To understand what's going on, consider the following program:
#lang at-exp racket
(define a "this")
(define (f . l)
(for-each displayln l))
@f{^@a}
which prints
^
this
What's going on is that the `{}` in at-exp notation will evaluate to a
list of strings. One for the "^", and one for `a`. `pregexp` accepts a
single string as argument (whereas `f` accepts as many as you want).
To make this work, you have to combine all these strings into one, as
you did using `~a`.
Vincent
On Fri, 28 Oct 2016 14:18:10 -0500,
David 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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