Source Edit

This module allows evaluating expressions safely against the following conditions:

nil dereferences
field accesses with incorrect discriminant in case objects

default(T) is returned in those cases when evaluating an expression of type T. This simplifies code by reducing need for if-else branches.

Note: experimental module, unstable API.

Example:

import std/wrapnils
type Foo = ref object
  x1: string
  x2: Foo
  x3: ref int

var f: Foo
assert ?.f.x2.x1 == "" # returns default value since `f` is nil

var f2 = Foo(x1: "a")
f2.x2 = f2
assert ?.f2.x1 == "a" # same as f2.x1 (no nil LHS in this chain)
assert ?.Foo(x1: "a").x1 == "a" # can use constructor inside

# when you know a sub-expression doesn't involve a `nil` (e.g. `f2.x2.x2`),
# you can scope it as follows:
assert ?.(f2.x2.x2).x3[] == 0

assert (?.f2.x2.x2).x3 == nil  # this terminates ?. early

Example:

import std/wrapnils
# ?. also allows case object
type B = object
  b0: int
  case cond: bool
  of false: discard
  of true:
    b1: float

var b = B(cond: false, b0: 3)
doAssertRaises(FieldDefect): discard b.b1 # wrong discriminant
doAssert ?.b.b1 == 0.0 # safe
b = B(cond: true, b1: 4.5)
doAssert ?.b.b1 == 4.5

# lvalue semantics are preserved:
if (let p = ?.b.b1.addr; p != nil): p[] = 4.7
doAssert b.b1 == 4.7

Imports

macros, options

Procs

func `[]`[T, I](a: Option[T]; i: I): auto {.inline.}: See top-level example. Source Edit
func `[]`[U](a: Option[U]): auto {.inline.}: See top-level example. Source Edit

Macros

macro `?.`(a: typed): auto: Transforms a into an expression that can be safely evaluated even in presence of intermediate nil pointers/references, in which case a default value is produced. Source Edit
macro `??.`(a: typed): Option: Same as ?. but returns an Option.
Example:

import std/options type Foo = ref object x1: ref int x2: int # `?.` can't distinguish between a valid vs invalid default value, but `??.` can: var f1 = Foo(x1: int.new, x2: 2) doAssert (??.f1.x1[]).get == 0 # not enough to tell when the chain was valid. doAssert (??.f1.x1[]).isSome # a nil didn't occur in the chain doAssert (??.f1.x2).get == 2 var f2: Foo doAssert not (??.f2.x1[]).isSome # f2 was nil doAssertRaises(UnpackDefect): discard (??.f2.x1[]).get doAssert ?.f2.x1[] == 0 # in contrast, this returns default(int)
Source Edit

Templates

template fakeDot(a: Option; b): untyped: See top-level example. Source Edit