Re: [Scheme-reports] mutable unicode strings

[Sam Tobin-Hochstadt <sam@x>]

On Wed, Jul 2, 2014 at 5:04 PM, Per Bothner <per@x> wrote:
> On 07/02/2014 12:34 AM, Sam Tobin-Hochstadt wrote:
>> Racket stores strings as arrays of 4-byte characters.
>
> On 07/02/2014 07:06 AM, Michael Montague wrote:
>> Foment uses 4 bytes to store each character in a string.
>
> The follow-up question is: When you have a Scheme reference
> to a string, presumably that is a pointer to a string object header
> containing at least a size and maybe some kind of typecode.  The
> actual data characters can be stored right next to the header (inlined),
> or the header can have a pointer to a separate array of data characters
> (indirected).  The latter makes it easy to resize the array by re-allocating
> it; doing it inlined makes resizing difficult.
>
> I would argue that the ability to resize a mutable string is so important
> that it justifies the slight overhead of indirection.  (Indirection may
> also be easier to implement, depending on how your memory allocation works.)
>
> In other words: Supporting string-replace! has no extra overheads beyond
> requiring an "indirect" representation.  The latter is forced anyway if
> you support mutability and full Unicode, unless you use 3- or 4-byte characters.
> Even if you do use 3- or 4-byte characters, indirection is worth it, because
> mutable fixed-size strings is an essentially-useless feature.

Racket provides mutable fixed-size strings with constant-time
string-ref and string-set! and no resizing. I think everyone agrees
that this is not a particularly great spot in the design space, but
it's where we ended up starting from earlier Scheme designs.

Sam

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