[racket-dev] Unique identifiers

What does unique mean in this context? Does probabilistically unique
work? If so could you form an identifier with the symbolic form
"unique-id-"+UUID?

On Fri, May 31, 2013 at 8:20 AM, Carl Eastlund <cce at ccs.neu.edu> wrote:
> I'm having trouble creating identifiers that are unique with respect to
> free-identifier=? and that survive marshaling to compiled code.  The normal
> source of fresh identifiers, generate-temporaries, only guarantees they are
> unique with respect to bound-identifier=?.  The obvious alternative, gensym,
> does not properly survive marshaling -- copies saved in different .zo files
> load as distinct identifiers.  I've tried a few alternative methods, none
> more successful than either of these.
>
> Below, there are a few short files that show the difficulties and can be
> used to test other methods.  The first, fresh.rkt, contains the procedure
> used to create fresh identifiers.  The file original.rkt creates a fresh
> identifier at compile time.  The file identical.rkt copies the identifier
> from original.rkt using quote-syntax.  The file different.rkt creates
> another fresh identifier at compile time.  The file check.rkt checks that
> the identifiers from original.rkt and identical.rkt are free-identifier=? to
> each other, and that the identifiers from original.rkt and different.rkt are
> not free-identifier=? to each other.  To run a test, first update fresh.rkt
> to use the appropriate method for creating identifiers, then run "raco make
> check.rkt".  Some of the methods work when simply running "racket
> check.rkt", but "raco make" marshals the identifiers to .zo files and
> exposes more problems.
>
> Can anyone suggest an implementation that would work here?
>
> ;;;;; fresh.rkt
> #lang racket
> (begin-for-syntax
>   (require racket/syntax)
>   (define (fresh)
>     ;; does not guarantee free-identifier=?
>     #;(generate-temporary)
>     ;; does not survive marshaling
>     #;(gensym)
>     ;; also does not survive marshaling
>     (begin
>       (define id0 (datum->syntax #false 'fresh))
>       (define ctx (syntax-local-make-definition-context))
>       (syntax-local-bind-syntaxes (list id0) #false ctx)
>       (internal-definition-context-seal ctx)
>       (internal-definition-context-apply ctx id0)))
>   (provide fresh))
>
> ;;;;; original.rkt
> #lang racket
> (require "fresh.rkt")
> (define-syntax (macro stx)
>   (with-syntax {[name (fresh)]}
>     #'(begin-for-syntax
>         (define original (quote-syntax name))
>         (provide original))))
> (macro)
>
> ;;;;; identical.rkt
> #lang racket
> (require "original.rkt")
> (define-syntax (macro stx)
>   (with-syntax {[orig original]}
>     #'(begin-for-syntax
>         (define identical (quote-syntax orig))
>         (provide identical))))
> (macro)
>
> ;;;;; different.rkt
> #lang racket
> (require "fresh.rkt")
> (define-syntax (macro stx)
>   (with-syntax {[name (fresh)]}
>     #'(begin-for-syntax
>         (define different (quote-syntax name))
>         (provide different))))
> (macro)
>
> ;;;;; check.rkt
> #lang racket
> (require "fresh.rkt" "original.rkt" "identical.rkt" "different.rkt")
> (begin-for-syntax
>   (unless (free-identifier=? original identical)
>     (error 'fresh "~v != ~v\n" original identical))
>   (when (free-identifier=? original different)
>     (error 'fresh "~v == ~v\n" original different)))
>
> --
> Carl Eastlund
>
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