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
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