Immutable tables
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- Subject: Immutable tables
- From: Dirk Laurie <dirk.laurie@...>
- Date: Fri, 9 Mar 2012 13:14:56 +0200
>
> Immutable tables and immutable variables would be a valuable
> addition.
Let's think about implementation.
Immutable tables, easy. Put something in the metatable to mark it.
Immutable variables, not so easy. Variables are just entries in _ENV,
and their names are just the keys. _ENV can be any table. You need
to keep the information "in this table, assignment to or deletion of
this key is not allowed". `Node` has no spare bits.
So, we may as well implement immutable tables and variables at
the Lua level. A field `__immutable` in the metatable will serve
nicely. If it is a table, it contains a list of immutable names.
If it tests `false` nothing about it is immutable. If it is `true`,
the whole table is immutable.
Of course, any table exploiting `__immutable` will have to be
a proxy table so you can catch __newindex.
Everything can go into a module. The user will see:
----
immutable = require "immutable"
reserved_names = immutable(_ENV,{})
reserved_names.x = true -- x is now immutable
----
I'll sit down and write the code next time I feel I really need these
immutable thingies. Which probably means never.