please how can i edit this code in order to define a constant twopower
function,which will look similar to the following definition:
(define twoplus ’{fun {n} { n 2}})
Of course, twopower should not implement the function n 2, but 2n, for n a
non-negative integer.
for example if we use this test : the result will be 32
(test (interp-expr (parse ‘ {with {f ,twopowerg {f 5}})) 32)
this is the original code:
#lang plai
(define-type F1WAE
[num (n number?)]
[add (lhs F1WAE?)(rhs F1WAE?)]
[sub (lhs F1WAE?)(rhs F1WAE?)]
[with (name symbol?) (named-expr F1WAE?) (body F1WAE?)]
[id (name symbol?)]
[app (fun-name symbol?) (arg F1WAE?)])
(define-type FunDef
[fundef (fun-name symbol?)
(arg-name symbol?)
(body F1WAE?)])
;; newest form of parse, with apply added in.
(define parse
(lambda (sexp)
(cond
[(number? sexp) (num sexp)]
[(symbol? sexp) (id sexp)]
[(list? sexp)
(case (first sexp)
[(+)(add (parse (second sexp))
(parse (third sexp)))]
[(-) (sub (parse (second sexp))
(parse (third sexp)))]
[(with) (with (first (second sexp))
(parse (second (second sexp)))
(parse (third sexp)))]
[else (app (first sexp)
(parse (second sexp)))]
)])))
;; interp: F1WAE listof(fundef) -> number
;; evaluates F1WAE expressions by reducing them to their corresponding
values
(define interp
(lambda (expr fundefs)
(type-case F1WAE expr
[num (n) n]
[add (l r) (+ (interp l fundefs) (interp r fundefs))]
[sub (l r) (- (interp l fundefs) (interp r fundefs))]
[with (bound-id named-expr bound-body)
(interp (subst bound-body
bound-id
(num (interp named-expr fundefs)))
fundefs)]
[id (v) (error 'interp "Free identifier")]
[app (fun-name arg-expr)
(local ([define the-fun-def (lookup-fundef fun-name
fundefs)])
(interp (subst (fundef-body the-fun-def)
(fundef-arg-name the-fun-def)
(num (interp arg-expr fundefs)))
fundefs))])))
;; helper functions
(define lookup-fundef
(lambda (fun-name fundefs)
(cond
[(empty? fundefs)
(error 'lookup-fundef "Function does not exist")]
[else (if (symbol=? fun-name (fundef-fun-name (first fundefs)))
(first fundefs)
(lookup-fundef fun-name (rest fundefs)))])))
(define subst
(lambda (expr sub-id val)
(type-case F1WAE expr
[num (n) expr]
[add (l r) (add (subst l sub-id val)
(subst r sub-id val))]
[sub (l r) (sub (subst l sub-id val)
(subst r sub-id val))]
[with (bound-id named-expr bound-body)
(if (symbol=? bound-id sub-id)
(with bound-id
(subst named-expr sub-id val)
bound-body)
(with bound-id
(subst named-expr sub-id val) ;; added to
take care of scope
(subst bound-body sub-id val)))]
[id (v)(if (symbol=? v sub-id) val expr)]
[app (fun-name arg-expr)
(app fun-name (subst arg-expr sub-id val))])))
;; test
(print "{double {double 5}}")
(interp (parse '{double {double 5}})
(list (fundef 'double
'n
(add (id 'n) (id 'n)))))
---------------------------------------
Firas .M. Abu Hassan
Web Developer
SKITCE
Alquds University
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