[racket] Why isn't at-exp the default?

Somewhat relatedly, have you ever considered making it possible for #lang languages to expose configuration parameters in the #lang line?

For instance, at-expressions can be marked with any unicode character. To use this functionality, however, one needs to set up a baby #lang that uses make-at-reader with the #:command-char parameter. This is what I did with my Pollen language so I could use the ◊ character for at-expressions by saying

#lang pollen

But someone who uses Pollen may not want that character. So in the interests of making easy language customizations easy, they might prefer to be able to do something like

#lang (pollen #:command-char [char])




	
On Nov 16, 2013, at 5:27 PM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> Back when we introduced `#lang racket`, we could have included
> @-notation by default, but I think we were focused on making `#lang
> racket` a small shift from `#lang scheme`. And `#lang scheme` was too
> long ago.
> 
> Backward compatibility rules out adding @-notation to `#lang racket`,
> but I agree that it would be better to include @-notation in future
> languages.
> 
> At Wed, 13 Nov 2013 22:06:11 -0500, Greg Hendershott wrote:
>> Lately my appreciation for `#lang at-exp ...` has grown. I've even
>> started to write things like:
>> 
>> (printf "The ~a and the ~a and the ~a\n" x y z)
>> 
>> instead as:
>> 
>> (displayln @~a{The @x and the @y and the @z})
>> 
>> When there's more than one or two items, this is a lot easier to write
>> (and change later). No more need to keep two lists in the same order.
>> 
>> Although I'm aware Ruby has something similar, I haven't pined for it.
>> It's just one example. Another is multi-line text fragments. In
>> general:
>> 
>> (define x @~a{One
>>              Two
>>              Three})
>> 
>> is waaaay nicer -- and editor indent friendly -- than reader "here"
>> strings like:
>> 
>> (define x #<<EOF
>> One
>> Two
>> Three
>> EOF
>> )
>> 
>> And there are more examples.
>> 
>> I'm not complaining about the need to type "at-exp".
>> 
>> But my question is, why hasn't at-exp at some point been made the
>> default reader for Racket?
>> 
>> Is it solely due to backward compatibility, the risk that existing
>> programs use @ in identifiers, like `(define @var)`?
>> 
>> Or is there some hidden cost or additional gotcha I should keep in mind?
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