On Feb 20, 2015, at 6:56 PM, Don Green <infodeveloper...@gmail.com> 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)) 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 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))) 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
