How to put Lisp/Scheme/Racket symbol at the end?

Question

A form in Lisp/Scheme/Racket has symbol put at first position. This code works in Racket:

(define (? a b) (if a (display b) 0))
(? #t "Hello")

But I want to simulate the a? b:0 statement in C/C++ and the question mark (the symbol) is supposed to be at the end, after a.

How to put ? and a and b in this order?

(define (a ? b) (if a (display b) 0))
(#t ? "Hello")

Do I need to use define-syntax or that kind of thing?

Answer 1

Let me offer an alternative solution via overriding the application syntax #%app. Here's an example of the ternary operator in C-like languages:

#lang racket

(require syntax/parse/define
         (only-in racket [#%app racket:#%app]))

(define-syntax-parser #%app
  [(_ test-expr {~datum ?} then-expr {~datum :} else-expr)
   #'(if test-expr then-expr else-expr)]
  [(_ xs ...)
   #'(racket:#%app xs ...)])

> ((= 42 42) ? "a" : "b")
"a"
> ((= 42 0) ? "a" : "b")
"b"

Because the application syntax has the lowest priority, other syntax will be considered first:

> (define-simple-macro (discard-all xs ...) 1)
> (discard-all ? 42 : 123)
1
> (if ? 1 : 2)
if: bad syntax ;; because if should have exactly three subforms, but the above line has four.

But as Ryan said, this is not a "good" Racket code in a sense that it defies Racket convention and no one (well, probably except you) will be able to understand your code.