afternoon code from friday

2018-10-05-afternoon.rkt

#lang typed/racket

(require typed/rackunit)


(define-type Value String)

(define-type ExprC (U ConcatC StrC))
(struct ConcatC ([l : ExprC] [r : ExprC])
  #:transparent)
(struct StrC ([s : String]) #:transparent)

(ConcatC (StrC "abc") (StrC "def"))

;; parse an expression
(define (parse [s : Sexp]) : ExprC
  (match s
    [(? string? str) (StrC str)]
    [(list l '+ r) (ConcatC (parse l) (parse r))]
    [other (error 'parse "ouch!")]))

(check-equal? (parse '"apple") (StrC "apple"))
(check-equal? (parse '{"abc" + {"d" + "ef"}})
              (ConcatC (StrC "abc")
                       (ConcatC (StrC "d")
                                (StrC "ef"))))


;; evaluate an expression
(define (interp [exp : ExprC]) : Value
  (match exp
    [(StrC s) s]
    [(ConcatC l r) (string-append (interp l) (interp r))]))

(check-equal? (interp (StrC "apple")) "apple")
(check-equal? (interp (ConcatC (StrC "abc")
                       (ConcatC (StrC "d")
                                (StrC "ef"))))
              "abcdef")

#;((check-equal? (interp (parse '"apple")) "apple")
(check-equal? (interp (parse '{"abc" + "def"}))
))