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
