value
    return

  # use value

Option chaining

To safely access fields and call procs, you can use the .? operator:

Note: in versions 0.3.x and 0.4.x, the operator was ?. instead of .?

var numbers: ?seq[int]
var amount: ?int

numbers = @[1, 2, 3].some
amount = numbers.?len
# amount now holds the integer 3

numbers = seq[int].none
amount = numbers.?len
# amount now equals int.none

Invocations of the .? operator can be chained:

import sequtils

numbers = @[1, 1, 2, 2, 2].some
amount = numbers.?deduplicate.?len
# amount now holds the integer 2

Fallback values

Use the |? operator to supply a fallback value when the Option does not hold a value:

x = int.none

let z = x |? 3
# z equals 3

Obtaining value with !

The ! operator returns the value of an Option when you're absolutely sure that it contains a value.

x = 42.some
let dare = !x     # dare equals 42

x = int.none
let crash = !x    # raises a Defect

Operators

The operators [], -, +, @, *, /, div, mod, shl, shr, &, <=, <, >=, > are all lifted, so they can be used directly on Options:

numbers = @[1, 2, 3].some
x = 39.some

let indexed = numbers[0]  # equals 1.some
let sum = x + 3           # equals 42.some

Results

Support for Result is considered experimental. If you want to use them you have to explicitly import the questionable/results module:

import questionable/results

You can use ?! make a Result type. These Result types either hold a value or an error. For example the type ?!int is short for Result[int, ref CatchableError].

proc example: ?!int =
  # either return an integer or an error

Results can be made using the success and failure procs:

proc works: ?!seq[int] =
  # always returns a Result holding a sequence
  success @[1, 1, 2, 2, 2]

proc fails: ?!seq[int] =
  # always returns a Result holding an error
  failure "something went wrong"

Binding, chaining, fallbacks and operators

Binding with the =? operator, chaining with the .? operator, fallbacks with the |? operator, and all the other operators that work with Options also work for Results:

import sequtils

# binding:
if x =? works():
  # use x

# chaining:
let amount = works().?deduplicate.?len

# fallback values:
let value = fails() |? @[]

# lifted operators:
let sum = works()[3] + 40

Without statement

The without statement can also be used with Results. It provides access to any errors that may arise:

proc someProc(r: ?!int) =
  without value =? r, error:
    # use `error` to get the error from r
    return

  # use value

Catching errors

When you want to use Results, but need to call a proc that may raise an error, you can use catch:

import strutils

let x = parseInt("42").catch  # equals 42.success
let y = parseInt("XX").catch  # equals int.failure(..)

Conversion to Option

Any Result can be converted to an Option:

let converted = works().option  # equals @[1, 1, 2, 2, 2].some