On Feb 20, 2015, at 10:45 PM, Alexander D. Knauth <[email protected]> wrote:
> > On Feb 20, 2015, at 6:56 PM, Don Green <[email protected]> wrote: > >> I looked in racket documentation and discussion archives for a >> thread-through function as illustrated below but without success. >> Any suggestions where I should look? OR please explain that line below only. >> Looks like it is declaring thread-safe variables x e and any others I care >> to list. Is that correct? >> >> ;Use a macro: >> (thread-through x e …) >> == >> (let* ([x e] …) x) >> ;—————————— > > If you have this macro definition: > > (define-simple-macro > (thread-through x e ...) > (let* ([x e] ...) x)) I probably should start with a simpler example. This: (thread-through x 1 (add1 x)) Will evaluate to the same thing as: (add1 1) This: (thread-through x 1 (add1 x) (number->string x)) Will evaluate to the same thing as: (number->string (add1 1)) This: (thread-through x ‘foo (symbol->string x) (string->list x) (reverse x) (list->string x)) Will evaluate to the same thing as: (list->string (reverse (string->list (symbol->string ‘foo)))) And If you wanted to do something like this: (map first (sort (map (λ (x) (list x (modulo (expt 11 x) 13))) '(0 1 2 3 4 5 6 7 8 9 10)) < #:key second)) > > Then you can do things like this: (contrived example) > > (thread-through > lst '(0 1 2 3 4 5 6 7 8 9 10) > (map (λ (x) (list x (modulo (expt 11 x) 13))) lst) > (sort lst < #:key second) > (map first lst)) ; '(0 7 4 2 3 5 9 8 10 1 6) And if you wanted to do something like this: (let ([tree 0]) (set! tree (list tree 1 tree)) (set! tree (list tree 2 tree)) (set! tree (list tree 3 tree)) tree) > > And this: (adapted from an example in > http://docs.racket-lang.org/guide/set_.html#%28part._using-set%21%29) > > (thread-through > tree 0 > (list tree 1 tree) > (list tree 2 tree) > (list tree 3 tree)) ; '(((0 1 0) 2 (0 1 0)) 3 ((0 1 0) 2 (0 1 0))) Or if you wanted to do something like this: (string->bytes/utf-8 (number->string (bytes-length #”foobar”) 16)) > > And this: (adapted from > http://pkg-build.racket-lang.org/doc/rackjure/index.html#%28part._.Threading_macros%29) > > (thread-through > x #"foobar" > (bytes-length x) > (number->string x 16) > (string->bytes/utf-8 x)) ; #"6" > > It has nothing to do with thread-safe variables. > It is similar in spirit to ~> from rackjure, or -> from clojure, or ~> or > thrush+ from point-free, if you want to look at those. > > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
