Re: Suggestion: Deprecate "Attempt to index a nil value" error; instead, return nil or create table

[Philippe Verdy <verdyp@...>]

Hmmm.. yes, but still "or nil" is not needed.

For such situation, where you expect a function to return a value or
an exception to catch in order to use an alternate (nil) value, I'd
use a utility function to use pcall():

function onerror(func, alt)
  local ok, value = pcall(func)
  if ok then return value end
  return alt
end

then:

v = onerror(function() return t[a][b][c][d][e] end, nil)

where you can choose which alternate value to use (not necessarily
nil, could be 0 or an empty string or anything else)

Le sam. 29 févr. 2020 à 19:07, Soni "They/Them" L. <fakedme@gmail.com> a écrit :
>
>
>
> On 2020-02-29 1:56 p.m., Philippe Verdy wrote:
> > I see no value at all in adding "or nil" (except that you'll change a
> > "false" stored value, returned without exception, into an "nil"). Then
> > why do you need "and t[a][b][c][d][e]" ? Wouldn' this be enough?:
> >
> >   _, v = pcall(function() return t[a][b][c][d][e] end)
> >
> > i.e. discard (_) the false/true status in the 1st return value of the
> > pcall, use the second value which is explicitly returned and will be
> > nil if the function does not return but throws an exception.
>
> actually it'll be the string "attempt to index a nil value"
>
> >
> > Le sam. 29 févr. 2020 à 17:43, Soni "They/Them" L. <fakedme@gmail.com> a écrit :
> > >
> > >
> > >
> > > On 2020-02-29 1:31 p.m., Egor Skriptunoff wrote:
> > > > On Sat, Feb 29, 2020 at 5:42 PM Italo Maia wrote:
> > > >
> > > >     the error when trying to index a null value is a good thing.
> > > >
> > > >
> > > >
> > > > Let's assume the statement
> > > > v = t[a][b][c][d][e]
> > > > Each of 5 indexing steps might result in nil.
> > > > Sometimes you want to raise "Attempt to index a nil value" exception,
> > > > sometimes you want to ignore the exception.
> > > >
> > > > If nil is indexable, then it's pretty easy to raise the exception
> > > > v = assert(t[a][b][c][d])[e]
> > > >
> > > > But if nil is non-indexable, it's not easy to ignore the exception
> > > > v = (((((t or {})[a] or {})[b] or {})[c] or {})[d] or {})[e]
> > > >
> > > > Indexable nil looks like a better alternative.
> > > >
> > > > Would indexable nil reduce runtime error detection?   No.
> > > > Almost always another exception will be raised soon:
> > > > "Attempt to perform arithmetic on a nil value" and the like.
> > > >
> > >
> > > v = (pcall(function() t[a][b][c][d][e] end) or nil) and t[a][b][c][d][e]
> > >
> > > slightly cleaner.
> > >
> >
>
>