[racket] Style or and/define

From: Carl Eastlund (cce at ccs.neu.edu)
Date: Tue Jun 11 13:15:08 EDT 2013

I don't like the idea of definitions inside and, at all.  I'll elaborate on

Internal definitions and for-effect expressions make sense to me when
computing a single result value, where the last form in sequence is the
result and everything else is just context for that.

They do not make sense to me in function arguments and other similar
contexts where, normally, each term's value contributes something to the
result.  Every expression in a function application has a result that is
used.  Every expression in an and form has a result that is used, if
evaluation doesn't stop earlier.

If we started adding definitions to and, or, &c., then suddenly I have to
wonder which terms are used as definitions and which as arguments.  Worse
yet, someone some day will want to put in an expression for effect in the
middle of an and, and then we'll have some real chaos.

I'm all for definitions anywhere they can be clearly seen as not part of
the result form.  Let's not put them in between arguments whose results
matter, please.

Carl Eastlund

On Tue, Jun 11, 2013 at 12:49 PM, Laurent <laurent.orseau at gmail.com> wrote:

> When I see what Robby is forced to write when following the Style:
> https://github.com/plt/racket/commit/09d636c54573522449a6591c805b38f72b6f7da8#L4R963
> I cannot help but think that something is wrong somewhere (it may not be
> the Style, and in case it wasn't clear I'm certainly not criticizing
> Robby's code).
> Using `let' and `and' instead, although being a bit better since it
> avoids all the [else #f], is not that big an improvement:
> (define (get-x-spot char-width)
>   (and
>    char-width
>    (let ([dc (get-dc)])
>      (and
>       dc
>       (let ([style (or (send (get-style-list) find-named-style "Standard")
>                        (send (get-style-list) find-named-style "Basic"))])
>         (and
>          style
>          (let*-values ([(fnt) (send style get-font)]
>                        [(xw _1 _2 _3) (send dc get-text-extent "x" fnt)])
>            (+ left-padding (* xw char-width)))))))))
> Actually I think here the right thing to do might be to allow for internal
> definitions inside `and':
> (define (get-x-spot char-width)
>   (and char-width
>        (define dc (get-dc))
>        dc
>        (define style (or (send (get-style-list) find-named-style
> "Standard")
>                          (send (get-style-list) find-named-style "Basic")))
>        style
>        (define fnt (send style get-font))
>        (define-values (xw _1 _2 _3) (send dc get-text-extent "x" fnt))
>        (+ left-padding (* xw char-width))))
> Isn't it *much* more readable? (shorter, avoid rightward drift, less
> parens, vertical alignment)
> Since it's not the first time I find the need for such internal
> definitions in `and', maybe this is something to consider for future
> addition to Racket? Or have some people already identified some problems
> with this idea?
> I've played a bit with it if you want to try by your own:
> https://gist.github.com/Metaxal/5758394
> (not sure I got it all good with syntax-parse though)
> Laurent
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