RE: [Yaml-core] eliminating the tilde and ''

[Oren Ben-K. <or...@ri...>]

Steve Howell [mailto:sh...@zi...] wrote:
> Take this YAML:
> 
> translations:
>   tilde: maps to None
>   quote quote: maps to empty string
> implementation: PyYaml
> 
> If you were to eliminate the 2nd and 3rd lines from the YAML 
> above,

  translations:
  implementation: PyYaml

> you might expect either a syntax error (like PyYaml 
> now gives) or an empty dictionary
> (after all, you just removed items from a dictionary).

There's no marker that it is a dictionary. All you see is that there's no
value.

> Instead, you'd get an
> empty string, or None, or something else that's not entirely 
> clear to me.

It seems pretty intuitive to me. A null pointer to a dictionary is equal to
a null pointer to a sequence and to a null pointer to a string... This works
in C/C++/Java/Perl and, I suspect, Python. "No value" is just that - no
value and hence no value kind.

If you want an empty dictionary/sequence/string, these are all honest
non-null objects and deserve some explicit syntax. In YAML you'd write them
as:

empty dictionary: {}
empty sequence: []
empty string: ""

All of these are very different from a null ("no value") in the internal
representation of any language I'm aware of (except perhaps /bin/sh).

> I like flexibility with implicit typing, but allowing 
> somebody to omit the placeholder altogether just seems like a 
> recipe for confusion.

We'd have to forbid writing this:

---
This:
...

Forbidding it is artificial. It has an obvious semantics, that 'This' has no
value associated with it (i.e., the value is a null).

I agree that using an explicit '~' is more readable, and I'd encourage
emitters to use it as a default. That's a matter of taste, however.

> It also forces a lookahead in the parser.

There already is lookahead in the parser:

What am I? :

# Looking ahead...

# Still looking ahead...

  - value # Ah, a sequence.

It has nothing to do with dis/allowing empty plain scalars.

Have fun,

	Oren Ben-Kiki