[racket] Style or and/define

Maybe I should have used let/ec? Or a define-based variant of that?

Robby

On Tuesday, June 11, 2013, Laurent wrote:

> I'm also open to other solutions, but I find the (and (let (and (let (and
> ...))))) dance really inconvenient (verbose and not readable).
>
> So maybe it can be made cleaner, like not use `define' but `let' (as I
> actually did), and maybe use a keyword as Ian does, to show that it is not
> a normal expression, e.g.:
> (define (get-x-spot char-width)
>   (and char-width
>        #:let dc (get-dc)
>        dc
>        #:let style (or (send (get-style-list) find-named-style "Standard")
>                        (send (get-style-list) find-named-style "Basic"))
>        style
>        #:let fnt (send style get-font)
>        #:let-values (xw _1 _2 _3) (send dc get-text-extent "x" fnt)
>        (+ left-padding (* xw char-width))))
>
> This way you would not need to care about the actual result of the
> `#:let's (and you could even add some `#:for-effect' actions if you like ;).
>
> Laurent
>
> On Tue, Jun 11, 2013 at 7:27 PM, Carl Eastlund <cce at ccs.neu.edu> wrote:
>
> I don't have a big problem with the version that uses let.  But my point
> isn't really about the code quality, it's about the can of worms being
> opened with the specific proposed solution.  I'm open to other solutions.
>
> Also, re: definitions in and, bear in mind that definition macros do all
> kinds of crazy things.  Some might expand into multiple forms, including
> for-effect expressions.  That's another reason it's dangerous to put
> definitions into abnormal contexts that interpret them as anything other
> than a sequence of definitions and effects.  You don't want spurious (void)
> or (values) or some such to spoil your conditional.
>
> Carl Eastlund
>
> On Tue, Jun 11, 2013 at 1:21 PM, Laurent <laurent.orseau at gmail.com> wrote:
>
> Interesting, I see your point (not yet sure I adhere to it though).
>
> Anyway don't you think there is a readability problem with the mentioned
> code?
>
> Laurent
>
>
> On Tue, Jun 11, 2013 at 7:15 PM, Carl Eastlund <cce at ccs.neu.edu> wrote:
>
> I don't like the idea of definitions inside and, at all.  I'll elaborate
> on why.
>
> 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
>          sty
>
>
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