Re: Proposal: Constant Tables (i.e. composed keys)
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- Subject: Re: Proposal: Constant Tables (i.e. composed keys)
- From: Dirk Laurie <dpl@...>
- Date: 2011年8月14日 11:18:48 +0200
On Sun, Aug 14, 2011 at 03:42:21AM +0200, Patrick Donnelly wrote:
> On Sat, Aug 13, 2011 at 6:50 PM, Dirk Laurie <dpl@sun.ac.za> wrote:
> > -- see PiL §13.4
> > local proxy = {}
> > local mt = {
> > __index = t,
> > __eq = ctab_eq,
> > __pairs = function() return pairs(t) end,
> > __ipairs = function() return ipairs(t) end,
> > __newindex = function()
> > error("attempt to update a read-only table",2)
> > end
> > }
> > setmetatable(proxy,mt)
>
> __pairs and __ipairs will return the original table allowing for
> modification. You need to return a custom iterator. Something like:
>
I also wrote:
> Warning: The safe thing is to make immutable tables only from table
> constants. If you have another reference to the table you are
> making immutable, that reference is not also immutable.
having in mind ordinary use by a possibly absent-minded but not
malicious programmer. But sure, that advice does not protect against
the sort of hacker who would write
s = imm {1,2,3}
_, original = pairs(s)
original[2] = 'Your PC is stoned'
print(s[2]) --> Your PC is stoned
I wondered why Roberto's PiL example of a read-only table does not
support pairs/ipairs. Now I know :-(
Dirk