Re: String indexing again

[HyperHacker <hyperhacker@...>]

On Mon, Dec 20, 2010 at 16:42,  <jgiors@threeeyessoftware.com> wrote:
>> ------------------------------
>> Date: Mon, 20 Dec 2010 14:03:58 -0700
>> From: HyperHacker <hyperhacker@gmail.com>
>> Subject: Re: String indexing again
>> To: Lua mailing list <lua-l@lists.lua.org>
>> Message-ID:
>>       <AANLkTi=voiYapisjqz6gvb-c1oXMjgTe=Ma-Apb48MJ4@mail.gmail.com>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>> On Mon, Dec 20, 2010 at 11:21,  <jgiors@threeeyessoftware.com> wrote:
>
> <...snip...>
>
>> > I think an argument against indexing of characters in a string is that
>> > it implies indexing is acceptable on the left-hand side of an
>> > assignment:
>> >
>> > s = "ABxDEF"
>> > s[3] = "C"    -- Oh-oh...
>> >
>> > If I am not mistaken, this cannot work in standard Lua (i.e. there is no
>> > way to make s contain "ABCDEF" after the assignment). The failure of
>> > assigning to a character in a string would probably be just as confusing
>> > as what you've mentioned above, especially if reading a character from a
>> > string (with an array index) is allowed.
>> >
>> > John Giors
>> > Independent Programmer
>> > Three Eyes Software
>> > jgiors@ThreeEyesSoftware.com
>> > http://ThreeEyesSoftware.com
>>
>> That could be done with __newindex. It might also allow s[3] = 65
>> (assigning by character value instead of string).
>>
> <...snip...>
>
> __newindex will not work since it does not have access to named variable
> "s" [in my example]. It only receives a pass-by-value copy of the
> string, and (as Drake Wilson replied) strings are immutable.
>
> The best way to see the problem is to try to write a __newindex which
> changes "s". Here is a Lua command-line example which demonstrates the
> issue [note: it is not a full-blown assignment handler]:
>
> Lua 5.1.2  Copyright (C) 1994-2007 Lua.org, PUC-Rio
>> m=getmetatable""
>> m.__newindex=function(str,i,c)
>>> print("string __newindex",str,i,c)
>>> str="ABCDEF"
>>> end
>> s="ABxDEF"
>> s[3]="C"
> string __newindex       ABxDEF  3       C
>> print(s)
> ABxDEF
>
>
> John Giors
> Independent Programmer
> Three Eyes Software
> jgiors@ThreeEyesSoftware.com
> http://ThreeEyesSoftware.com
>
>
>
>

Yes, I realize the problem there. Can't think of a sane way to resolve
that... still, I don't see why we can't support read-only indexing
with a brief explanation in the manual of why assignment isn't
possible.

-- 
Sent from my toaster.