perl-pre-rfc-not-exists.md

new internal value not_exists

Abstract

Introduce new internal SV value PL_sv_not_exists acting like PL_sv_undef in most cases (in every case when feature is not enabled).

PL_sv_not_exists will be used when value is not initialized yet.

• when key lookup doesn't exist (without autovivification)
• when optional parameter is not provided
• when variable is not initialized

Motivation

• to be able to distinguish between uninitialized value and value initialized to undef
• hash slices - to be able to distinguish between "key didn't exist in master hash" vs. "key set to undef in master hash"
• to prevent autovivification (overlaps with RFC-0021 but optional chaining is implicit)
• leads to: Data::Path-like extension capable to set value to not_exist-ing lookup (with necessary autovivification)

Intended behaviour

Some overview of changes in language (lot of boring details omitted)

• new feature not-exists
• new internal constant SV PL_sv_not_exists will be treated like PL_sv_undef except of PL_sv_not_exists specific behaviour
• every dereference of not_exists will result into not_exists
• new postfix operator ?
  • returning not_exists for undef, not_exists, builtin::false
• new postfix operator !
  • returning undef when argument is not_exists
• exists
  • will accept also scalar (holding not_exists value)
• delete
  • will accept also scalar
  • will set value to not_exists

Related

• SQL and its NULL
• Javascript and its undefined
• previous thread https://perl.markmail.org/message/lupj45g6bxkw2hrl
• PerlX::Maybe

Affected RFCs

• RFC-0021 - optional chaining this pre-RFC wraps intended usage of optional chaining completely with small difference, instead of single operator ?-> there will two operators ? ->