Let's take a look at this error:
"format-id: undefined;
cannot reference an identifier before its definition
phase: 1"
Since format-id is undefined, we need to require the corresponding module.
The docs say the identifier is from racket/syntax.
Now
(require racket/syntax) will import racket/syntax to phase 0 (runtime)
and
(require (for-syntax racket/syntax)) will import racket/syntax to
phase 1 (compile time).
In general: the body of a define-syntax runs on compile time, so all
identifiers needs to required using (for-syntax ...).
If in doubt: (require racket/syntax (for-syntax racket/syntax)).
Note: If you want to play with, say format-id in the repl, note that
the repl is in phase 0 - which means (require racket/syntax) is
needed before format-id can be used in the repl.
/Jens Axel
2017-01-12 18:37 GMT+01:00 Erich Rast <er...@snafu.de>:
> Thanks for your your help. Unfortunately, I haven't managed to get my
> head around the phases and the question of when symbols are quoted or
> evaluated. I tried Matthew Buttericks change but it yields an error:
>
> "format-id: undefined;
> cannot reference an identifier before its definition
> phase: 1"
>
> So I tried to implement Matthias Felleisen's suggestion (see below), but
> now I get "home-dir: unbound identifier in module in: home-dir". Why
> does this occur and how can I fix this? And is there a way to avoid
> quoting db-path when using defpref?
>
> Best,
>
> Erich
>
> --------
> #lang racket
> (require framework/preferences)
> ;"constants.rkt")
> (provide (prefix-out pref: init) (prefix-out pref: db-path))
>
> (define default-datapath (build-path (find-system-path 'home-dir)
> "Documents" "va"))
>
> (define *defaults* '())
>
> (define (lookup-key ident)
> (string->symbol (string-append "va:" (symbol->string ident))))
>
> (define-syntax pref
> (syntax-rules ()
> [(pref ident) (let ((key (lookup-key ident)))
> (unless (preferences:default-set? key) (init))
> (preferences:get key))]
> [(pref ident value) (let ((key (lookup-key ident)))
> (unless (preferences:default-set? key) (init))
> (preferences:set key value))]
> [(pref ident expr test?)
> (set! *defaults* (cons (list (lookup-key ident) expr test?)
> *defaults*))]))
>
> (define-syntax defpref
> (syntax-rules ()
> [(defpref ident default test?) (begin
> (define ident
> (case-lambda
> [() (pref ident)]
> [(value) (pref ident value)]))
> (pref ident default test?))]))
>
> (defpref 'db-path default-datapath path?)
>
> (define (init)
> (make-directory* default-datapath)
> (for-each
> (lambda (triple)
> (preferences:set-default (first triple) (second triple) (third
> triple))) *defaults*))
>
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--
--
Jens Axel Søgaard
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