Re: Bug with unset variables

[Felipe Contreras <felipe.contreras@xxxxxxxxx>]

On Thu, Nov 12, 2020 at 2:46 AM Roman Perepelitsa
<roman.perepelitsa@xxxxxxxxx> wrote:
> On Wed, Nov 11, 2020 at 9:42 PM Felipe Contreras
> <felipe.contreras@xxxxxxxxx> wrote:

> > In all these it should be obvious what would be the sensible default, but this:
> >
> >   typeset var
>
> Both bash and zsh are consistent. Regardless of the presence or
> absence of a storage specifier, bash leaves the variable unset while
> zsh sets it to the "natural" value of the parameter's storage type. By
> natural I mean neutral w.r.t. +=. If `typeset -i var` was setting
> `var` to zero while `typeset var` was leaving it unset, that would be
> inconsistent. In my opinion this would be worse than the behavior of
> bash and zsh.

No. Zsh is not consistent. I did not type /typeset var=''/, I typed
/typeset var/.

> The fact that unset parameters are also called "null" in ksh/bash/zsh
> invites confusion when comparing them to languages that can have
> parameters with null *values*. Those null values are first-class
> citizens. You can pass them as arguments to functions, store them in
> arrays, etc. Shells don't have null *values*, they just have unset
> parameters.

This is distinction without a difference, like saying we are not lost,
we just don't know where we are. Conceptually it is the same thing,
you are just using a different word for it. It's wordplay.

An unset variable is for all intents and purposes a variable with a null value.

> Most languages (in fact, all languages I know) either don't have the
> notion of an unset variable with function scope, or automatically give
> all declared variables values. The closest equivalent to ksh/bash/zsh
> I'm aware of is elisp because it also has dynamic typing and dynamic
> scope. elisp has the same notion of an unset variable as ksh/bash/zsh
> (they are called void in elisp). You can declare local variables with
> `let` and unset them with `makunbound`. These behave like `typeset`
> and `unset` in zsh -- in order to create an unset variable with
> function scope, you need to declare it and then unset. Declaring the
> variable without value won't do.

So you don't know JavaScript (one of the most popular languages today)?

  > var v
  > typeof v
  'undefined'

Even in Python and Ruby the way you "declare" variables without a type
is by setting them to the equivalent of the null value. This has
*exactly* the same effect as "local x". Once again it's a distinction
without a difference.

In Swift you can declare a variable with the type "Any", and by
default it has the nil value.

Virtually all languages have a way of declaring a variable with a
local scope, and *all* of them (including shell) have an idiom to do
it without assigning an empty string (except zsh).

> In sum, what zsh does makes sense to me and feels natural and
> consistent with other languages I know.

Carrying luggage without wheels also felt natural.

Humans can get used to anything.

You cannot tell me that if I originally have this:

  func () {
    [[ -n "$1" ]] && var=$1
    dosomething ${var-other}
  }

And I want to change the scope of the variable, so it's not set
globally (which can be done in plenty of languages), and then I do
this:

  func () {
    typeset var
    [[ -n "$1" ]] && var=$1
    dosomething ${var-other}
  }

It makes sense to *change* the behavior of the code.

Can you?

> That isn't to say that I
> consider the behavior of ksh/bash incorrect. It's a bit surprising but
> sensible. I could definitely get used to it. The strongest argument
> for changing zsh is consistency with ksh and bash. The strongest
> argument against it is that it'll break a lot of existing zsh code.
> It's not my call but to me this looks like a no-go.

This is a false dichotomy.

Adding a setopt option for the new behavior doesn't break a lot of
existing zsh code.

Cheers.

-- 
Felipe Contreras