<div dir="ltr">Yes, expanding to let* will indeed be better for the optimizer. The optimizer, IIRC, currently uses a "You using set!? I give up" approach, and if you ever mention set!-ing the variable, the compiler will then insert typechecks every time the variable is used, slowing down things, and also box anything stored in it. While if a variable is not assigned to, usages of it do not need to check its type over and over. At least that's what I understood.</div><div class="gmail_extra"> <div class="gmail_quote">On Thu, Feb 19, 2015 at 3:07 PM, Alexander D. Knauth <<a href="mailto:alexander@knauth.org" target="_blank">alexander@knauth.org</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Because of this: <a href="http://docs.racket-lang.org/guide/set_.html#%28part._using-set%21%29" target="_blank">http://docs.racket-lang.org/guide/set_.html#%28part._using-set%21%29</a> Would this be better: (thread-through x e …) == (let* ([x e] …) x) <div class="HOEnZb"><div class="h5"> On Feb 19, 2015, at 1:37 PM, Matthias Felleisen <<a href="mailto:matthias@ccs.neu.edu">matthias@ccs.neu.edu</a>> wrote: > > I recommend a macro for these situations, it's almost always what you want: > > (thread-through x e_0 e ...) > == > (let ([x e_0]) > (set! x e) > ... > x) > > (you can also use let* here). > > This brings across what you're really doing. > > > > On Feb 19, 2015, at 1:32 PM, Jens Axel Søgaard <<a href="mailto:jensaxel@soegaard.net">jensaxel@soegaard.net</a>> wrote: > >> We have let and let*, but only define. I miss define* at times. >> >> This is an error: >> >> (block >> (define x 4) >> (define x (+ x 1)) >> (+ x 2)) >> >> With a define* it becomes: >> >> (block >> (define x 4) >> (define* x (+ x 1)) >> (+ x 2)) >> >> which (should) expand to: >> >> (block >> (define x 4) >> (block >> (define x (+ x 1)) >> (+ x 2))) >> >> /Jens Axel >> >> 2015-02-19 19:24 GMT+01:00 Laurent <<a href="mailto:laurent.orseau@gmail.com">laurent.orseau@gmail.com</a>>: >>> But in such situations you can be vicious and abuse `let` to have your >>> `define`s: >>> (let () >>> (define x 3) >>> (define y 4) >>> (list x y)) >>> >>> It's even more vicious if you use `let*` instead of `let`, but quite less if >>> you use `begin` instead ;) >>> >>> >>> >>> On Thu, Feb 19, 2015 at 5:49 PM, Matthias Felleisen <<a href="mailto:matthias@ccs.neu.edu">matthias@ccs.neu.edu</a>> >>> wrote: >>>> >>>> >>>> In some places, you are allowed only one expression, and for that >>>> situation, you need let*. >>>> >>>> >>>> On Feb 19, 2015, at 12:40 PM, Don Green <<a href="mailto:infodeveloperdon@gmail.com">infodeveloperdon@gmail.com</a>> >>>> wrote: >>>> >>>>> What is/are the reason(s) for choosing the 'let*' construct over the >>>>> 'define' construct? >>>>> >>>>> (define (print-two f) >>>>> (let* ([_ (print (first f))] >>>>> [f (rest f)] >>>>> [_ (print (first f))] >>>>> [f (rest f)]) >>>>> f)) >>>>> >>>>> (define print-two >>>>> (lambda (f) >>>>> (print (first f)) >>>>> (set! f (rest f)) >>>>> (print (first f)) >>>>> (set! f (rest f)) >>>>> f)) >>>>> >>>>> (void (print-two '(1 2))) ;=> 12 >>>>> >>>>> ____________________ >>>>> Racket Users list: >>>>> <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> >>>> >>>> >>>> ____________________ >>>> Racket Users list: >>>> <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> >>> >>> >>> >>> ____________________ >>> Racket Users list: >>> <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> >>> >> >> >> >> -- >> -- >> Jens Axel Søgaard > > > ____________________ > Racket Users list: > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> ____________________ Racket Users list: <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> </div></div></blockquote></div> </div>