Re: [racket-users] Building regexen in at-exp

[Eli Barzilay]

On Fri, Oct 28, 2016 at 3:18 PM, 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

I see that the problem was pointed to you several times, but I didn't
see suggestions for a convenient replacement.  I often have something
like this:

#lang at-exp racket
(define px (compose pregexp string-append))   ; <--- like this
(define a "this")
@px{^@a}  ; works fine, and looks like a variant of #px"..."


[[ You can also do this for a one-time-use:

@(compose pregexp string-append){^@a}

but that's almost as bad as one of

@pregexp{@string-append{^@a}}
@pregexp[@string-append{^@a}]
(pregexp @string-append{^@a})

]]

-- 
   ((x=>x(x))(x=>x(x)))  Eli Barzilay:
   http://barzilay.org/  Maze is Life!

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Re: [racket-users] Building regexen in at-exp

[Vincent St-Amour]

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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[racket-users] Building regexen in at-exp

[David Storrs]

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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