*@pregexp{^@a}* is read as *(pregexp "^" a)* [you can test this by
evaluating the quoted form, '*@pregexp{^@a}* ], but the function pregexp
expects a single string as its first argument (and, apparently, a function
or #f as its optional second argument).
More generally, the body part of an @-expression is read as several strings
(i.e. a list, or several arguments to a function), not a single string.
You could get what you want by writing, for example, *(pregexp @~a{^@a})*
On Fri, Oct 28, 2016 at 2:41 PM David Storrs <david.sto...@gmail.com> wrote:
> On Fri, Oct 28, 2016 at 3:24 PM, Ken MacKenzie <deviloc...@gmail.com>
> wrote:
> > In a future version of what I am working on I see regex being an
> excellent solution. My search is against a file I load into a list at
> program initialization. However for other parts of this to load a list
> based on a partial file filtered with regex may be a better solution.
>
> So you're doing something like "load the whole file, then search for
> all lines that start with prefix X"?
>
> You might try something like this:
>
> (define prefix #px"^foo") ;; Construct this however you like
>
> (for ((line (file->lines "/path/to/file)))
> (when (pregexp-match prefix l)
> (do-the-thing-with line)))
>
> Code not tested.
> >
> > There is also the case that I eventually switch to a DB backend, but at
> prototype stage I have been skipping that.
> >
> > Sorry not an answer to your query, but I think I could make use of the
> answer so inserting myself into the thread.
> >
> > Ken
> >
> > On Friday, October 28, 2016 at 3:18:14 PM UTC-4, David K. Storrs wrote:
> >> tl;dr : Why is the following an error?
> >>
> >> #lang at-exp racket
> >> (define a "this")
> >> @pregexp{^@a} ;; Should produce #px"^this" but errors out
> >> @pregexp{@(~a "^" a)} ;; This works but is clumsy
> >>
> >> Long version:
> >>
> >> The at-exp language
> >> (http://www.greghendershott.com/2015/08/at-expressions.html and
> >> https://docs.racket-lang.org/scribble/reader-internals.html) allows
> >> for (among other things) more convenient construction of regexen, like
> >> so:
> >>
> >> (pregexp "\\d\\d\\.\\d\\d") ;; base racket. Ugh.
> >> @pregexp{\d\d\.\d\d} ;; at-exp...ah, much better
> >>
> >> I started to reply to Ken MacKenzie's recent post about string
> >> prefixes with a suggestion that, although string-prefix was what he
> >> wanted in this case, a regex would be a more general solution. When I
> >> went to test the code I was suggesting, I was surprised to find it
> >> didn't work as expected. I thought maybe "^@" was a function or
> >> special form in Racket, but a quick search of the docs revealed
> >> nothing. I tried various forms of quoting inside the at-exp but
> >> nothing worked.
> >>
> >> What am I missing?
> >
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