meow.rkt

#lang racket

(require (rename-in racket/base [+ r:+]))
(provide +)

(define (join e1 e2)
  (cond
    [(and (number? e1) (number? e2))
     number?]
    [(and (number? e1) (symbol? e2))
     symbol?]
    [(and (number? e1) (string? e2))
     string?]
    [(and (number? e1) (void? e2))
     number?]
    [(and (symbol? e1) (number? e2))
     symbol?]
    [(and (symbol? e1) (symbol? e2))
     symbol?]
    [(and (symbol? e1) (string? e2))
     string?]
    [(and (symbol? e1) (void? e2))
     string?]
    [(string? e1)
     string?]
    [(string? e2)
     string?]))

(define (to f e)
  (match f
    [(? (curry eq? number?))
     (if (void? e) 0 e)]
    [(? (curry eq? symbol?))
     (string->symbol (format "~a" e))]
    [(? (curry eq? string?))
     (format "~a" e)]))

(define (add e1 e2)
  (match (join e1 e2)
    [(? (curry eq? number?))
     (r:+ (to number? e1) (to number? e2))]
    [(? (curry eq? symbol?))
     (lambda (e1 e2) (string-append (symbol->string (to symbol? e1))
                                            (symbol->string (to symbol? e2))))]
    [(? (curry eq? string?))
     (string-append (to string? e1) (to string? e2))]
    [else (error "meow" (join e1 e2))]))

(define +
  (lambda rest
    (cond
      [(null? rest)
       (void)]
      [(null? (cdr rest))
       (car rest)]
      [else
       (add (add (car rest) (cadr rest)) (apply + (cddr rest)))])))

(module+ test
  (require rackunit)
  (check-equal? (+ 2 2) 4)
  (check-equal? (+ 2 "") "2")
  (check-equal? (+ "hello" " " " world") "hello world")
  (check-equal? (+ "hello" " " 'world) "hello world")
  (check-equal? (+ "hello" 4 'world) "hello4world")
  (check-equal? (+ 4 'world) '4world))

wat.rkt

#lang sweet-exp racket
require "meow.rkt"

{"Hello, " + "world." + " What have you done." + " What did you do to my parens?!"}
{"Hello, " + 2 + 'lol}