Re: RO-strings in Guile

[Mikael Djurfeldt <mdj@nada.kth.se>]

Jim Blandy <jimb@red-bean.com> writes:

> Here's what I want to do: "Efficient substrings via copy-on-write."
> 
>     The SUBSTRING primitive should work by sharing space with its
>     source string, but both should be marked as shared, so that if
>     either is side-effected, Guile will actually copy the string
>     first.
> 
>     This gets us the semantics of a copy, with the performance of
>     sharing.  C code needs to be a little careful, calling a
>     SCM_BREAK_SHARING macro before mutating a string, but we've got
>     restrictions like this already.  Code review solves this
>     problem. 
> 
>     We can get rid of read-only strings.
> 
>     We can get rid of the MAKE-SHARED-SUBSTRING primitive.
> 
>     This will need to interact nicely with threads, but I believe
>     there are ways around that.
> 
> If we have the above, then we can implement symbol->string using
> sharing, making it efficient enough that separate primitives which
> accept symbols as strings (like const-string-append) will be
> unnecessary.

This is an excellent proposal.