Not that I have any idea of what's going on, but interestingly, Typed Racket's second ->* example has (1)(3)(4). The use of list* may be possible if one follows the type (List* String Natural x) in the example.
https://docs.racket-lang.org/ts-reference/type-ref.html?#%28form._%28%28lib._typed-racket%2Fbase-env%2Fbase-types-extra..rkt%29._-~3e%2A%29%29 Shu-Hung On Fri, May 7, 2021 at 10:53 AM 'John Clements' via Racket Users <[email protected]> wrote: > > Background: I teach a PL course, using Shriram’s PLAI. Many of the > assignments require students to maintain an environment mapping symbols to > values. Shriram illustrates a nice easy way to do this, as a list of > two-element structures. You can also use an immutable hash. Fine. So I’m > grading a student submission, and I come across this: > > (: extend-environment* (-> Environment (Listof (List Symbol Value)) > Environment)) > (define (extend-environment* env kv) > (let loop ([kv : (Listof (List Symbol Value)) kv] > [env env]) > (cond > [(empty? kv) env] > [else > (let* ([kv-pair (first kv)] > [new-env (KVEnvironment (first kv-pair) (second kv-pair) env)]) > (loop (rest kv) new-env))]))) > > > ;; Returns a new environment with old-env as parent > (: extend-environment (->* (Environment Symbol Value) #:rest-star (Symbol > Value) Environment)) > (define (extend-environment env key value . kv) > (define kv* : (Listof (List Symbol Value)) > (let loop : (Listof (List Symbol Value)) > ([kv : (Rec x (U Null (List* Symbol Value x))) (list* key value kv)]) > (if (empty? kv) > '() > (let* ([take-two (list (first kv) (second kv))] > [drop-two (rest (rest kv))]) > (list* take-two (loop drop-two)))))) > (extend-environment* env kv*)) > > This solution uses > > 1) a named let, > 2) the list* function, > 3) a ->* type with a #:rest-star argument, and > 4) A custom rec type for alternating symbols and values. > > To cap it all off, the student DOESN’T EVEN USE the extra hardware. This is > the only use of extend-environment in the code: > > (extend-environment e name arg) > > So, uh, any idea where this code came from? > > :) > > John > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/ee31119c-aab4-48e4-94dd-39272dd34724%40mtasv.net. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAMTzy%2BZDuZbSW3ga%2BeDdiXhfgJSZpNS2u_nhB%2Bj9u40tYKXJKQ%40mail.gmail.com.
