[racket] Style or and/define

I've already extended and for my own purposes. Particularly, I sometimes want to use the result of an expression that's in the and without breaking out of the and, so I added a keyword #:bind id before any expression to bind id to the result for the remainder of the and. I would add defines to this extended and, but that requires a local-expand. Not sure if I want to do that.
-Ian
----- Original Message -----
From: "Sean McBeth" <sean.mcbeth at gmail.com>
To: "Laurent" <laurent.orseau at gmail.com>
Cc: "Racket Mailing List" <users at lists.racket-lang.org>
Sent: Tuesday, June 11, 2013 1:11:26 PM GMT -05:00 US/Canada Eastern
Subject: Re: [racket] Style or and/define




Okay, there is one difference between our examples, in that yours will short-circuit and mine won't. However, if that is not a concern and you're looking more for readability, you could even use local define 

(define (get-x-spot char-width) 
(define dc (and char-width (get-dc))) 
(define style (and dc (or (send (get-style-list) find-named-style "Standard") 
(send (get-style-list) find-named-style "Basic")))) 
(define fnt (and style (send style get-font))) 
(when fnt 
(define-values (xw _1 _2 _3) (send dc get-text-extent "x" fnt)) 
(+ left-padding (* xw char-width)))) 





On Tue, Jun 11, 2013 at 1:04 PM, Sean McBeth < sean.mcbeth at gmail.com > wrote: 



Would let* mostly achieve this? 

(define (get-x-spot char-width) 
(let* ([dc (and char-width (get-dc))] 
[style (and dc (or (send (get-style-list) find-named-style "Standard") 
(send (get-style-list) find-named-style "Basic")))] 
[fnt (and style (send style get-font))]) 
(when fnt 

(define-values (xw _1 _2 _3) (send dc get-text-extent "x" fnt)) 
(+ left-padding (* xw char-width))))) 






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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