Error: usage of 'isNil' is a user-defined error

[Arnaz87]

test.nim

var a: string = "foo"
echo a.isNil

$ nim -r c test.nim
Hint: used config file '/etc/nim/nim.cfg' [Conf]
Hint: system [Processing]
Hint: test [Processing]
test.nim(2, 8) Error: usage of 'isNil' is a user-defined error

OS: Manjaro (Arch Linux)

$ nim -v
Nim Compiler Version 0.19.6 [Linux: amd64]
Compiled at 2019-05-14
Copyright (c) 2006-2018 by Andreas Rumpf

active boot switches: -d:release -d:nativeStackTrace

I can't understand how is it that a function as common as that doesn't work. Am I doing something wrong?

btw I noticed the version says it's compiled for an amd, but I'm running an x86-64

[kaushalmodi]

It's because a string cannot be nil. You need to compare with "" or look into using options stdlib module.

https://scripter.co/notes/nim/#nil-string

[Arnaz87]

I suggest removing the function on strings from the standard library since it doesn't make sense. Also on seqs which also seem to not work anymore.

[Arnaz87]

Works with ref

type Obj = ref object
  v: string

var a: Obj = Obj(v: "foo")
echo a.isNil

Doesn't work with seq

var a: seq[int] = @[1]
echo a.isNil

[kaushalmodi]

Looks like the isNil procs for strings and seqs still exist so that --nilseqs:on works during the transition period of this deprecation:

Nim/changelogs/changelog_0_19_0.md

Lines 180 to 181 in bce908f

	- ``nil`` for strings/seqs is finally gone. Instead the default value for
	these is ``"" / @[]``. Use ``--nilseqs:on`` for a transition period.

Nim/lib/system.nim

Lines 2828 to 2838 in bce908f

	when defined(nimNoNilSeqs2):
	when not compileOption("nilseqs"):
	{.pragma: nilError, error.}
	else:
	{.pragma: nilError.}
	else:
	{.pragma: nilError.}
	proc isNil*[T](x: seq[T]): bool {.noSideEffect, magic: "IsNil", nilError.}
	proc isNil*[T](x: ref T): bool {.noSideEffect, magic: "IsNil".}
	proc isNil*(x: string): bool {.noSideEffect, magic: "IsNil", nilError.}