[SOLVED] How to Implement Ternary Conditional in a Racket Language Interpreter with Structs?

How to Implement Ternary Conditional in a Racket Language Interpreter with Structs?

I’m working on an interpreter for a simple language in Racket, and I need to implement a ternary conditional (Expr ? Expr : Expr) similar to JavaScript. My language supports basic operations like addition, subtraction, and equality, and now I’m adding booleans and this ternary operation.

here is my current code for parse ;; parse: Expr -> AST ;; Parses “custom Lang” Expr to AST

(define (parse expr)
  (match expr
    [(? number?) (num expr)]                      
    [(? string?) (str expr)]                      
    ['TRUE (bool #t)]                            
    ['FALSE (bool #f)]                           
    [`(+ ,x ,y) (add (parse x) (parse y))]        
    [`(- ,x ,y) (sub (parse x) (parse y))]       
    [`(=== ,x ,y) (equals (parse x) (parse y))]  
    [`(,test '? ,true ': ,false)                 
     (condt (parse test) (parse true) (parse false))]
    [_ (error "Invalid custom Lang expression")]))

Im learning racket atm, and I only have previous java experience. im assuming this is like the class in java, and im defining what can be parsed? and then I would have a run fn that takes parsed input and output results.

here is my run

;; run: AST -> Result
;; Computes the result of running a customLang AST
(define (run ast)
  (match ast
    [(num n) n]
    [(str s) s]
    [(bool b) b]
    [(add x y) (cus+ (run x) (run y))]
    [(sub x y) (cus- (run x) (run y))]
    [(equals x y) (equal? (run x) (run y))]
    [(condt test true-branch false-branch)
     (if (truthy? (run test)) (run true-branch) (run false-branch))]
    [_ NaN]))

so I understood this as if true run 1st, if false run 2nd. and my parse seem to have correct setup. but when I try to run the tests

   ;; JS "truthy" true
   (check-equal? (evalcustom '(10 ? 100 : 200)) 100)

   ;; JS "truthy" false
   (check-equal? (evalcustom '((- 100 100) ? "a" : "b")) "b")))

it could not recognize and just outputs my error msg that I created. Im confused on how to make this structure and AST tree thing worked together and why they should work. therefore I could not figure out how to debug my issue.

thanks for help I really appreciate it.

Solution

You have an extra level of quotes in the pattern.

[`(,test '? ,true ': ,false)                 
     (condt (parse test) (parse true) (parse false))]

should be

[`(,test ? ,true : ,false)                 
     (condt (parse test) (parse true) (parse false))]

to match the list you're providing; the current pattern would need '(10 '? 100 ': 200) in order to match.