Thanks for the explanation Robby. It's actually not bad, just a bit more
verbose, but it can then be tailored easily:
(define-syntax-rule (let/ec/check check body ...)
(let/ec return
(define (check v)
(unless v (return #f)))
body ...
))
(define/private (get-x-spot char-width)
(let/ec/check check
(check char-width)
(define dc (get-dc))
(check dc)
(define style (or (send (get-style-list) find-named-style "Standard")
(send (get-style-list) find-named-style "Basic")))
(check 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))))
I think it's quite ok then. Good idea.
Laurent
On Wed, Jun 12, 2013 at 12:15 PM, Robby Findler <ro...@eecs.northwestern.edu
> wrote:
> It would have looked like this. "ec" means escaping continuation: it is
> much weaker than a real continuation, something very much like 'return' or
> 'break' in other languages. Except that in Racket you have a little bit
> more fine-grained control over where you end up escaping out of (it isn't
> tied to a function or a loop). In this case, tho, you'd want something that
> returns from the function. The code would look like this:
>
> (define/private (get-x-spot char-width)
> (let/ec return
> (unless char-width (return #f))
> (define dc (get-dc))
> (unless dc (return #f))
> (define style (or (send (get-style-list) find-named-style "Standard")
> (send (get-style-list) find-named-style "Basic")))
> (unless style (return #f))
> (define fnt (send style get-font))
> (define-values (xw _1 _2 _3) (send dc get-text-extent "x" fnt))
> (+ left-padding (* xw char-width))))
>
> Robby
>
>
> On Wed, Jun 12, 2013 at 2:49 AM, Laurent <laurent.ors...@gmail.com> wrote:
>
>> I don't use continuations sufficiently to tell whether it would have been
>> better or not.
>> Anyway, I wasn't complaining at all about what you wrote or should have
>> written -since in general I really only care about what I write myself- but
>> about what I should have written if following the Style this way. The
>> existence of `and-let*' shows there is another solution, so I'm happy with
>> that.
>>
>> Thank you all,
>> Laurent
>>
>>
>>
>> On Tue, Jun 11, 2013 at 9:22 PM, Robby Findler <
>> ro...@eecs.northwestern.edu> wrote:
>>
>>> 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 <c...@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.ors...@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 <c...@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.ors...@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
>>>>
>>>>
>>
>
____________________
Racket Users list:
http://lists.racket-lang.org/users
