Because of this: http://docs.racket-lang.org/guide/set_.html#%28part._using-set%21%29 Would this be better: (thread-through x e …) == (let* ([x e] …) x)
On Feb 19, 2015, at 1:37 PM, Matthias Felleisen <[email protected]> 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 <[email protected]> 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 <[email protected]>: >>> 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 <[email protected]> >>> 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 <[email protected]> >>>> 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: >>>>> http://lists.racket-lang.org/users >>>> >>>> >>>> ____________________ >>>> Racket Users list: >>>> http://lists.racket-lang.org/users >>> >>> >>> >>> ____________________ >>> Racket Users list: >>> http://lists.racket-lang.org/users >>> >> >> >> >> -- >> -- >> Jens Axel Søgaard > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
